A 30 H magnet with poles of about 10 cm. square, is connected
to a 12V storage battery. After power is applied an aluminum ring is dropped
into the field of the magnet. The ring falls slowly when entering or leaving
the field and fast when totally inside the poles. This demonstration illustrates
Faraday's and Lenz's Law and is shadow projected. This demo is also shown
with a smaller electromagnet, connected to 125VDC, and the TV projection system.
The area through which the magnetic field "flows" increases as the ring is placed in the field. Thus, the magnetic flux changes over time, inducing an emf, which in turn induces a current. The force of the magnetic field on the current acts to slow the ring as it falls.
When the ring is in the middle of the field, there is no longer a change in magnetic flux because the entire area of the ring is within the magnetic field. Therefore, there is no induced current or "braking force." The ring falls quickly.
When the ring is once again leaving the magnetic field, the induced current is in the opposite direction, this time opposing the decreasing magnetic flux. Once again, the force of the magnetic field on the current acts to slow the ring.
Special Cart: 6C-207g
See chapter 29, starting page 993, University Physics, for information on electromagnetic induction.
Young, Hugh D., Roger A. Freedman, and A. Lewis Ford. Sears and Zemansky's University Physics: With Modern Physics, 12th ed. San Francisco: Pearson Addison-Wesley, 2008.