Lenz' law: North pole of bar magnet moves away from a coil

[subhradeep mahata]

Homework Statement

You have a circular coil, and two metal plates a and b. The north pole of a bar magnet is moved away from the coil at a constant velocity(assume the bar magnet and the coil lies along the x axis). On which plate will the excess positive charge appear?

Homework Equations

The Attempt at a Solution

Since the north pole is moved away, a south pole will be 'induced' in coil , which will result in a clockwise current, so a-b should behave as a cell with positive terminal a and negative terminal b. But my book says that excess positive charge will appear at b. Where am i wrong?[/B]

 

[Charles Link]

You need to have a changing flux in the coil=I think the statement of the problem is slightly inaccurate and the perpendicular to the plane of the coil needs to face the north pole of the magnet. It is sort of apparent what they are trying to show with their sketch, so we can proceed from there. Do the magnetic flux lines from the north pole of the magnet go into the plane of the loop?=(assume the magnet is held above the loop with the north pole facing the loop. What happens when the magnet is pulled farther away from the loop?=do the flux lines decrease in intensity? What direction does the current need to flow in the loop to create a magnetic field to offset this effect?

 

[subhradeep mahata]

That's what i am stuck at...according to me the current should flow in the clockwise direction

 

[Charles Link]

If it flows clockwise with positive charge flowing into capacitor plate "b", the positive charge flows into $b$. ("a b" is not a battery=it is a set of capacitor plates). $\\$ Additional comment: The EMF $\mathcal{E}$ occurs inside the loop, but it is not indicated. It would be in the direction from a to b.

 

[subhradeep mahata]

Can you please elaborate a little?

 

[Charles Link]

See my additional comment in post 4.

 

[subhradeep mahata]

If the current flows clockwise, how can you say that positive charge will flow in plate b?

 

[Charles Link]

That is how a capacitor works. Draw a capacitor in series with a battery or other EMF. That's what happens.

 

[subhradeep mahata]

Do you mean something like this?
The plate through which current enters behaves as a positive plate?

 

[Charles Link]

Yes. And the EMF $\mathcal{E}$ of the battery proceeds around the loop in the direction of current flow, proceeding inside the battery from the minus terminal to the plus terminal. The (positive) current flows into the previously uncharged capacitor giving it a positive charge on the upper plate, and positive current flows out of the previously uncharged lower plate, leaving behind a negative charge.

 

[subhradeep mahata]

Thank you very much Charles Link, i understood it.