# Lenz's law: determining current direction

1. Dec 26, 2013

### tdod

http://img853.imageshack.us/img853/1150/n5pj.png [Broken]

I a can't seem to find out how to determine the direction the current will flow in... I would really appreciate an explanation.

Thanks!

Last edited by a moderator: May 6, 2017
2. Dec 26, 2013

### CAF123

Use the Lorentz force law to determine the direction of the force on the mobile electrons within the metal loop.

3. Dec 26, 2013

### Philip Wood

There are many ways to do this. First, here's one based directly on energy conservation.

As the leading side - call it "L" - of the loop enters the field an emf is induced in the loop so there will be a current in the loop, since it is a closed circuit. Thus L will be a current-carrying conductor in a magnetic field, and will therefore experience a 'motor effect' (or magnetic Lorentz) force. This must be to the left, opposing whatever agency is pushing the loop to the right. [If the motor effect force were to the right, the loop would accelerate to the right on its own accord, gaining both kinetic energy and internal energy due to joule heating - both 'for free'.]

Since the field is out of the page and the motor effect force is to the left, the current in L must be downwards (Fleming's left hand motor rule), so the sense of current in the loop is clockwise.

Now here's another way of using Lenz's law… As the loop enters the field, the flux linking the loop changes from zero to some value out of the page. The current in the loop must produce an flux opposing this change in flux, that is INTO the page. Using the right hand grip or corkscrew rule, this means the current in the loop is clockwise.

OR use CAF123's method!

If you've followed this, you should be able to show for yourself that there's no current when the whole loop is in the field, even though it's moving, and that the current is in the opposite sense when only the trailing edge of the loop is still in the field.

4. Jan 1, 2014

### Philip Wood

tdod: Did these explanations help?

5. Jan 1, 2014

### davenn

another post and runner LOL

Dave