Waste Water Story Class 7 Notes Science Chapter 18

Waste Water Story Class 7 Notes Science Chapter 18

CBSE Class 7 Science Notes Chapter 18 Waste Water Story Pdf free download is part of Class 7 Science Notes for Quick Revision. Here we have given NCERT Class 7 Science Notes Chapter 18 Waste Water Story. https://www.cbselabs.com/waste-water-story-class-7-notes/

CBSE Class 7 Science Notes Chapter 18 Waste Water Story

Waste Water Story Class 7 Notes

Water is an elixir of life. We all use water and in that process of using, we make it dirty. The water rich in a lather, mixed with oil and other pollutants that go down the drains from sinks, showers, toilets, laundries is dirty. It is called wastewater.

We use water every day for drinking, bathing, washing clothes, cooking food, washing utensils, flushing toilets, mopping the floors, etc. We cannot think of our life without water. We use lots of water daily which is passed on to the drainage system every day.

Water, Our Lifeline
Water is needed by all forms of life. Clean water is a basic need of human beings. Unfortunately, clean water is not available to a large segment of human populations.

The water which is unfit for human consumption becomes the source of many water-borne diseases which ultimately lead to loss ( of human life. It is estimated that one billion human beings do not get safe drinking water. So, realising the urgency we celebrate 22nd March as World Water Day to bring awareness amongst people for safe water, fit for human consumption.
Thus, the water is cleaned by removing pollutants before it enters a waterbody or is reused. This process of wastewater treatment is commonly known as sewage treatment which takes place in several steps as discussed later.

On the World Water Day, i.e. 22 March 2005, the General Assembly of the United Nations proclaimed the period 2005-2015 as the International Decade for action on Water for Life. All efforts made during this decade aim to reduce by half the number of people who do not have access to safe drinking water.

Sewage
It is wastewater released by homes, industries agricultural fields and other human activities. It also includes rainwater that has run down the street during a storm or heavy rain and it is liquid waste. Most of its water has dissolved and suspended impurities called contaminants.

Wastewater Story Class 7 Notes

Composition of Sewage
The following components make the sewage:

  • The organic impurities present in sewage are human faeces, animal wastes (like animal dung), urea (as urine), oil, fruits and vegetable wastes, pesticides, herbicides, etc.
  • The inorganic impurities present in sewage are nitrates, phosphates and metals.
  • The nutrients present in sewage are nitrogen and phosphorus.
  • The bacteria present in sewage include those bacteria which cause water-borne diseases such as cholera and typhoid.
  • The other microbes present in sewage are Protozoa which cause a water-borne disease called dysentery.

Water Freshens Up: An Eventful Journey
In a house (a public building) generally, there are two sets of pipes, i.e. one set of pipes brings clean drinking water into the house and the other set of pipes takes away wastewater (sewage) from houses. For proper sanitation, a well-maintained sewage system is required.

Sewerage System
The pipes which carry away wastewater or sewage from houses and other buildings are buried under the ground. An underground pipe which carries away dirty drainage water and waste matter is called sewer. The provision of drainage at a place by laying sewers under the ground is called sewerage. Actually, sewerage is an underground network of interconnected pipes called sewers that carries the sewage from the place where it is produced to the sewage treatment plants, where it is processed.

Manholes
A manhole is a covered vertical hole in the ground, pavement or road, above the underground sewer pipeline through which a worker can go down up to the sewer pipes for inspection, cleaning, etc. Manholes are provided at every 50-60 m distance in the main sewer pipeline. Manholes are also provided at the junction of two or more sewers and at points where there is a change in the direction of the sewer line.

Class 7 Science Chapter 18 Notes

Treatment of Polluted Water
Perform the following activity. It will help you to understand the processes that take place at the wastewater treatment plant.

Wastewater Treatment Plant (WWTP)
A place where wastewater or sewage from houses and other buildings is brought for processing is called wastewater treatment plant.
Treatment of wastewater involves physical, biological and chemical processes depending on the nature of contaminants.

1. Physical Process (Screening)
The bar screens first remove the large rubbish objects like rags, sticks, cans, polybags, napkins, sanitary towels, etc., from the wastewater.
Waste Water Story Class 7 Notes Science Chapter 18 img 1
After passing through the bar screen, wastewater is taken to a tank called grit and sand removal tanks. In this, sand and grit settle down slowly at the bottom of the tank as water passes slowly through this tank. The settled sand and impurities are removed from the tanks from time to time.

2. Biological Process
The first sedimentation tank is sloped towards the centre. Solid like faeces settle at the bottom and is called sludge which is removed by a scraper. Oils and grease float at the surface of the water are removed by a skimmer. The biogas produced (by anaerobic bacteria) in the process can be used as fuel or can be used to produce electricity. Here, water gets cleared of rubbage, oil, grease, etc and we get clarified water which is sent to aeration tank now. In the aeration tank, the watery waste already contains bacteria (aerobic) in it. The compressed air bubbles are passed through this waste to provide 02 to the bacteria to increase bacterial activity which ultimately digests human waste, food waste, soaps and other unwanted and harmful matter still remaining in the wastewater leaving behind fairly pure water.

3. Chemical Process
The water after aeration tank is allowed to stand in a second sedimentation tank. Here, the microbes present get settled at the bottom at the tank in the form of activated sludge which is about 97% water. At this stage, water has very low level of organic matter suspended matter and does not contain many harmful things. It is safe for human consumption but is disinfected by chlorine or ozone before distributing it.

Ncert Class 7 Science Chapter 18 Notes

Become an Active Citizen
Waste generation is a natural part of human activity but we can limit the type of waste and quantity of waste produced. Often we have been repelled by an offensive smell. The sight of open drains is disgusting. The situation worsens in the rainy season when the drains start overflowing. We have to wade through the mud pools on the roads. Most unhygienic and unsanitary conditions prevail. Flies, mosquitoes and other insects breed in it.

We should be an enlightened citizen and approach the municipality or the gram panchayat. We should also insist that open drains should be covered. If the sewage of any particular house makes the neighbourhood dirty. We should request them to be more considerate about other’s health.

Note: We should plant Eucalyptus trees all along sewage ponds. These trees absorb all surplus wastewater rapidly and release pure water vapour into the atmosphere.

Wastewater Story Notes

Better House Keeping Practices
We must minimise and manage waste at our houses before its disposal in the following manner:

  • Cooking oil and fats should not be thrown down the drain. They can harden and block the pipes. In an open drain, the fats clog the soil pores reducing its effectiveness in filtering water. Throw oil and fats in the dustbin.
  • Used tea leaves, solid food remains, soft toys, cotton, sanitary towels, etc., should also be thrown in the
    dustbin. These wastes choke the drains. They do not allow the free flow of oxygen. This hampers the degradation process.
  • The chemicals like paints, solvents, insecticides, medicines and motor oils should not be thrown in drains as they kill helpful microbes which digest the organic waste.

Sanitation and Disease
Contaminated water and poor sanitation practices are the major causes of the number of infectious diseases in our country. Safe sewage disposal facilities are still not available at many cities and villages in India. So, people go to open places and defecate. This causes the increase in insect-vector population which transmit diseases like cholera, typhoid, meningitis, etc.

