## Friday, July 06, 2007

### Another Question on Electromagnetic Induction

The following question is in continuation of yesterday’s question on electromagnetic induction. In fact I had tried to post it yesterday itself but there were some technical problems.

Here is the question which is meant for checking whether you have understood Lenz’s law clearly:

(1) A current carrying solenoid and a conducting ring are arranged coaxially, the ring being at a distance ‘d’ from one end of the solenoid. By varying the output voltage of a variable voltage source feeding the solenoid, the current ‘I’ in the solenoid can be varied. Pick out the correct statement:

(a) When the current ‘I’ increases, there is an attractive force between the ring and the solenoid

(b) When the current ‘I’ decreases, there is a repulsive force between the ring and

the solenoid

(c) When the current ‘I’ increases, there is a repulsive force between the ring and the solenoid

(d) When the current ‘I’ increases, there no force between the ring and the solenoid

(e) When the current ‘I’ decreases, there no force between the ring and the solenoid

When the current in the solenoid increases, the magnetic flux linked with the ring will increase. This increment in magnetic flux produces an induced current in the ring. By Lenz’s law, the system has to oppose this. The ring and the solenoid will try to move away so that the magnetic field at the ring and the flux through the ring are reduced. So. there is a repulsive force in this case [Option (c) is the correct one].

When the current in the solenoid decreases, the solenoid and the ring will have to come closer to oppose the decrement in flux. In that case there will be an attractive force. [ So, option (b) is incorrect]

You can find additional typical multiple choice questions (with solution) on electromagnetic induction by clicking on the label ‘electromagnetic induction’ either below this post or on the side of this page (or any page).