CBSE Class 12 Physics Paper 2018 (page 3)

CBSE Class 12 Physics Paper 2018

Page 3

Question 11: Four point charges Q, q, Q and q are placed at the corners of a square of side 'a' as shown in the figure.

[figure to be inserted]

Find the
(a) Resultant electric force on a charge Q, and
(b) Potential energy of this system.

OR part of Question 11: 

(a) Three point charges q, –4q and 2q are placed at the vertices of an equilateral triangle ABC of side 'l' a shown in the figure. Obtain the expression for the magnitude of the resultant electric
force acting on the charge q.

[figure to be inserted]
(b) Find out the amount of the work done to separate the charges at infinite distance.

Question 12:
(a) Define the term 'conductivity' of a metallic wire. Write its SI unit.
(b) Using the concept of free electrons in a conductor, derive the expression for the conductivity of a wire in terms of number density and relaxation time. Hence obtain the relation between current density and the applied electric field E.

Question 13:
A bar magnet of magnetic moment 6 J/T is aligned at 60° with a uniform external magnetic field of 0·44 T. Calculate

(a) the work done in turning the magnet to align its magnetic moment

(i) normal to the magnetic field,

 (ii) opposite to the magnetic field, and

(b) the torque on the magnet in the final orientation in case (ii).

Question 14:
(a) An iron ring of relative permeability μr has windings of insulated copper wire of n turns per meter. When the current in the windings is I, find the expression for the magnetic field in the ring.
(b) The susceptibility of a magnetic material is 0·9853. Identify the type of magnetic material.
Draw the modification of the field pattern on keeping a piece of this material in a uniform magnetic field.


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