How Does Moving a Magnet Near a Coil Demonstrate Lenz's Law?

[UrbanXrisis]

In physics class today, we did a project that I don't quite understand. We layed a small 50-turn coil flat on the table and held a magnet in a vertical position near the coil. We had the Galvanometer hooked up to make a clockwise current. The class then moved the magnet close to the coil and observed the current. Then moved the magnet away. We needed to conclude the generation of a current as a function of the magnet motion. Anyone explain to me what the purpose of this experiment was?

 

[Doc Al]

You've just demonstrated that a changing magnetic field creates an electric field. (It's the electric field that creates the current.)

Some things you may have noted: Current only exists when the magnet is moving--a stationary magnetic field does not create an EMF. The direction of the current (clockwise versus counter-clockwise) is different when you move the magnet toward the coil versus away from the coil.

 

[UrbanXrisis]

I noticed that when I moved the magnet away, it produced a negative chage and a positive charge when I moved it closer. What does that signify?

 

[Doc Al]

Lenz's Law

I noticed that when I moved the magnet away, it produced a negative chage and a positive charge when I moved it closer. What does that signify?

You mean current, not charge. Move the magnet one way, you get one EMF; move it the opposite way, the EMF is reversed. Reverse the EMF and the current is reversed as well.

The induced EMF creates a current, which in turn creates another magnetic field. (Moving charges--current--create a magnetic field, right?) One thing to point out is that the induced current always creates a field that opposes any change to the field that the moving magnet is creating. Let's say you move the north pole of the magnetic towards the center of the coil, thus trying to increase the field going into the coil. The induced current will then respond to create a magnetic field opposing that of the magnet. When you pull the magnet away, the opposite occurs: you are reducing the field, so the induced current tries to increase it. This is called Lenz's Law.

Lenz's law is a consequence of conservation of energy--you have to push or pull the magnet--it takes energy to induce a field.