# Important Questions for Class 12 Chapter 4 - Moving Charges and Magnetism

Important questions based on NCERT syllabus for Chapter 4 - Moving Charges and Magnetism:

Question-1: A horizontal overhead power line carries a current of 90 A in east to west direction. What is the magnitude and direction of the magnetic field due to the current 1.5 m below the line?

Solution:
Given:
I = 90 A
R = 1.5 m
Considering Amperian loop and using the above formula
B = [(2 × 10^‒7) × (90)] / 1.5
Or B = 1.2 × 10^‒5 T
Direction is towards south.

Question-2: A 3.0 cm wire carrying a current of 10 A is placed inside a solenoid perpendicular to its axis. The magnetic field inside the solenoid is given to be 0.27 T. What is the magnetic force on the wire?

Solution:
Given:
l = 3 cm
I = 10 A
B = 0.27 T
θ = 90o
Then the force
F = I. l B. sin θ
Substituting the values, we have
F = 0.081 N.

Question-3: A closely wound solenoid 80 cm long has 5 layers of windings of 400 turns each. The diameter of the solenoid is 1.8 cm. If the current carried is 8.0 A, estimate the magnitude of B inside the solenoid near its centre. 4.9 A square coil of side 10 cm consists of 20 turns and carries a current of 12 A. The coil is suspended vertically and the normal to the plane of the coil makes an angle of 30º with the direction of a uniform horizontal magnetic field of magnitude 0.80 T. What is the magnitude of torque experienced by the coil?

Solution:
Given:
L = 80 cm
N = 2000
r = 0.9
I = 8 A
Enclosed current Ie = 8 x 2000 = 16000 A
Since, BL =μo Ie
Putting the values
B = μo (16000/0.8) = 0.025 T