12th Sept Test

8/4/2019 12th Sept Test

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Class – XII

Subject – PHYSICS

TIME: 3 Hrs. MM: 70

General Instructions:

All the questions are compulsory.

The Question paper consists of four section A, B, C and D. Section A

contains 8 questions of 1 mark each, section B is of 10 questions of 2

marks each, Section C has 9 questions of 3 marks each whereas Section

D is of 3 questions of 5 marks each.

There is no overall choice. However, an internal choice has been

provided in one question of 2 marks, one question of 3 marks and all the

three questions of 5 marks weight age. A student has to attempt only

one of the alternatives in such questions.

Wherever necessary, the diagrams drawn should be neat and clean and

properly labeled.

SECTION A

1. State Coulomb’s law of electric force.

2. What is relaxation time?

3. What is the effect of speed on the charge and mass of a charged particle?

4. What is the charge on the nucleus of a helium atom?

5.

Draw electric field lines to represent a uniform electric field.6. Write the characteristics on electric lines of force.

7. What is equipotential surface?

8. A wire of resistivity ρ is stretched to three times its initial length, what will be its new

resistivity.

SECTION B



9. If p.d. v applied across a conductor is increased to 2v, how will the drift velocity of the

electrons change?

10. A 10Ω thick wire is stretched so that its length becomes three times. Assuming that there

is no change in its density on stretching. Calculate the resistance of new wire.

11. What is dipole moment? Is it a scalar quantity? Explain.

12. Prove I=Anevd.where symbols have usual meaning.

13. You are given 8 Ω resistors. What length of wire of resistance 120 Ωm-1

should be joined

in parallel with it to get a value of 6 Ω?

14. Three resistance 3Ω, 6Ω and 9Ω are connected to a battery. In which of them will the

current be maximum if

a) They all are connected in parallel

b) They all are connected in series .Give reason.

15. A wire of resistance 4R is bend in the form of circle .What is the effective resistance

between the ends of diameter? Or

Derive expression for drift velocity of electrons.

16. Two wires A and B have same lengths and same material, have their cross sectional areas

1:4, what would be the ratio of resistances of these wires.

17. Plot of current I versus time interval is given below. Find the charge that flows through the

wire during this time period

18. Amount of charge passing through the cross section of a wire is q(t)=at 2+bt+c.Write the

dimensional formula for a, b and c. If the values of a, b and c in SI unit are 6, 4, 2

respectively. Find the value of current at t=6 seconds.

SECTION C

19. Derive an expression for electric potential at a point due to a dipole.

20. Name the carriers of electric current in1) a bar made of silver

2) hydrogen discharge tube

3) a germanium semiconductor

4) a wire made of alloy nichrome

5) Super conductor.

6) Salt water



21. n small drops of same size are charged to V volts each. If they coalesce to form a signal

large drop, then what will be the potential of final drop?

Or

State and prove ohms law.

22. Derive expression for capacitance of a parallel plate capacitor with dielectric inserted.

23. A uniform wire of resistance “R” is shaped into a regular “n” sided polygon,

Where “n” is even. Find the equivalent resistance between

1) Opposite corners of the polygon

2) Adjacent corners of the polygon.

24. What is emf of a cell? State the factors of which its value depends derive the relation

between E, V, r & R. Prove E > V. is their any exception to this rule?

25. Derive expression for electric field on axial line of dipole.

26. A point charge Q= +4.60 µC is held fixed at the origin. A second point charge q= +1.20

µC with a mass of 2.80 x 10-4

kg is placed on the x-axis, 0.250 m from the origin. a.)What

is the electric potential energy U of the pair of charges? (Take U to be zero when charges

have infinite separation). b.)The second point charge is released from rest. What is its

speed when its distance from the origin is i) 0.500 m? ii) 5.00?m iii)50.0 m?

27. Resistance between the points A and B is given in the following figure, reduce the

following network to get the equivalent resistance.

SECTION D

28. Derive expression for potential difference when n cells of emf E1, E2…….. En, are connected

in series. Their internal resistances are r1, r2……….rn, respectively. State and prove when thisconfiguration yields maximum current.

OrA small negatively charged oil drop is held stationary in an electric field between twohorizontal parallel plated as shown. -10 kg.



(a) What are the two forces acting on the drop and in which direction do they act?

(b) The drop is stationary. What can be said about the two forces?(c) What is the charge on the oil drop?

29. Explain the principle of a capacitor V. Derive an expression for the energy stored in a

capacitor. In which form is energy stored in a capacitor?

Or

Four capacitors are connected as shown in the figure. What is the equivalent capacitancesbetween A and B? (M=µ) what is energy of each capacitor if 28 V is connected across

AB.

30. State Gauss theorem and hence derive expression for electric field due to a metal sphere.Draw graph showing variation of electric field with distance for a uniformly charged

metallic sphere.

OR

A 300pF capacitor is charged by a 200V supply. It is then disconnected from the supply

and is connected to another uncharged 300pF capacitor. How much energy is lost in the

process? Where does this energy go?

12th Sept Test

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