Untreated human excreta is a health hazard which causes soil pollution and water pollution also. The river water and groundwater are sources of water for drinking for many people. So, the contaminated water can spread many diseases especially water-borne.

Class 7 Science Wastewater Story Notes

Vermi-processing Toilets
In the vermi-processing toilets, human excreta is treated by earthworms in a pit. The earthworms usually eat up all organic matter present in human excrete and turn it into compost. These are tow water use toilets for the safe processing of human.

Alternate Arrangement for Sewage Disposal
Low cost outside the sewage disposal system has been developed to take care of places where there is no sewage system, e.g. rural areas, isolated buildings. These are described below:
(i) Septic tanks: Septic tank is a low-cost onsite sewage disposal system. Septic tanks are suitable where there is no sewerage made. These tanks need cleaning every four to six months.

A septic tank usually consists of a big, covered underground tank made of concrete having an inlet pipe at one end and on outlet pipe at the other end. The toilet seat is connected to the inlet pipe of the septic tank. The human excreta from the toilet seat enters into the septic tank through the inlet pipe. The solid part of excreta keeps on collecting at the bottom of the septic tank in the form of a sludge whereas watery waste remains above it.

The anaerobic bacteria breakdown most of the solid organic matter present in human excreta due to which the volume of solid waste is reduced too much. The digested solid waste keeps on depositing at the bottom of septic tank. The watery waste is also cleaned by anaerobic bacteria. The excess water goes out of the septic tank through the outlet pipe and get absorbed in soil.

(ii) Composting pits: These are self-sustained human waste disposal units which is not connected to a sewer line or a septic tank. A composting toilet breaks down and dehydrates human waste to compost.

(iii) Chemical toilets: These toilets have limited storage capacity for human waste and need to be emptied periodically. These are the toilets which use chemically treated reservoir located just below the toilet seats. The chemicals reduce the foul smell coming out of human excrete and carry out partial disinfection of human waste.

Sanitation at Public Places
In our country, fairs are organised periodically. A large number of people participate in them. In the same way, railway stations, bus depots, airports, hospitals are very busy places. Thousands of people visit them daily which generate large amount of waste. It must be disposed of properly otherwise epidemics could break out. The government has laid down certain standards of sanitation but unfortunately, they are not strictly enforced. We should not scatter litter anywhere. If there is no dustbin in sight, we should carry the litter at home and throw it in the dustbin.

Waste Water Story Class 7 Notes Pdf

Conclusion
We all have a role to play in keeping our environment clean and healthy. We must realise our responsibility in maintaining the water sources in a healthy state. Adopting good sanitation practices should be our way of life. As an agent of change your individual initiative will make a great difference. Influence others with your energy ideas and optimum, A lot con be done if people work together. There is great power in collective action.
As an active citizen, we have many responsibilities regarding sanitation. These can be listed as follows:

  • To ensure that our surroundings are clean.
  • To ensure that the sewerage system in our house is properly managed.
  • If any leakage or an open drain in the sewerage system is present, then it should be reported to the municipality or the gram panchayats to insist that the open drain must be covered properly and several air and water-borne diseases can be prevented.

Note: Mahatma Gandhi said, ‘No one needs to wait for anyone else to adopt a humane and enlightened course of action’.

We hope the given CBSE Class 7 Science Notes Chapter 18 Waste Water Story Pdf free download will help you. If you have any query regarding NCERT Class 7 Science Notes Chapter 18 Waste Water Story, drop a comment below and we will get back to you at the earliest.

Water: A Precious Resource Class 7 Notes Science Chapter 16

Water: A Precious Resource Class 7 Notes Science Chapter 16

CBSE Class 7 Science Notes Chapter 16 Water: A Precious Resource Pdf free download is part of Class 7 Science Notes for Quick Revision. Here we have given NCERT Class 7 Science Notes Chapter 16 Water: A Precious Resource. https://www.cbselabs.com/water-a-precious-resource-class-7-notes/

CBSE Class 7 Science Notes Chapter 16 Water: A Precious Resource

Water A Precious Resource Class 7 Notes Chapter 16

Water is one of the most common and useful substances around us. Water is essential for the existence of all forms of life.
After knowing the importance of water, awareness is being created by different organisations of the world. 22nd March is celebrated as World Water Day to attract the attention of everybody towards the importance of conserving water. The year 2003 was recognised as ‘International year of freshwater’.

By doing such activities, we spread the message of conservation of natural resources of water and make people understand that there will be no life without water on the earth. We also believe that “if you have water, you can think of the future”. Before we discuss why water is getting scarce, we must know how much water is available for use on our planet.

Class 7 Science Chapter 16 Notes

Water Available for Use
If we take a picture of earth from outer space, it appears blue because of presence of water in the form of sea and ocean. About 71% of surface of the earth is covered with water. Of the total water present on earth, 97.4% is in the seas and oceans but it is not fit for human consumption. Freshwater in a usable form is present in just a small fraction of all water present on the earth.
Most of us assume that there is plenty of water all over the earth. But infact this all water is not suitable for human consumption, not even fit for plants and other forms of life.

Water: A Precious Resource Class 7 Notes Chapter 16

Different Forms of Water
Water exists in three forms. These three forms of water are as follows:

  • Snow or ice (solid) exists on the earth in the form of ice caps at the poles of the earth, glaciers and snow covered mountains. These are the main sources of water on earth.
  • Water (liquid) is present in oceans, lakes, rivers and even
    underground water in the earth’s upper layers.
  • Clouds (gas) are found in the form of water vapour present in the atmosphere. On condensation, it turns into droplets and precipitates on earth’s surface in the form of rain.

The continuous recycling of these forms of water takes place and the amount of water on the earth is maintained of constant. Most of the urban areas have a system of water supply whereas underdeveloped/undeveloped areas depend on resources like rivers, lakes, ponds, handpumps, etc.

Class 7 Water A Precious Resource Notes Chapter 16

Groundwater as an Important Source of Water
The wells, tubewells and handpumps are the main sources of water for many people. The water in these sources is the groundwater. It is the upper level of underground water which occupies all the spaces in the soil and rocks and form a water table as shown in the figure below:
Water A Precious Resource Class 7 Notes Science Chapter 16 img 1
The upper limit of groundwater is called water table. It represents the depth of water filled area at a given place. The water table rises and falls depending upon the amount of rainwater that seeps into the groundwater and how much groundwater is drawn out for irrigation and industry. The seeping down of rainwater into the groundwater is called infiltration. The groundwater thus gets recharged by this process.

At some places, the groundwater is stored between layers of hard rock below the water table. This is aquifer which contains the groundwater usable by tubewells and handpumps. These sources however, have limited sources of water and may get exhausted if used excessively.

Class 7 Water: A Precious Resource Notes Chapter 16

Depletion of Water Table
Water drawn from underground water gets replenished by seepage of rainwater. The water table does not get affected as long as we draw the same amount of water as is replenished by natural resources like rain.

However, there is a number of factors which cause depletion of water table at a very fast rate which is really a matter of concern for every one of us. Increase in population, industrial and agricultural activities are some common factors affecting water table. Scanty rainfall, deforestation and decrease in the effective area for seepage of water may also deplete the water table.

