Lenz's Law and Eddy Currents

Nature resists change in flux.

Lenz's law says the induced current flows in the direction that opposes the change that caused it. This is conservation of energy at work: if the induced current helped the change, energy would appear from nowhere.

Drop a magnet down a copper tube and it falls slowly, because swirling eddy currents in the tube create a field that resists the motion. Engineers use eddy-current braking in trains and laminate transformer cores to keep these currents small.

• The minus sign in Faraday's law expresses Lenz's law.
• Eddy currents waste energy as heat in solid cores.
• They are useful in braking and induction heating.

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