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I know that when electrodynamics comes into play, Newton's third law is not always valid in a "classical way". I mean that there are cases where the action-reaction principle is not stricly true, since we also have to consider the momentum which is stored by the electromagnetic field.

My question is about electrical machines, I mean all devices that provide the conversion between electrical energy and mechanical energy (and vice versa). Does the action-reaction principle always hold for all these machines? Said another way, is it safe to say that the torque produced on the rotor is always equal and opposite to the torque produced on the stator? Is there any type of machine that is an exception to this rule, because of the its electrodynamic nature?

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  • $\begingroup$ the action reaction principle does not hold for the Biot-Savart differential forces between differential current elements that are non-physical but the action-reaction principle for forces based on the Biot-Savart law holds between full current loops that are physical. $\endgroup$
    – hyportnex
    Jan 27, 2023 at 0:54

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If you have used an electric drill with a reasonably large drill bit or with a disc sanding attachment you would have felt a torque in the opposite direction to the direction of rotation of the bit/disc which is an example of Newton’s third law in action.

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