EC-5a  Lenz's Law

OBJECTIVES:

1. To observe the eddy currents induced by the magnetic field in a moving conductor.
2. To follow how the eddy current intensity depends on the shape of the conductor.

   YOU NEED TO KNOW: You must understand and be able to use the Right Hand Rule in order to determine the direction and sense of the forces involved.

APPARATUS:

A Permanent Magnet; four different paddle pendulums; two small red and green magnets. (There is only a single one of these set-ups per room and so, if it is in use, proceed to the next section first.)

Figure 1: The magnet and the paddle pendulum.

PROCEDURE I: (10 min)

Using one of the small magnets determine the direction of the magnetic field between the jaws of the permanent magnet. Observe and describe the behavior of the four pendulums when dropped between the jaws of the permanent magnet. Reverse the magnet. Observe and describe the behavior of two pendulums when dropped between the jaws of the permanent magnet. Set pendulum #1 between the jaws of the magnet. Move the magnet rapidly in a direction parallel to the pendulum plate (i.e. without touching jaws). Describe what happens.

Figure 2: Paddle moves in the + y direction

QUESTIONS:

Q1:

Use the right hand rule to determine the motion of electrons at the moment when pendulum # 4 enters the magnetic field region. Is it from A to B? Or from B to A?

Q2:

What is the direction of the current flowing in the pendulum? Is it from A to B? Or from B to A?

Q3:

Use the right hand rule to determine the direction of the force exerted on the current that flows in the arm of the pendulum by the magnetic field. Is it along the positive or the negative y axis?