From this figure:

enter image description here

And many others related to electromagnetic induction, the main component of motion would be the conductor perpendicular to a magnetic field.

I know it's possible to have it the other way around, fixed conductor and move the magnetic field source(e.g magnets, electromagnets), but I'm curious of the type of force the magnet/electromagnet would experience when current flows to the stationary conductor.

In a typical generator(where the wire rotates around a fixed magnet(s)) there is an induced EMF(vBL) and current will flow to the load, and due to the induced EMF is opposing the change in magnetic field flux, the Lorentz force acting on the wire will oppose the applied motion(v), what about the case when the magnet rotates around a fixed conductor? Will it experience a form of force opposing it's motion(besides eddy currents)? When the conductor has current flowing?


2 Answers 2


Yes, a magnet that is moving such that it creates a current in a non-moving wire will feel a force resisting that motion.

You can make a generator with spinning magnets instead of spinning wires like this one...

enter image description here

As the magnets move, they induce a current in the wire. The induced current itself make a magnet field on the opposite direction, opposes the rotation of the magnets.

Keep in mind that the lines of magnetic force must be cutting though the wire to induce a current in it, so just rotating a magnet around a wire such that one of its poles is always pointing toward the wire will not induce a current, and therefore will not create a back force.

Also keep in mind that it is the current flowing in the wire that creates the field that creates the back force, so the wire must be in a circuit or else no force. You can demonstrate this by spinning an unconnected stepper motor with your fingers, and then shorting the wires and spinning it again. It is harder to turn when the wires are shorted. Most stepper motors have fixed coils and turning magnets btw.

If you are interested in electromagnetics, I'd highly recommend this book...

enter image description here

Electricity and Magnetism 3rd Edition by Edward M. Purcell

Yo can find a free PDF version on google.

  • \$\begingroup\$ The magnitude of that force would be weak though, I mean if we used Ampere's law to calculate the current and magnetic field that wire has(for current flow) it would not create a significant force as the Lorentz force on a wire I think... \$\endgroup\$
    – Pupil
    Dec 5, 2015 at 20:26
  • \$\begingroup\$ Depends on how strong the magnets are and how fast they are moving. No difference from moving wires and static magnets. The reason the force is large in every day motors is because the wires are coiled - the force form each turn of wire add to the net force. Same would be true of moving magnets who's lines of force cut though static coils of wire. If you look at you above diagram though the lens of general relativity, Imagine you are sitting on the conductor. Now it is the magnetic field that is moving and all the forces are the same. \$\endgroup\$
    – bigjosh
    Dec 5, 2015 at 20:42
  • \$\begingroup\$ Thank you for that book recommendation, I will start reading it soon. There is one thing that is confusing me when thinking about a motor, reverse the setup we dicussed, making the magnets fixed, and a conductor movable, when current flows within the conductor it will experience the Lorentz force moving it away from the magnets, even though there is current flowing into that conductor and it's own magnetic field... aside from the Lorentz force acting on the wire there is also(depending on the polarity) a magnetic attraction/repulsion? I'm confused at that point. \$\endgroup\$
    – Pupil
    Dec 6, 2015 at 13:11

A conductor carrying a current will obviously create a magnetic field around itself. If that field interacts with the field of a nearby permanent magnet the magnet will feel a force (push or pull) dependent on the matching or opposing field polarities. You are basically just creating another magnet with the current carrying conductor.

As for induced currents, the generated currents also generate their own additional magnetic fields that just happen to oppose the field that created them in the first place. This is why a shorted generator is very hard to spin.


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