Increasing Population
As our population increases, we need more water for drinking, washing, cooking and cleaning. We also need more number of houses, offices, shops and roads. This means more construction work and construction itself uses lots of water itself. Most of the time, it is the groundwater. Besides this, there is less open area which can seep in the rain water into the ground.
So, rise in population also increases use of water. This results in depletion of groundwater level to alarmingly low levels (in many cities).

Water Class 7 Science Notes Chapter 16

Increased Industrialisation
Water is used by all industries. Almost everything that we use needs water somewhere in its production process. The number of industries is increasing continuously. Most of the water used in the industries is drawn from groundwater. This results in depletion of water.

Agricultural Activities
More food is needed to meet the requirement as there is an increase in population. This puts more pressure to the agriculture practices. In our country, farmers depend on rain, canal water or groundwater for irrigation. Canals are found only at a few places. Since rain is often erratic, canals also suffer frequently from lack of water. Therefore, farmers have to use groundwater for irrigation. This results in depletion of groundwater.

Class 7 Science Ch 16 Notes

Deforestation
Large scale deforestation has occurred to accommodate the growing population to grow food for them and to provide space for industries. Overgrazing by our animals has also destroyed large amount of vegetation. The green coverage of vegetation slows down the flow of water on land and increases the absorption of water by the soil. Cutting down of trees and vegetation, therefore interferes with the natural processes by which seepage takes place and the groundwater is recharged and causes depletion of water.

Distribution of Water
The distribution of water over the globe is quite uneven due to the number of factors. Some places have a good amount of rain. On the other hand, these are deserts which have scanty rainfall. Some regions have excessive rains which cause floods while some others have very little rainfall which causes drought. India is a vast country and the rainfall is not the same everywhere. Therefore, some regions in our country may have floods while others may suffer from droughts at the same time.

Class 7 Science Chapter 16 Notes Pdf

Water Resources in India
India receives a lot of precipitation (rain and snow) in comparison to the rest of the world. The average annual precipitation in India is 1170 mm as compared to the world’s average of 700 mm. The rain map of India showing average rainfall in the different parts of our country.
Water A Precious Resource Class 7 Notes Science Chapter 16 img 2

Note:
1. Government of India, Copyright 2007.
2. Based upon Survey of India map with the permission of the Surveyor General of India.
3. The territorial waters of India extend into the sea to a distance of twelve nautical miles measured from the appropriate baseline.
4. The external boundaries and coastlines of India agree with the Record/Master Copy certified by Survey of India.

Water Class 7 Notes Chapter 16

Water Management
It is the activity of planning, developing, distribution and managing the optimum use of water resources. It is a subset of water cycle management. Water supply pipes leaking and a lot of water gushing out of the pipes are the wastage of water. It is the responsibility of the civic authorities to prevent such wastage of precious water. Mismanagement or wastage may also take place at the level of individuals also. All of us knowingly or unknowingly waste water, we should also take care for it. Some of the steps which can be taken for the proper management of water are given below:

  1. Rainwater harvesting
  2. Bawris
  3. Drip irrigation

1. Rainwater Harvesting
Most of the rainwater just flows away. This can be skillfully used to recharge the groundwater. The modern buildings of schools, offices, homes can install a rainwater harvesting system, so as to store rainwater in their own premises for future use.
Water A Precious Resource Class 7 Notes Science Chapter 16 img 3

A Case Study
Bhujpur in the Kutch area of Gujarat has a very erratic rainfall. The only source of freshwater lies underground because rivers in this area do not have water throughout the year. Over the years, demand for water has grown. As a result, the water table has gone down alarmingly.
In 1989, the villagers along with a non-governmental organisation, decided to harvest rainwater.
Eighteen chech-dams were built on Ruhmavati river and its tributaries where water percolates through the soil and recharge the aquifers. ^ According to farmers, the wells have water now and the water that flowed into the sea and was wasted has become available for irrigation.

2. Bawris
The bawris is age old method of collecting water. These structures are still found in old buildings, palaces and forts. With time, the bawris fell into disuse and garbage started piling in these reservoirs. However, because of the acute shortage of water, the bawris are being revived. Today the situation is that inspite of scanty rains those places are managing their water needs well.
Water A Precious Resource Class 7 Notes Science Chapter 16 img 4

3. Drip Irrigation
It is a method of watering plants by use of narrow tubings which deliver water directly to the base of a plant. This minimises wastage of water. The mechanism of drip irrigation is shown in the figure given below:
Water A Precious Resource Class 7 Notes Science Chapter 16 img 5

Role for Saving Water
You can be a leader to show people about water management skills. If any pipeline and tap water is leaking there, immediately report to authorities like ‘JAL BOARD’ (in Delhi) to prevent water loss. Educate people about water wise habits which can be developed gradually and will last life long once developed.

Water-wise Habits

  • Turn off the tap while brushing, shaving and washing hand. Open when need. This will check the excess flow of water into drains.
  • Use mug and water in the bucket for bathing instead of using showers.
  • Mop the floor instead of washing.
  • Irrigate potted plants with used water for washing rice and dal in the kitchen while cooking.
  • Check no tap or pipe is leaking.

Class 7 Chapter 16 Science Notes

Effect of Water Scarcity on Plants
We grow many plants in pots in our homes. These are called potted plants. The potted plants are watered regularly. If the potted plants are not watered even for a few days, the plants will ‘wilt’ (become limp) and ultimately ‘dry up’. If potted plants are not given water for a considerable time, they will die. Thus, sufficient water is essential for maintaining the life of plants. Plants need water to obtain nutrients from the soil and to make food by the process of photosynthesis.
The various effects of water scarcity on plants are

  • Water scarcity will affect nutrient uptake from soil by the plants.
  • The rate of photosynthesis will decline, so oxygen evolved will be less.
  • Rate of transpiration will also decline, so water vapours released in the atmosphere by transpiration will be less, it will disturb the water cycle.

So, in brief, we can say the shortage of water will lead to a shortage of food, shortage of oxygen and shortage of rain also.

We hope the given CBSE Class 7 Science Notes Chapter 16 Water: A Precious Resource Pdf free download will help you. If you have any query regarding NCERT Class 7 Science Notes Chapter 16 Water: A Precious Resource, drop a comment below and we will get back to you at the earliest.

Forests: Our Lifeline Class 7 Notes Science Chapter 17

Forests: Our Lifeline Class 7 Notes Science Chapter 17

CBSE Class 7 Science Notes Chapter 17 Forests: Our Lifeline Pdf free download is part of Class 7 Science Notes for Quick Revision. Here we have given NCERT Class 7 Science Notes Chapter 17 Forests: Our Lifeline. https://www.cbselabs.com/forests-our-lifeline-class-7-notes/

CBSE Class 7 Science Notes Chapter 17 Forests: Our Lifeline

Forest Our Lifeline Class 7 Notes Chapter 17

In ancient times, we used to live in forests. It met all our requirements for leading a normal life at that time. Now-a-days, we live in cities and towns far off from forests, so we really do not know the importance of forests in our life. It is a natural renewable resource a habitat (home) to many forms of wildlife like bear, bison, jackal, deer, porcupine, elephant, etc.

