Exploration 29.1: Lenz's Law

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Lenz's law is the part of Faraday's law that tells you in which direction the current in a loop will flow. Current flows in such a way as to oppose the change in flux. The magnetic field created by the current in the loop opposes the change in the magnetic flux through the loop's area (position is given in meters, time is given in seconds, and magnetic field strength is given in tesla). Restart.

Consider the initial configuration. The center has a field-free region, and the sides have a linearly increasing magnetic field into the computer screen (blue) and out of the computer screen (red). The deeper colors represent a stronger field. Drag the loop from the white (field-free region) into the blue.

  1. While you drag it, which way does the current in the loop flow? (right arrow means clockwise current; left arrow means counterclockwise current).
  2. Sketch the field that the current in the loop generates.
  3. In the center of the loop, does this field (created by the induced current) point into or out of the computer screen?
  4. So, as you drag the loop to the right, the external field in the loop increases in which direction? The field generated by the current points in which direction? According to Lenz's law, these two directions should be opposite.

Now, take the loop over to the far right and then move it slowly to the white region.

  1. Explain why the direction of the current points the way it does.
  2. What if you take the loop from the center to the left (into the red region)? Explain what you expect to happen and then try it.
  3. Can you tell the difference between moving a loop from a blue to a white region and moving from a white region to a red region? Why or why not?
  4. Try the two other configurations, Configurations A and B (in which the magnetic field is hidden). Describe the magnetic field as completely as possible.
  5. Once you've completed your descriptions, decide which of the magnetic fields (Fields 1, 2, or 3) matches Configuration A and Configuration B.
Check your answers to (i) by adding a loop to a field animation:

Download PDF Worksheet

Exploration authored by Anne J. Cox.
Script authored by Mario Belloni and Wolfgang Christian and modified by Anne J. Cox.