0
$\begingroup$

I'm new to the wonderful world of physics, and I'm really confused about magnetism and induced currents.

Let's say we have 2 circular closed loops that each carry a current $I$ clockwise and the loops are placed on top of one another. Now if the current in the top loop was to decrease to $0$, by Lenz's Law, what happens to the current in the bottom loop?

I said that if the current was to decrease to $0$ in the top loop, then an induced magnetic field will be created that's coming in the plane as opposed to out the plane (looking at the loops from the top). It will be in the same direction as the original magnetic field created from the first loop. So because the original magnetic field is nonexistent, then the only current in the bottom loop would be the induced one (which is clockwise by R.H. rule), so the current would decrease.

Again, I really don't know how to approach this question it's really confusing for me. Any help is greatly appreciated.

$\endgroup$
1
  • $\begingroup$ @MonkeysUncle has a good answer. Let me point out some problems with your question. 1.) You mention the direction of the original magnetic field, but then say that the original magnetic field is nonexistent. How's that possible? 2.) You say that the induced current would be clockwise as is the original current. How can you add two currents that move in the same direction and end up decreasing the current? $\endgroup$
    – garyp
    Jul 11 '16 at 12:03
1
$\begingroup$

A constant current in a wire creates a static magnetic field. So at the start of the problem, you have $B_1$ from the top loop, and $B_2$ from the bottom loop. They are both in the same direction. The question also asks you to use Lenz's Law which states when an induced current flows, it always acts to oppose the change which produced it. In other words, Lenz's Law states that induced current will act to try and maintain the status quo of the original setup.

If you take the current in the top wire to zero, this is equivalent to saying that we are turning $B_1$ off. Because $B_1$ was created by a clockwise current, by Lenz's Law loop 2 will get an induced current in the clockwise direction to create a magnetic field in the same direction $B_1$ was pointing. Thus the current in the bottom loop actually increases to compensate for the lost current in the top loop.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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