Question

Use Lenz's law to answer the following questions concerning the direction of induced currents. Express your answers in terms of the letter labels a and b in each part of the figure below. 

(a) What is the direction of the induced current in the resistor R in Figure a when the bar magnet is moved to the left? 

a to b 

b to a 

The magnitude is zero.

 (b) What is the direction of the current induced in the resistor R after the switch S in Figure b is closed? 

a to b 

b to a 

The magnitude is zero. 

(c) What is the direction of the induced current in the resistor R when the current I in Figure c decreases rapidly to zero? 

 a to b

 b to a 

The magnitude is zero.

Answer 1

(a) The magnetic flux through the coil decreases as the bar magnet moved to the left. From Lenz's law, the magnetic field in the coil induced in such a way to increase the magnetic flux. This induced magnetic field in the coil will induce a counter clock wise current in the coil. Thus, the current in the resistor passes from a to b.

(b) A clock wise current produced in the coil creates a magnetic field that is directed to the right. From Lenz's law, the induced magnetic field through the resistor coil is directed to the left. From right hand thumb rule, the induced current through the resistor passes from \(\mathrm{b}\) to a.

(c) The current in the wire increases the magnetic flux through the coil. From Lenz's law, the induced magnetic field will induced in such a way to decrease the original magnetic flux. From right hand rule, the induced current passes from a to \(\mathrm{b}\).

Answer 2

Lenz Law :

the direcrion of induced current due to magnetic field change

opposes the cause of its generation. ie

I = - N*(d$\Phi$/dt)

here,

d$\Phi$/dt = rate of change of magnetic flux

(a)

a to b.

(b)

b to a.

(c)

a to b.