directed toward you. Remember: The current must flow so that it creates a magnetic

field inside the loop that opposes this decrease in flux.

Lenz's law

Magnetic field of induced current

opposes change in flux

In what direction will the current

flow?

Counterclockwise

29.10 - Eddy currents

Eddy currents: Electric currents created when a

solid conductor moves through a magnetic field.

We have used a relatively simple configuration to analyze the current created by

moving a wire in a magnetic field. The moving wire was a straight segment, and an

overall rectangular circuit contained three other straight segments.

Here, we use eddy currents to illustrate a significant point: Even when a circuit is not

well defined, currents can still be caused to flow in a conductor at rest in a time-varying

magnetic field or passing through a nonuniform magnetic field. For instance, the

rectangular block to the right is made of a conducting material. When it moves into or

out of the magnetic field shown, currents flow in it, but not in straight lines. These types

of current flow are called eddy currents, and they flow in a complex pattern that

resembles a whirlpool. The diagram in Concept 1 visually approximates the eddy

currents by depicting them as circular.

The existence of eddy currents can be vividly demonstrated with two differing pieces of

conducting material. One piece is a solid block attached to a string, swinging in and out

of a magnetic field. This motion generates eddy currents in the block. The magnetic

fields of the eddy currents oppose the change in flux that causes them. In essence, the

block becomes an electromagnet whose motion is opposed by the external magnetic

field. The opposing force quickly damps (reduces) the motion.

A second block, with slots cut through it, swings through the same field. Because the

slots prevent the formation of significant large-scale eddy currents, a much weaker

opposing magnetic field is created. There is less force to damp the motion, and the

block swings relatively freely.

Like any phenomenon that removes mechanical energy from a system, causing it to

dissipate as heat, eddy currents can be either an undesirable source of inefficiency, or

in the right circumstances, a valuable tool. One useful application of eddy currents is to

the braking of trains.

If the upper half of a spinning train wheel is subjected to a strong magnetic field, each

portion of the wheel will rotate into and out of this field. The changing magnetic flux in

that portion of the wheel will induce eddy currents that apply a countertorque to the

Eddy currents

Caused by changing flux in solid

conductors

Eddy currents damp motion

Slotted block

Slots minimize eddy currents

Oscillation minimally dampened

(^546) Copyright 2007 Kinetic Books Co. Chapter 29