Motor and dynamo

Problem

Explain in a few lines how the electric motor and the dynamo work, highlighting the reversal of the energy balance (electrical to mechanical in the motor, mechanical to electrical in the dynamo).

Solution

Motor. A current in a loop immersed in a field $\vec B$ experiences a torque $\vec M=\vec\mu\times\vec B$ that makes it rotate. The commutator reverses the current every half turn, so the torque always keeps the same sense of rotation and the motion is continuous. The device converts electrical energy into mechanical energy.

Dynamo. The loop is made to rotate mechanically in the field. The change of magnetic flux through the loop induces, by Faraday’s law, an electromotive force that drives current in the circuit. The device converts mechanical energy into electrical energy.

Motor and dynamo are the same device, with the energy balance reversed.

$\ev{\text{motor: el.}\to\text{mech.};\quad \text{dynamo: mech.}\to\text{el.}}$

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Topics: Magnetismo Concepts: Dipolo magnetico · Forza elettromotrice