Many trees like sal, teak, semal, Sheesham, neem, Palash, fig, khair, amla, bamboo, kachnar and many others are also found in the forests. The forest also contains insects, butterflies, honeybees and birds which help in palliation in the flowering plants of the forest in pollination.

Structure of a Forest
The plants (trees, shrubs and herbs) make different layers in the forest which are described below:
1. Canopy
The uppermost branches and leaves of tall trees which act as a roof over the forest ground is called canopy. It is the highest layer of vegetation in the forest. The branch part of a tree above the stem is known as the crown of the tree.

2. Understorey
The different horizontal layers formed due to different types of crowns in the forest is called understorey. The constituents of understorey can be described as follows:
Forests Our Lifeline Class 7 Notes Science Chapter 17 img 1
(i) Top layer It constitutes the gaint and tall trees followed by shrubs and tall grasses.
(ii) Shrub layer It has many shrubs and bushes of approximately 1-2 metres of height from the forest floor. It makes dense layer at some places of forest where enough sunlight is present.
(iii) Herb layer Just below the shrub layer occurs the herb layer of plants. It is the lowest layer of vegetation in the forest (having leafy plants). Most of the plants in herb layer have short lifespan.
(iv) Forest floor Plants found here are as small as mosses, liverworts, lichens. It has many kinds of insects, worms, toad stool, etc. Most of the forest floor is covered with dead and decaying plant matter, and animal waste.

Class 7 Science Chapter 17 Notes

Components of the Forest
The living organisms found in the forest are plants, animals, decomposers and scavengers. The non-living environment of the forest provides nutrients, water and carbon dioxide for the growth of the plants.
1. Plants
Green plants are living organisms also called autotrophs as they produce food by photosynthesis (by absorbing nutrients water from soil, CO2 from air and sunlight as energy source). They provide food to all living organisms which live in the forest. They are called producers (of food).

2. Animals
Forests have many animals and they are called consumers (of food). The animals which eat only plants/their parts are called herbivores (herb eating) whereas flesh-eating animals are called carnivores. All animals are called heterotrophs because they depend on other organisms for food.

3. Decomposers
Mostly these organisms are microorganisms like bacteria and fungi. They feed on dead plants and animals and thus are called saprotrophs. These organisms are also called decomposers as they breakdown dead parts of plants and dead bodies of animals into simple substances. They play a very important role in sustaining the forests.

4. Scavengers
Those animals which eat dead animals are called scavengers, e.g. vultures, crows, jackals, hyena, some insects (ants, beetles, termites, woodlice, maggots, millipedes and earthworms), etc. Scavengers are the cleaning agents of our environment. But these are not decomposers as they do not breakdown complex dead organic matter into simple ones.

Forests Our Lifeline Class 7 Notes Chapter 17

Importance of Forests
The forests provide us with a large number of products. They also purify air and water quality and maintain soil moisture and climate. So, they are called lifeline.

1. Forests Provide Many Useful Products
The various things which are obtained from the forests are called forest products. Forests give us a large number of useful products. Some of the important products which we get from the forests are wood, honey, gum, sealing wax (or lac), catechu (kattha), fruits, oils, spices, natural rubber, cork, dyes, medicinal plants and fodder for cattle. Perhaps the most important product obtained from forests is the wood (which is obtained by cutting down the forest trees). The wood obtained from forests is used for a large number of purposes in our day-to-day life.

2. Forests Maintain Balance between Oxygen and Carbon Dioxide
Plants in the forest release oxygen during photosynthesis. This provides all animals including us with oxygen to breathe and helps to maintain the ratio of oxygen to carbon dioxide in the atmosphere. That is why, forests are called green lungs.

If the amount of carbon dioxide increases in the atmosphere, it would result in an increase in earth’s temperature. Plants in the forest intake carbon dioxide from the atmosphere during photosynthesis. Hence, they help to maintain the right amount of carbon dioxide in the atmosphere.

3. Forests Maintain Water Cycle
The forest trees suck water from the soil through their roots and release water vapour into the air through transpiration. This water vapour helps in the formation of clouds and bring rain on the earth. Thus, forests bring sufficient rainfall on the earth. In fact, about half the rain which falls in forest areas comes from the transpiration of forest trees themselves. In this way, forests help in maintaining a perfect water cycle in nature and meet our freshwater requirements.
Forests Our Lifeline Class 7 Notes Science Chapter 17 img 2

4. Forests Prevent Occurrence of Flood
The forest acts as a natural absorber of rainwater and allows it to seep. It helps to maintain the water table throughout the year. Forests not only help in controlling floods but also help to maintain the flow of water in the streams so that we get a steady supply of water.

On the other hand, if trees are not present, rain hits the ground directly and may flood the area around it. Heavy rain may also damage the soil. Roots of trees normally bind the soil together, but in their absence, the soil is washed away or eroded.
The different kinds of plants grow together in the forest making different levels of layers and habitat for wild animals. In the absence of plants/trees, the soil will not hold water which will cause flood and erosion.

5. Forests Provide Habitat for Wildlife
The different types of vegetation present in a forest provide food and shelter to animals, birds and insects which live in the forest. This makes a food chain.

Forests: Our Lifeline Class 7 Notes Chapter 17

Food chain
Food chain can be defined as a sequence of living organisms in which one organism feeds on another.
A typical chain in grassland is: grass → deer → lion
A typical food chain in a pond is: algae → small fish → large fish

Flow of Energy in a Food Chain
The sun is the ultimate source of energy for everything on the planet. Green plants or producers are able to harness the energy of the sun to make food. In a food Flow ener0y in food chain chain, energy from plants (producers) is passed on from one organism to another. From the producers, the energy goes to primary consumers (herbivores) and is then passed on to secondary consumers (carnivores). Thus, producers are always at the beginning of the food chain.
Forests Our Lifeline Class 7 Notes Science Chapter 17 img 3

Dynamic Living Entity
By harbouring greater variety of plants, the forest provides great opportunities of food and habitat for the herbivores. Larger number of herbivores means increased availability of food for a variety of carnivores. The wide variety of animals helps the forest to regenerate and grow. Decomposers help in maintaining the supply of nutrients to the growing plants in the forest. Therefore, the forest is a dynamic living entity. There is a continuous interaction between soil, water, air, plants and animals in a forest.

6. Forests can Regenerate on their Own
The dead parts of trees and plants, dead animals and animal wastes (like animal dung or droppings) keep on collecting on the forest floor. Decomposers (fungi and bacteria) degrade them into simple organic substances which are usable by plants in the form of humus. The hummus makes the forest soil fertile by providing the nutrients. The animals, birds of forests, wind and water disperse the seeds of trees and plants on the forest soil. These seeds obtain nutrients from the soil and germinate to form seedlings and ultimately grow to form the forest vegetation.

