2
$\begingroup$

The permanent magnet is being moved away from the ring. How to determine the direction of the inductional current in the ring?

Here's what I've tried. Magnetic field lines go from the North pole of the magnet to its South pole. The ring is attracted to the magnet => the ring pole near the magnet and the magnet pole near the ring is opposite. According to Lentz's law, the magnetic field of the ring resists the magnetic field of the magnet.

But now I'm stuck trying to understand whether the direction of the inductional current in the ring is clockwise or counterclockwise. How can I determine this?

$\endgroup$

1 Answer 1

2
$\begingroup$

So the direction of the B field is right-to-left in your drawing - going into the S pole of the magnet. As the magnet gets farther away to the left, the field at the ring (coil) gets weaker but it still points to the left.

Faraday's law tells us that the resulting changing magnetic flux cutting the coil will induce a voltage. This voltage will induce a current in the coil in such a way (Lenz's law) that the flux the current produces will oppose the original change that created it.

$$\oint_{L}\mathbf{E}\cdot d\mathbf{l} = -\frac{\partial\mathbf{}}{\partial t}\int_A \mathbf{B}\cdot d\mathbf{A}$$

So, which way would current need to flow in the loop such that the flux it produces would oppose the decreasing of the B field produced by the moving magnet?

$\endgroup$
4
  • $\begingroup$ Thank you. :) So, the direction is counterclockwise, isn't it? $\endgroup$ Oct 4, 2020 at 15:59
  • 1
    $\begingroup$ Yes (looking at the loop from the left). $\endgroup$ Oct 4, 2020 at 16:23
  • 1
    $\begingroup$ thank you very much! $\endgroup$ Oct 4, 2020 at 17:21
  • 1
    $\begingroup$ My pleasure Demian $\endgroup$ Oct 4, 2020 at 17:22

Not the answer you're looking for? Browse other questions tagged or ask your own question.