# Lenz's Law and determining direction of a current

1. Mar 29, 2014

### stillcold

Hey guys,

Background: I'm having trouble determining the direction of a current in a wire in response to changing magentic flux. Although I understand which direction of the magnetic field should be for the induced current given the magnetic flux scenarios, I am having trouble determining the direction of the current using the right hand rule.

Question: How do you determine the direction of a current in a wire in the scenario of Len'z law? Because the induced current in a wire creates a magnetic field where one side of the wire has the magnetic field going in and the other one going out, and both clockwise and counter-clockwise directions of induced current provide the same two vectors of magnetic field but on different sides of the wire, so which direction of induced current do you pick when trying to determine the magnetic field since both configurations provide the two magnetic field vectors in both direction? Is there a way of determining the primary direction or something?

Thanks.

2. Mar 30, 2014

### Simon Bridge

It is the change in the flux through the wire loop that induces the current.
The current has to oppose the change.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

Also have a look at:
http://www.launc.tased.edu.au/online/sciences/physics/Lenz's.html [Broken]
... top example shows the situation you are dealing with.

Lets say you have a conducting circle centered in the x-y plane centered at the origin, and orient everything so the +z axis points "upwards".

There is a B field in the +z direction - so the flux goes in from the -z side (the bottom) and out through the +z side (the top).

The induced current is zero, because the B field is not changing.

If the strength of B increases in time, then that is equivalent to a north pole approaching from the bottom or a south pole approaching from the top.

Thus the induced magnetic field must look like a south pole from the top and a north pole from the bottom in order to oppose the change. The same current direction will do both at the same time.

Using the RH screw rule: For S at the top and N at the bottom, your thumb must point down (from south to north). So the current circulates clockwise when viewed from the top.

If the B field decreases, then that is equivalent to a north pole retreating from the bottom or a south pole retreating from the top. So the loop must have a north pole on top and a south pole on the bottom. So your thumb points upwards etc...

Last edited by a moderator: May 6, 2017