Forest Our Lifeline Class 7 Pdf Notes Chapter 17

Forest Conservation
Paper is made from wood pulp that is produced from the wood of forest trees. So, to make paper, many trees have to be cut down from the forests. If all of us keep on collecting old newspapers, magazines, books, notebooks, etc., and send them to paper mills for recycling through a junk dealer (kabaddi wala), we will be able to save many forest trees from being cut down.
Some of the other ways to conserve forests are also given below:

  • Excessive cutting down of forest trees should not be allowed by the government to conserve forests.
  • More trees should be planted in the forest in place of cut down trees to conserve forests.
  • Paper products such as old newspapers, magazines, books, notebooks, etc., should be recycled to conserve forests.

A large number of forest trees are being cut down every day to meet the various demands of the increasing population. This is called deforestation. Following are the consequence if forests disappear:

  • Increase of the earth’s temperature If there are no trees and plants, their will be no photosynthesis. So, no C02 of the atmosphere will be used. This will increase the level of C02, resulting in the increase of earth’s temperature.
  • No food and shelter to wildlife In the absence of trees, plants and animals will not get food and shelter. So, this will disrupt the whole cycle of life and gradually life might disappear from the land area of the earth.
  • There will be more floods The trees plant roots help in holding the soil during rains and also soil is able to hold water. In the absence of trees, the soil will not hold water which will cause floods.
  • Deforestation endanger the environment The continuous deforestation is threatening the different form of life including human beings. So, there is necessity to think and set to conserve forests. Natural calamities like floods, cyclones, hail forms are more in the absence of trees and forest. People become homeless when such disaster occurs.

We hope the given CBSE Class 7 Science Notes Chapter 17 Forests: Our Lifeline Pdf free download will help you. If you have any query regarding NCERT Class 7 Science Notes Chapter 17 Forests: Our Lifeline, drop a comment below and we will get back to you at the earliest.

Light Class 7 Notes Science Chapter 15

Light Class 7 Notes Science Chapter 15

CBSE Class 7 Science Notes Chapter 15 Light Pdf free download is part of Class 7 Science Notes for Quick Revision. Here we have given NCERT Class 7 Science Notes Chapter 15 Light. https://www.cbselabs.com/light-class-7-notes/

CBSE Class 7 Science Notes Chapter 15 Light

Light Class 7 Notes Chapter 15

In this world, we usually see a variety of objects. Sometimes, we are unable to see anything in a dark room but on lighting up the room, we are able to see the things in the room. Since, it is an obvious question arising that what makes thing visible.

So, its answer is light. Light is a form of energy which enables us to see objects from where it comes or reflected. We can detect light with our eyes.

Light Travelling along a Straight Line
By seeing the phenomena around us like a beam of sunlight enters a room through a narrow opening or a hole and beams of light coming out from the headlamps of cars, scooters, engines, torch, etc.
From the above examples, we can conclude that light travels along a straight line.

Class 7 Light Notes Chapter 15

Reflection of Light
There are some certain situations in which a mirror or shiny surfaces like stainless steel plate, shining steel spoon act as a mirror, can change the direction of light that falls on it. So, this process of change in direction of light by a mirror is called a reflection of light. The surface of the water can also act as a mirror and can also change the path of light and that is why we see the reflection of trees or buildings in the water.

Image of An Object
Generally, when we look into a mirror, then we see our face. Actually, what we see in the mirror it is exactly a reflection of our face, hence it is known as an image of our face. In this case, our face is the object and what we see in the mirror is its image. The image of our face seen in the mirror is formed where light rays, after reflection from the mirror, seems to originate from. The image of our face appears to be situated behind the mirror.
There are two types of images:
Real image: It is an image which can be obtained on a screen, e.g. the image formed on a cinema screen. When the light rays coming from an object actually meet at a point after reflection from the mirror, then it results in the formation of a real image.

Virtual image: It is an image which cannot be obtained on a screen, e.g. image formed by a plane mirror. When the light rays coming from an object appear to meet after reflection from the mirror, then it results in the formation of virtual image. It is not possible to form a virtual image on the screen because light rays actually do not pass the screen or cannot be received on a screen.

Class 7 Science Chapter 15 Light Notes

Characteristics of the Images Formed by a Plane Mirror
Now, we will describe the various characteristics of the images formed in a plane mirror by taking the example of the image of the candle.
(i) When we see the mirror, the image of candle appears to be formed behind the mirror.

(ii) Now, put a vertical screen behind the plane mirror (where the image of candle appears to be situated), then we will notice that the image of candle cannot be formed on the screen. Even if the screen is placed in front of the plane mirror, then the image of candle cannot be formed on the screen. Since, the image of candle formed in the plane mirror cannot be formed on a screen, which means that the image of candle in the plane mirror is a virtual image.

(iii) If we see the figure, then we will find that the length and breadth of the image of the candle and its flame to be the same as that of the original candle and its flame. The image of candle in the plane mirror is of the same size as the original candle.

(iv) Also if we see the figure, then we will find that the candle has a flame at the top and the image of candle also has a flame at the top. So, the top of the candle remains at the top in the image. In the same way, the bottom of candle remains at the bottom in an image. Such an image is called an erect image (or upright image). Therefore, the image formed by a plane mirror is erect.

Class 7 Science Chapter 15 Notes

Side Inversion (Right ⇔ Left)
When we see our image in a plane mirror, is it exactly like us? There is an interesting difference between us and our image. Let us find out this difference with the help of an example.

If we stand in front of a plane mirror and lift our right hand, then we see our image lift its left hand. And if we lift our left hand, then the image appears to lift its right hand.

This means that the right side of our body becomes the left side in the image while the left side of our body becomes the right side of the image. It appears as if our image has been ‘reversed side ways’ with respect to your body. The effect of reversing the sides of an object and its image is called lateral inversion.

So, we say that image formed in a plane mirror is laterally inverted.
So, we can understand why the word AMBULANCE is written as 3DMAJU9MA. When a driver of a vehicle ahead of an ambulance look in his/her rear view mirror, then he/she can read AMBULANCE written on it and give way to it. So, it is the duty of everyone of us to allow an ambulance to pass without blocking its way.

Spherical Mirrors
All the mirrors are not straight like plane mirror as some of the mirrors are curved mirror. There is a common example of a curved mirror, i.e. spherical mirror. A mirror whose reflecting surface is the part of a hollow sphere of glass is known as a spherical mirror.

Light Chapter Of Class 7 Pdf Notes Chapter 15

Image Formed by Spherical Mirror
It is a fact that spherical mirrors form images of the objects placed in front of them. So, these images are formed, when light rays coming from the object fall on the mirror, get reflected and converge or diverge. We can use a spoon in order to understand the image formation by a spherical mirror.

The inside surface of a hollow sphere of glass is bent in or concave but the outside surface is bulging out or convex. So, the spherical mirrors are of two types:

  • Concave mirror
  • Convex mirror

e.g. A shining steel spoon represents both a convex mirror as well as a concave mirror. As the front side (or inner side) of a spoon is bent inward, so the front side of a shining spoon represents a concave mirror while the back side (or outer side) of a spoon is bulging outward, so the back side of a shining spoon represents a convex mirror as shown in figure.

