Kendiya Vidyalaya Rbnm, Salboni

KENDIYA VIDYALAYA RBNM, SALBONI

Session 2017-2018

PERIODIC TEST I

Class: XII Subject: Physics (042)

Time: 1hr. 30min. Full Marks: 40

Instructions:

1) All questions are compulsory. There are 17 questions in all.

2) This question paper has five sections: section A, section B, section C, section D, section E.

3) Section A contains five question of one mark each.

Section B contains five questions of two marks each.

Section C contains five questions of three marks each.

Section D contains five questions of four marks each.

Section E contains five questions of six marks each.

4) There is no overall choice. However an internal choice has been provided in question of section E. You have to attempt only one of the choices in such question.

5) You may use the following values of physical constants wherever necessary:

e = 1.6 x 10-19C

µ0 = 4π x 10-7mA-1

ε0 = 8.854 x 10-12 C2 N-1 m-2

1/4πε0 = 9 x109 Nm2C-2

me = 9.1 x 10-31 kg

mp = 1.673 x 10-27 kg

mn = 1.675 x 10-27 kg

Section A

1) Two small ball of same type having charge q= 10µC each. Now they are suspended from the ceiling of a roof at a particular point using two insulated thread. In equilibrium the angle between the thread is 600. Calculate tension of thread.

600

2) Q charge is provided to a conducting sphere of radius R. now potential at the points A, B, C of the sphere are VA, VB, VC respectively. A is the centre of the sphere, B is on the sphere and C is a point at a distance R/2 from the origin. Then what is the relation between VA,VB& VC.

3) Write the underlying principle of a moving coil galvanometer.

4) Two wires one of copper & other of manganin have same resistance & equal length. Which wire is thicker & why?

5) What is electrostatic shielding? How is this property used in actual practice?

Section B

6) In a two dimensional system two point charge q1 = 2µC & q2 = 1 µC are situated along Y axis & X axis from the centre at a distance of b =1cm & a =2cm respectively. If direction of electric field intensity makes an angle θ with position X axis at a point P(a,b) then calculate tan θ .

7) A parallel plate capacitor each with plate area ‘A’ & separation d is charged to a potential difference V. The battery used to charge it is then disconnected. A dielectric slab of thickness d & dielectric constant k is now placed between the plates. What change if any will take place in (i) charge on plates (ii) electric field intensity (iii) capacitance of the capacitors.

Justify your answer in each case.

8) A small metal ball has suspended within the two plates of a big parallel plate capacitor by a weightless thread. If ball has a charge +q & upper plate of capacitor also have +ve charge. Then what is the time period of oscillation of this system? [ l= length of thread ]

9) What is the change of resistance of a ammeter if its range has increased by n times?

10) What is Bohr magneton? What is the angle of dip at a place where the horizontal & vertical components of earth magnetic field are equal?

Section C

11) (i) Name the machine which uses crossed electric& magnetic fields to accelerate the ions of high energies. With the help of a diagram, explain the resonance condition.

(ii) What will happen to the motion of charged particle if the frequency of the AC voltage is doubled?

30Ώ

12) (a) E /\/\/\/\/\ F

40Ώ 40V

A /\/\/\/\/\/\ | C

80V 20Ώ

B | /\/\/\/\ D

From the figure calculate the current in the arms AC of the given current using Kirchhoff’s law.

(b) Calculate magnetic field at an axial point distance d from the centre of a circular coil of radius R carrying current I.

13) (i) Prove, σ = (ne2τ / m), where σ = Conductivity of material, n = number density of charge, τ = relaxation time, m = mass of electron, e = charge of electron.

(ii) A voltage source of internal resistance r delivers same power when connected separately to the loads R1 & R2. Show that r = √( R1 R2).

14) (i) Prove that electrostatics susceptibility χ = ε0( k-1) k = dielectric constant.

(ii) 1µF

1µF 1µF

A B

2µF

Calculate equivalent capacitance between A & B.

15) (i) State Gauss Theorem.

(ii) Two small identical electric dipole AB & CD each of dipole moment P are kept at an angle of 1200. What is the resultant dipole moment of this combination?

Section D

16) During a thunder storm the live wire of the transmission line fell down on the ground. A group of boys passing through noticed it & some of them wanted to place the wire by the side. As they were approaching the wire & trying to lift it, Hari noticed it & immediately pushed them away to prevent them from touching the wire. Two of them got hurt in the process. Hari took them to a doctor to get medical aid. Now answer:

(i) Write two values which Hari displayed during the incident.

(ii) Why is it that a bird can sit over a suspended live wire without any harm where as touching it on the ground can give a fatal shock?

(iii) The electric power from a power plant is set up to a very high voltage before transmitting it to distant consumer. Why?

(iv) State the work of live , neutral & earthing wire.

Section E

17) (i) In the circuit diagram given below, AB is uniform wire of resistance 15Ώ & length 1m. It is connected to a cell E1 of emf 2V and negligible internal resistance and a resistance R. the balance point with another cell E2 of emf 75mV is found at 30cm from end A. Calculate the value of R.

E1 R

/\/\/\/\/\/\

A X B

(ii) Why is potentiometer preferred over a voltmeter for comparison of emf of cells.

(i) Depict magnetic field lines due to two straight long parallel conductor carrying stedy current in the same directions.

(ii) Write the expression for the magnetic field produced by one of the conductor over the other. Deduced an expression for the force per unit length.

(iii) Determine the direction of this force.

(iv) In figure given below, wire PQ is fixed while square loop ABCD is free to move under the influence of current flowing in them. In which direction does loop begin to move or rotate?

AB = d

A B

Ia d

D C