CBSE Sample Papers for Class 12 Physics Solved 2016 Set 6

Section A

1.If the horizontal and vertical components of the earth’s magnetic field are equal at a certain place, what would be the angle of dip at that place?

2.Draw the graph, showing the variation of resistivity with temperature for silicon.

3. Which has larger susceptibility; iron or copper?

4. A piece of copper and the other of germanium are cooled from room temperature to 100 K. What will be its effect on their conductivities?

5. In the figure shown below, X, Y represent parallel plate capacitors having the same area of plates and the same distance of separation between them. What is the relation between the energies stored in the capacitors?

Section B

6. Identify the parts X and Y in the following block of a generalised communication system

7. Write the type of biasing which is used in the following devices in their working.

Photodiode
Light emitting diode

8. The work function, for a certain metal is 4.2 eV Will this metal give photoelectric emission for incident radiation of wavelength is 330 nm?

9. Give reason.

Lighter elements are better moderators for a nuclear reactor than heavier elements.
In a natural uranium reactor, heavy water is preferred moderator to ordinary water.

How does the size of a nucleus depend on its mass number? Hence, explain why the density of nuclear matter is independent of the size of nucleus?

10. Write the truth table for the circuit shown in Fig. (a) and Fig. (b) consisting of NOR gate only. Identify the logic operation (OR, AND, NOT) performed by the two circuits.

Section C

11. A capacitor made of two circular plates each of radius, r = 12 cm and charging current is constant I =015 A.

Calculate the rate of change of electric field between the plates.
Find the displacement current across the plates.
Is Kirchhoff’s first rule valid at each plate of capacitor? Explain.

A student connects a long air core coil of manganin wire to a 100 V DC source and records current of 1.5 A. When the same coil is connected across a 100 V 50 Hz AC source, the current reduces to 1.0 A.

(i) Give reason for this observation. (ii) Calculate the value of reactance of the coil.

14. Give reason for the following.

Why is ground wave transmission of signals restricted to a frequency of 1500 kHz?
How does the length of a dipole antenna vary with increase of the frequency of the carrier wave?
How does the effective power radiated by an antenna vary with wavelength?

15. In Young’s experiment, the width of fringes obtained with light of wavelength 6000 A is 2.0 mm. What will be the fringe width, if the entire apparatus is immersed in a liquid of refractive index 1.33?

16. With the help of a diagram explain Rutherford a-particle scattering experiment.

17. In the accompanying diagram, the direct image is formed by the lens (f = 10 cm) of an object placed at O and that formed after reflection from the spherical mirror is formed at the same point O’. What is the radius of curvature of the mirror?

18. Write the function of base region of a transistor. Why is this region made thin and slightly doped?

In the given diagram, a piece of pure semiconductor, S in series with a variable resistor, R and source of constant voltage, V. Would you increase or decrease the value of R to keep the reading of ammeter, A constant, when semiconductor, S is heated? Give reason.

19. Careful measurement of the electric field at the surface of a black box indicates that the net outward flux through the surface of box is 8.0xl0³ Nm²C^_1.

What is the net charge inside the box?
If the net outward flux through the surface of the box was zero, what can you conclude about the presence of charges inside the box.

20. A rectangular loop shaped wire KLMN is moved with a uniform velocity, v at right angle to a uniform magnetic field, B as shown in the figure.

What is the magnitude of current induced in the loop?
Will there be any work done by the loop? Give reason for your answer.

21. Draw a labelled ray diagram of an astronomical telescope, forming the image at infinity. An astronomical telescope uses two lenses of powers 10D and ID.

State with reason, which lens is preferred of objective and eyepiece.
Calculate the magnifying power of the telescope, if the final image is formed at the near point.

22. Define the Q value of a nuclear process. When can a nuclear process not proceed spontaneously? If both the number of protons and the number of neutrons are conserved in a nuclear reaction, in what way is mass converted into energy (or vice-versa) in a nuclear relation?

Section D

23. In a winter night, Sonu was sitting with his grandmother, she told him “When I was of your age, paisa coins were used like one paisa, two paisa etc., and those paisa, coins were having great value at that time.”

Sonu asked his grandmother, “From which metal was the coin made up of ?”

Grandmother replied, “I don’t know about it, as I am not educated and I even never tried to know about that”.

But Sonu’s elder brother Monu was standing behind and listening to the conversation between them, he told to Sonu that those coins were made up of aluminium, magnesium alloy. Grandmother felt happy with her grandsons.

Write about Monu’s intellegency.
State law of conservation of charge.
A paisa coin weighs 0.75 g and is square shaped with diagonal 17 mm. Suppose it is electrically neutral. If the coin is only made up of aluminium, find the magnitude of equal number of positive and negative charges.

Section E

Answers

Section A

1.If the horizontal and vertical components of the earth’s magnetic field are equal at a certain place, what would be the angle of dip at that place?

Ans.

2.Draw the graph, showing the variation of resistivity with temperature for silicon.

Ans.

3. Which has larger susceptibility; iron or copper?

Ans. Iron is a ferromagnetic substance while copper is diamagnetic, the susceptibility of iron is much larger.

4. A piece of copper and the other of germanium are cooled from room temperature to 100 K. What will be its effect on their conductivities?

Ans.

Section B

6. Identify the parts X and Y in the following block of a generalised communication system

Ans. X is message signal or information source. Y is a transmission channel.

7. Write the type of biasing which is used in the following devices in their working.

Photodiode
Light emitting diode

Ans. (i) Photodiode is used in reverse biasing.

(ii) Light Emitting Diode (LED) is used in forward biasing.