Light Notes Class 7 Chapter 15

Concave Mirror (Converging Mirror)
The mirror whose reflecting surface is concave (and polished surface is convex) is called a concave mirror.
The concave mirror reflects the parallel rays of light in such a way that after reflection, all the rays converge (or meet) at one point called focus in front of the mirror. Since a concave mirror converges a beam of parallel light rays. Therefore, a concave mirror is also known as a converging mirror.
Light Class 7 Notes Science Chapter 15 img 1

Light Chapter Of Class 7 Notes Chapter 15

Image Formed by a Concave Mirror When the Object is Far Off
A concave mirror forms a real image of the sun. We can understand the formation of the image by a concave mirror when the object is far off by an activity.

Therefore, we can conclude that the image formed by a concave mirror is much smaller than the object (highly diminished) and real because it can be obtained on a sheet of paper (which is a kind of screen).
So, when an object is placed at a far off distance front a concave mirror, then image formed by a concave mirror is

  • real
  • inverted
  • much smaller than the object.

Image Formed by a Concave Mirror When the Object is Placed Close to Concave Mirror
Let us perform an activity to understand the formation of image by a concave mirror when the object is placed close to the concave mirror.

Since the image can be observed only by looking into the concave mirror and cannot be formed on the screen, therefore, the image is virtual. If we look at the image in the concave mirror, we find it to be the same side up as the candle, so the image is erect. And if we compare the size of the candle and its image, then we will find that the image is larger than the candle. Therefore, the image is larger than the object (enlarged or magnified).

Hence, we can conclude that when an object is placed close to a concave mirror, the image formed by the concave mirror is

  • virtual
  • erect
  • larger than the object (enlarged or magnified).

Uses of Concave Mirrors

  • To see the large image of teeth of a patient, concave mirrors are used by the dentist.
  • In torches, headlights of vehicles and searchlights to get a strong, straight beam of light, etc., concave mirrors are used as reflectors.
  • To see a large image of the face, then concave mirrors are used as shaving mirrors.

Class 7 Science Chapter 15 Light Notes Pdf

Convex Mirror (Diverging Mirror)
The mirror whose reflecting surface is bulging or convex (polished surface is concave) is called the convex mirror. After reflection from the convex mirror, the parallel rays of light are spreading out. When the parallel rays of light spread out, we can say that the rays of light are diverging.
Light Class 7 Notes Science Chapter 15 img 2
Now, we can say that a beam of parallel light rays diverges (spreads out) after reflection from a convex mirror.
Since a convex mirror diverges a beam of parallel light rays, therefore, it is also known as a diverging mirror.

Light Chapter Class 7 Notes Chapter 15

Image Formed By a Convex Mirror
Let us perform an activity to understand the formation of an image by a convex mirror.
The image of the candle can be seen only by looking into the convex mirror and cannot be formed on a screen. It is a virtual image. If we look at the image in the convex mirror, we will find that it is the same side up as the candle. So, the image is erect. And if we compare the size of the candle and its image, the image appears to be smaller. Therefore, the image is smaller in size than the object (or diminished). Even if we change the distance of candle (object) from the convex mirror, we will notice that in every case, the image of the candle formed by the convex mirror remains virtual, erect and smaller in size than the candle.
So, we can conclude that whatever be the distance of the object from a convex mirror, the image formed by a convex mirror is always

  • virtual
  • erect and
  • smaller than the object (or diminished).

Uses of Convex Mirrors

  • To see the traffic at the rear side or backside on the road, convex mirrors are used as rear view mirrors or side view mirrors in vehicles such as cars, scooters, buses, etc.
  • Big convex mirrors are used as shop security mirrors. By installing a convex mirror in the shop, the shop owner can keep an eye on the customers.

Image Formed by Lenses
Since a lens is a piece of transparent glass bound by the two spherical surfaces. Lenses are transparent so that light can pass through lenses. Lenses are of two types:

  • Convex lens
  • Concave lens

Get some lens and try to touch them, we will find that some are thicker in the middle than at the edges and some are thinner in the middle than at the edge.
Light Class 7 Notes Science Chapter 15 img 3

Class 7 Science Light Notes Chapter 15

Convex Lens (Converging Lens)
The convex lens is the lens which is thicker in the middle than at the edges. A beam of parallel rays of light falls on a convex lens from the left side. After passing through the convex lens, the beam of parallel rays of light converges at a point as shown in the figure given below. Hence, a convex lens is a converging lens.
Light Class 7 Notes Science Chapter 15 img 4

Image Formed by a Convex Lens
The nature and size of the image formed by a convex lens depend on the distance of the object from the convex lens.

Thus, we can conclude that when an object is placed at a far off distance from a convex lens, then the image formed by the convex lens is real, inverted and much smaller than the object (or highly diminished).

Now, change the distance of the candle from the lens and try to obtain the image of the candle flame every time on the screen by moving it. So, is it possible to get in any position of the object for which image was erect and, magnified? Yes, it is possible when the candle is placed very close to the convex lens.

Uses of Convex Lenses

  • Convex lenses are used as a magnifying glass.
  • In the manufacturing of spectacles, camera, microscope, telescope and binoculars, convex lenses are used.

Concave Lens (Diverging Lens)
A concave lens is a lens which is thinner in the middle than at the edge. A parallel beam of light falls on a concave lens as shown in the figure. After passing through the concave lens, the rays of light are diverging (or spreading out).
Light Class 7 Notes Science Chapter 15 img 5
Since a concave lens diverges light rays falling on it, therefore, it is also called a diverging lens.

Image Formed by a Concave Lens
In the case of a convex lens, we have studied that the nature of image formed depends on the distance of the object from the convex lens. But, this is not followed in the case of a concave lens.
Let us perform an activity to understand the formation of an image by a concave lens.

Uses of Concave Lenses

  • In order to see the image of the person standing outside, concave lenses are used in the peepholes in the door of hotel rooms.
  • Concave lenses are used in making spectacles.

Sunlight: White or Coloured
We might have noticed a rainbow which usually appears after the rain when the sun is low in the sky. An arc of seven colours seen in the sky is known as the rainbow. The seven colours of a rainbow are red, orange, yellow, green, blue, indigo and violet. We might also have seen that when we blow soap bubbles, they appear colourful. Similarly, when light is reflecting from the surface of a Compact Disc (CD), we can see many colours. The rainbow is produced by the dispersion of sunlight by tiny raindrops suspended in the atmosphere.

Dispersion of Light
In the year 1665, Newton discovered by his experiments with glass prisms that white light (like sunlight) consists of a mixture of lights of seven colours. Newton found that if a beam of white light is passed through a glass prism, then the white light splits to form a band of seven colours on a white screen. The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, is known as a spectrum of white light. The seven colours of the spectrum are Red, Orange, Yellow, Green, Blue, Indigo and Violet.
Light Class 7 Notes Science Chapter 15 img 6
So, dispersion of light is the phenomenon of splitting up of white light into seven colours on passing through a transparent medium like a glass prism. The formation of a spectrum of seven colours indicates that white light is a mixture of seven colours. White light can be sunlight. So, now we can say that sunlight consists of seven colours.
We can mix these colours to get white light. This can be done by using Newton’s disc, let us try this.

We hope the given CBSE Class 7 Science Notes Chapter 15 Light Pdf free download will help you. If you have any query regarding NCERT Class 7 Science Notes Chapter 15 Light, drop a comment below and we will get back to you at the earliest.

