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Q&A

# How exactly do eddy currents slow down objects moving though a magnetic field

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When a pendulum made of a conducting material moves through a magnetic field, it's a well-established fact that it experiences a retarding force, thus slowing it down, however, I'm unable to understand why this takes place, as expected the magnetic field doesn't retard it(one can verify the same via the right hand rule). Is this because of an induced electric field, if so, how can one find the direction of this field?

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As the pendulum swings, it experiences a changing magnetic field from the externally fixed magnets. Any changing magnetic field causes eddy currents in a conductor. Since these conductors aren't perfect (the conductor has non-zero resistivity), energy will be dissipated by eddy currents. That energy comes from the kinetic energy of the pendulum.

Since kinetic energy of the pendulum is transformed into heat, the net effect is that a magnetic field has a dampening effect on the motion of conductors moving thru the field. Since the amount of energy lost is proportional to the speed of the motion, it feels like viscous friction to the pendulum.

Note that this does not happen with super-conducting materials. Since the resistivity is 0, the eddy currents don't dissipate energy as heat, and therefore don't diminish with time on their own. In fact, they build up to create a magnetic field opposite of what was applied. This is how a magnet can levitate over a block of super-conducting material.

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