8. The work function, for a certain metal is 4.2 eV Will this metal give photoelectric emission for incident radiation of wavelength is 330 nm?

Ans.

9. Give reason.

Lighter elements are better moderators for a nuclear reactor than heavier elements.
In a natural uranium reactor, heavy water is preferred moderator to ordinary water.

How does the size of a nucleus depend on its mass number? Hence, explain why the density of nuclear matter is independent of the size of nucleus?

Ans.

(i) A moderator slows down fast neutrons released in a nuclear reactor. The basic principle of mechanics is that the energy transfer in a collision is the maximum when the colliding particles have equal masses. As, lighter elements have mass close to that of neutrons, lighter elements are better moderators than heavier elements.

(ii) Ordinary water has hydrogen nuclei which have greater absorption capture for neutrons; so ordinary water will absorb neutrons rather than slowing them. On the other hand, the heavy hydrogen nuclei (fH) have negligible absorption capture for neutrons,so they share energy with neutrons and neutrons are slowed down.

10. Write the truth table for the circuit shown in Fig. (a) and Fig. (b) consisting of NOR gate only. Identify the logic operation (OR, AND, NOT) performed by the two circuits.

Ans.

Section C

11. A capacitor made of two circular plates each of radius, r = 12 cm and charging current is constant I =015 A.

Calculate the rate of change of electric field between the plates.
Find the displacement current across the plates.
Is Kirchhoff’s first rule valid at each plate of capacitor? Explain.

12. A student connects a long air core coil of manganin wire to a 100 V DC source and records current of 1.5 A. When the same coil is connected across a 100 V 50 Hz AC source, the current reduces to 1.0 A.

(i) Give reason for this observation. (ii) Calculate the value of reactance of the coil.

Ans. When DC source is connected across the coil, the hindrance in current is due to pure resistance of coil, because inductance does not offer any hindrance to steady DC current. But when AC source is connected across the coil, the hindrance is offered due to both resistance, R and inductance, L of the coil, hence current is reduced.
Ans.

Ans.

14. Give reason for the following.

Why is ground wave transmission of signals restricted to a frequency of 1500 kHz?
How does the length of a dipole antenna vary with increase of the frequency of the carrier wave?
How does the effective power radiated by an antenna vary with wavelength?

Ans.

16. With the help of a diagram explain Rutherford a-particle scattering experiment.

Ans.

18. Write the function of base region of a transistor. Why is this region made thin and slightly doped?

Ans. The lease region of the transistor allows the majority carriers of the emitter to pass through it to the collector region.The majority carriers of the emitter are the minority in the base. Thus, base donates the region which accepts minority carriers from the emitter.

A thin and slightly doped base region contains a smaller number of majority charge carriers. This reduces the recombination rate of electrons and holes at the base-emitter junction. Most of the majority charge carriers coming from emitter into base immediately get collected by the collector. This reduces base current and increases both collector current and current gain of the transistor.

We shall increase the value fo resistance R, joined in series with pure semiconductor, S in order to maintain the ammeter reading constant when semiconductor is heated.

On heating the semiconductor more charge carriers are available due to higher thermal energy and, consequently, resistance of the semiconductor piece decreases. In order to maintain constant current, the total resistance of the circuit must remain constant.

Hence, external resistance will raise.

19. Careful measurement of the electric field at the surface of a black box indicates that the net outward flux through the surface of box is 8.0xl0³ Nm²C^_1.

What is the net charge inside the box?
If the net outward flux through the surface of the box was zero, what can you conclude about the presence of charges inside the box.

Ans.

20. A rectangular loop shaped wire KLMN is moved with a uniform velocity, v at right angle to a uniform magnetic field, B as shown in the figure.

What is the magnitude of current induced in the loop?
Will there be any work done by the loop? Give reason for your answer.

Ans. Magnitude of current induced in the loop is zero, as there is no change in magnetic flux through the loop.

(ii) No, work is not done by the loop as there is no change in magnetic flux through the loop. According to Lenz’s law, there is no force to oppose the motion.

21. Draw a labelled ray diagram of an astronomical telescope, forming the image at infinity. An astronomical telescope uses two lenses of powers 10D and ID.

State with reason, which lens is preferred of objective and eyepiece.
Calculate the magnifying power of the telescope, if the final image is formed at the near point.

Ans.

Ans. The G-value of a nuclear process refers the energy release in the nuclear process which can be determined using Einstein’s mass energy relation E = mc². The Q-value is equal to the difference of mass of products and reactance multiplied by square of velocity of light. (1) The nuclear process does not proceed spontaneously when Q-value of a process is negative or sum of masses of product is greater than sum of masses of reactant. Since, mass defect occurs in nucleus which converts into energy as per Einstein’s mass energy relation E = mc²and produces binding energy. This energy binds nucleons together due to nuclear forces inspite of repulsive Coulombian forces.

Section D

Sonu asked his grandmother, “From which metal was the coin made up of ?”

Grandmother replied, “I don’t know about it, as I am not educated and I even never tried to know about that”.

Write about Monu’s intellegency.
State law of conservation of charge.
A paisa coin weighs 0.75 g and is square shaped with diagonal 17 mm. Suppose it is electrically neutral. If the coin is only made up of aluminium, find the magnitude of equal number of positive and negative charges.

Ans. (i) Monu’s frequent answers indicates his presence of mind with sharp memory and deep general knowledge . And, at his very young age all this proves him a brilliant child.

(ii) Total electric charge on any isolated system must remain conserved. Therefore, electric charge can neither be created nor be destroyed.

Section E

Ans.

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