Electric Current and Its Effects Class 7 Notes Science Chapter 14

Electric Current and Its Effects Class 7 Notes Science Chapter 14

CBSE Class 7 Science Notes Chapter 14 Electric Current and Its Effects Pdf free download is part of Class 7 Science Notes for Quick Revision. Here we have given NCERT Class 7 Science Notes Chapter 14 Electric Current and Its Effects. https://www.cbselabs.com/electric-current-and-its-effects-class-7-notes/

CBSE Class 7 Science Notes Chapter 14 Electric Current and Its Effects

Electric Current And Its Effects Class 7 Notes Chapter 14

The most convenient source of energy is electricity. Electricity has a very important role because it is used to run many electrical appliances like an electric bulb, television, a stereo system, refrigerator, washing machine, computers, etc., and we cannot think our life without making use of electricity.

Electricity is produced at power stations from where it is brought to our homes through the thin wire and electric poles networks or underground cables (or wires). Here, we can define the electric current as of the flow of electricity through a conductor (wires, cables).

Actually, in everyday life, the word electricity and electric current are used in the same sense. There is another source of electricity, i.e. electric cell or battery. Now, in order to obtain electricity from a cell or battery, we have to connect it into a circuit. So, let us study about the electric circuit.

Class 7 Science Chapter 14 Notes

Electric Circuits
A continuous conducting path (consisting of wires, bulb, switch, etc.) between the two terminals of a cell or battery along with an electric current flows, is known as an electric circuit.
e.g. take a cell having a positive terminal (+) and a negative terminal (-). Now try to connect the positive terminal of the cell to one end of the switch with a piece of copper wire and other ends of the switch to one end of bulb holder with another piece of copper wire.
The negative terminal of the cell is connected directly to the other end of the bulb holder with a wire (as shown in the figure), so this kind of setup is known as an electric circuit.
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 10

Circuit Diagram
A circuit diagram tells us how the various components in an electric circuit have been connected by using the electrical symbols of the components.
(i) When the bulb glows In an electric circuit when the switch is closed, then the switch is said to be in the ON position. And when the switch in a circuit is open, then the switch is said to be in the OFF position. So, in an electric circuit, a bulb lights up only when the switch is in the ON position and at that time, we can say that the electric circuit is complete because the current flows throughout the circuit instantly (as shown in the figure) electric circuit

(ii) When the bulb does not glow While checking the circuit notice that sometimes the bulb does not glow even when the switch is in the ON position. This condition can occur only if the bulb gets fused, i.e. its filament breaks. It is a difficult and time-consuming job to draw the electric circuit by making the actual drawings of cell, battery, switch, bulb, etc.
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 2
So, the scientists have devised a very easy way to represent the component of an electric circuit (cell, battery, switch, bulb, etc.) with the help of symbols, which are easy to draw.

Some of the commonly used symbols for electric components are shown here:
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 3

Class 7 Electric Current And Its Effects Notes Chapter 14 

Electric Cell and Battery
The common source of electricity to run the number of devices, e.g. torches, radio, electric clocks and watches, toys, etc., is an electric cell, but sometimes a single cell is not sufficient to run many devices as they require high voltage to run all these appliances. The cell provides much less electricity as compared to that provided by the electric supply line, e.g. in the case, a single electric cell which provides only 1.5 V of electricity, whereas electricity from the power station is supplied to our home at very high voltage of 220 V.

Combination of Electric Cells
Since the higher voltage can be obtained by combining a number of cells in series. So, when the positive terminal of one cell is joined with the negative terminal of the other cell, then the cells are said to be joined in series (as shown in figure given below). So, a battery can be defined as the group of cells joined together in the series, e.g. in a torch, the cells are placed one after the other. But in many devices, cells are not placed one after the other, e.g. in a TV remote control, the two cells are placed side by side (or parallel to each other) instead of single one. So, two or more cells connected in side by side manner are said to be joined in parallel. This combination is also known as battery.
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 4
If we combine two cells by keeping the positive terminal of one cell in contact with the positive terminal of the other cell or negative terminal of one cell in contact with the negative terminal of the other cell, then the battery obtained will not work.

The batteries which are used in cars, buses, trucks, inverters, etc., are also made of cells. There is a special feature of car battery is that its cells can be recharged. Ordinary cells, however, cannot be recharged.

Electric Current And Its Effects Class 7 Notes Pdf Chapter 14

Connection of Cells/Battery
‘+’ and ‘-‘ symbols are printed in the battery in order to have an exact placement of the cells in their respective battery compartment.

  • The switch or key can be placed anywhere in the circuit.
  • The circuit is complete and it is said to be closed only when the switch is ON.
  • The circuit is incomplete and it is said to be open, only when the switch is OFF.

There is a thin wire in the bulb, called filament which glows when an electric current passes through it. So, if the bulb gets fused, then its filament gets broken.
Note: Never touch a lighted electric bulb connected to the mains as it may be very hot and can damage your hands.

Class 7 Chapter 14 Science Notes

Heating Effect of Electric Current
Production of heat in an electric device due to the flow of electric current is called the heating effect of electric current. We have seen an electric heater used for cooking, an electric bulb or room heater. So, when these appliances are switched ON after connecting to the electric supply, then their elements become red hot and release the heat. This happens due to the heating effect of electric current.

The degree to which a material opposes the passage of current through itself is known as its resistance. Actually, when an electric current passes through a high resistance wire, the electric energy gets converted into heat energy and this heat energy heats up the wire.

Element
All electrical heating devices consist of a coil of wire called an element. When these appliances are switched ON after connecting to the electric supply, then their elements become red hot and release the heat. There are some electric appliances such as immersion heaters, hotplates, irons, geysers, electric kettles, hair dryers, etc., which have elements inside them.

Ncert Class 7 Science Chapter 14 Notes

Factors on Which the Heating Effect of Current Depends
There are two factors on which the heating effect of current depends:
(i) Resistance of wire: Greater the resistance of a wire, greater will be the heat produced in it by a given
current, e.g. if we choose two wires, one of copper and other is nichrome of equal length and equal thickness and pass them the equal amount of current through them for the same duration, then we will notice that nichrome wire will become hotter in comparison to the copper wire. It is due to the reason that the resistance of nichrome wire is more than that of the copper wire.
That is why the nichrome wire is used to make heating elements of electric heating appliances such as electric room heater, electric iron, etc. The resistance of a wire depends on the material of the wire, length of wire and thickness of the wire.

(ii) Magnitude of current passed through a given wire: If the magnitude of current passed through a given wire is greater, then the heat produced in it will also be greater, e.g. if a normal amount of current flows through the copper wires of household electric wiring, then the wires do not become much hot but if a large current flows in the same wiring accidentally, then the wires become extremely hot and a fire may be started.

Electric Current And Circuits Class 7 Notes Chapter 14

Applications of the Heating Effect of Current
Some of the important applications of heating effect of electric current are

  • For the production of light, the heating effect of electric current is utilised in the electric bulbs.
  • For the working of electrical heating appliances such as water heater, electric room heater, electric iron, etc., the heating effect of electric current is utilised.
  • The heating effect of electric current is utilised in a safety device called ‘electric fuse’.
    Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 5

Class 7 Science Ch 14 Notes

Compact Fluorescent Lamps (CFLs)
An electric bulb is basically used for producing light but it also releases the heat which is not desirable because a major part of the electricity consumed by the filament of a bulb is converted into heat and results in the wastage of electricity. So, this wastage can be decreased by using fluorescent tube light or CFLs (as shown in the figure) in place of the bulbs.
However, before purchasing bulbs, tubes orCFL’s we should look for the ISI marl? (ISI- Bureau of Indian standard). It is because the ISI mark ensures that the appliance is safe and wastage of energy is minimum.
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 6

Electric Fuse
A safety device which works on the heating effect of current and prevents electric fires or damage to electrical appliances due to excessive flow of current is known as the fuse. This safety device consists of a short length of a thin wire of tin plated copper having a low melting point and this wire has a much greater resistance than the rest of the electric wiring in the house.

So, due to this, if the current in the electric wiring suddenly increases too much, then the fuse wire gets heated, then it melts and breaks the circuit which means that the current flowing in the household circuit will stop. This prevents fire in the house and also the damage of various electrical appliances like refrigerator, fans, tube lights, TV, etc. When a fuse gets blown (breaks), a new fuse has to be fitted in its place in order to restore the electricity supply in the household circuit.
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 7
Note: We should not use a thick wire as a fuse wire because it will have low resistance and thus it will not get heated to its melting point ‘ when a large current passes through it.

Electricity Class 7 Notes Chapter 14

Cause of Large Current Flow in Household Electric Wiring
An extremely large current can flow in the household electric wiring circuits under two circumstances overloading and short circuit. We might have read reports in the newspaper about fires caused by short circuits and overloading. Now, let’s study these two terms.

Overloading
It is a situation when too many electrical appliances are connected to a single socket, they draw an extremely large amount of current from the household circuit. The flow of large current due to overloading may heat the copper wires of household wiring to a very high temperature and fire may be started.

Short Circuit
Electric current is supplied by household through two insulated wires which run together and reach each and every electrical appliances. One insulated wire is called live wire and the other insulated wire is called neutral wire and both these wires are necessary for the working of an electrical appliance (say an electric iron).

So, if in case the plastic insulation of the live wire and the neutral wire gets worn due to wear and tear, then the two naked wires touch each other. So, this touching of live wire and neutral wire directly is known as a short circuit. Due to which a large current flows through the household wiring and this large’ current may heat the wires to a dangerously high temperature and a fire may be started.
Note: Always use proper fuses which have been specified for particular applications carrying ISI mark Never use just any wire or strip of metal in place of a fuse.

Electric Current Class 7 Notes Chapter 14

Miniature Circuit Breakers (MCBs)
This ore increasingly being used these days in place of fuses. MCB does not work on the heating effect of current as it works on the magnetic effect of current. These switches outomaticoUy turn OFF when the current in a circuit exceeds the safe limit. We turn them ON and the circuit ¡s once again completed.
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 8

Magnetic Effect of Electric Current
If the electric current passes through a wire, then the current carrying wire behaves like a magnet. This phenomenon is known as the magnetic effect of current. It was discovered by a scientist Hans Christian Oersted who found that when an electric current is passed in a wire, then the compass needle placed near it got deflected from its usual North-South position.

A straight wire carrying an electric current produces a magnetic effect. The magnetic effect is increased only if we use a long coil of wire instead of a straight wire. Even further the magnetic effect is increased if the coil of wire is wound around an iron rod and then current is passed through it.

Electromagnets
It is a magnet made by using electric current. An electromagnet works on the magnetic effect of current. An electromagnet consists of a coil of insulated wire wrapped around a piece of iron which is magnetised only when an electric current is passed through the coil.
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 9
This magnet consists of a long coil of insulated copper wire wound around an iron rod and when the two ends of the coil get connected to a cell, then a current passes through the coil and produces a magnetic effect. The magnetic effect magnetises the iron rod. In this way, the iron rod becomes an electromagnet. The magnetism of an electromagnet remains as long as the current is flowing in its coil. So, if we switch OFF the current in the coil, then all the magnetism of the iron rod disappear and it will no longer behave like a magnet.
There are two factors through which an electromagnet can be made stronger, i.e.

  • By increasing the amount of current used in the coil.
  • By increasing the number of turns forming the coil.

Uses of Electromagnets

  • These magnets are used in electrical appliances such as an electric bell, electric fan, electric motor.
  • These magnets have their utilisation in electric generators where the very strong magnetic field is required.
  • For deflecting electron beam of the picture tube of TV electromagnets are used.
  • For the magnetic separation of iron ores from the earthly substances, electromagnets are used.
  • For preparing strong permanent magnets, electromagnets are used.

Advantages of Electromagnets over Permanent Magnets
An electromagnet is a temporary form of the magnet because its magnetism is only for the duration of current flowing in its coil. Actually, an electromagnet is better than a permanent magnet in many respects. There are some of the advantages of the electromagnets over the permanent magnets which are stated as follows:

  • The magnetism of an electromagnet can be switched ON or switched OFF as desired. While it is not possible with a permanent magnet.
  • By increasing the number of turns in the coil and by increasing the current passing through the coil an electromagnet can be made very strong. On the other hand, a permanent magnet cannot be made so strong.

Electric Bell
An electric bell works on the magnetic effect of current. It has an electromagnet in it. Let us study its construction and working as well.

Construction of Electric Bell
The electric bell has a U-shaped electromagnet. There is a small iron bar called armature which is h^d in front of the poles of the electromagnet. The lower end of the iron bar is attached to a flat spring and the flat spring is itself fixed to a metal bracket. The upper end of the iron bar has a clapper attach to it. A metal gong is fixed near the clapper.

Working of Electric Bell
In order to ring the bell, first of all, we press the push button switch in order to ring the bell. So, when we press the switch, then the electric circuit of the bell is completed and a current passes through the coil of the electromagnet and it gets magnetised. The electromagnet attracts the iron armature towards itself.

So, as the armature moves towards the poles of the electromagnet, the clapper attached to it strikes the gong and produces a ringing sound. It implies that the bell rings.
Electric Current and Its Effects Class 7 Notes Science Chapter 14 img 10
When the armature moves towards the magnet, its contact with the contact screw is broken. Due to this, the electric circuit breaks and no current flows in the electromagnet coil. The electromagnet loses its magnetism for a moment and the armature is no longer attracted by it. The flat spring brings back the iron armature to its original position and the clapper also moves away from the gong.

As soon as the armature comes back and touches the contact screw the circuit is completed and current starts flowing in the electromagnet coil again. The electromagnet attracts the iron armature once again and the clapper strikes the gong again producing a ringing sound.

So, this process of ‘make and break’ of the electric circuit continues as long as we are pressing the switch. Due to this, the armature vibrates forwards and backwards rapidly each time making the clapper strike the gong. Thus, the clapper strikes the gong rapidly producing an almost continuous sound.

We hope the given CBSE Class 7 Science Notes Chapter 14 Electric Current and Its Effects Pdf free download will help you. If you have any query regarding NCERT Class 7 Science Notes Chapter 14 Electric Current and Its Effects, drop a comment below and we will get back to you at the earliest.