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Consider a simple electrical circuit made up of a battery, an incandescent bulb, and wire. The battery and bulb are equal in mass and are on opposite sides of a circle made up by the wire. Lastly, the circuit is operating and floating freely in microgravity.

Since an electromotive force propels objects with mass (electrons) around the circuit, can we expect the circuit, given enough battery life, to eventually rotate in the opposite direction of the electrons due to Newton’s third law of motion?

D. Martin
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    An early effort to detect this effect was made by Einstein and de Haas, but since electron spin had not been discovered yet, their prediction was doomed to be wrong. The story is well-told by Galison in his How Experiments End. – rob Mar 15 '19 at 00:37
  • You ask if current creates acceleration. An alternate answer refers to how acceleration creates current. https://physics.stackexchange.com/a/397315/55662 – BowlOfRed Mar 15 '19 at 06:34
  • The speed of electrons through a wire is fairly constant usually. Electrical resistance is like friction in that way. So the only net force is at the very beginning. Given the tiny mass of electrons and the tiny speed they get up to and the large mass of the circuit.... Hang on, I'm doing the math.... you'd need to wait about 4 hours for a reasonable circuit loop to turn a barely perceptible half millimeter. That's assuming that nothing like air resistance slows it down and assuming a reasonably large electron drift velocity to EMF ratio. That also assumes nothing quantum happens – Jim Nov 05 '20 at 14:44
  • I must apologize for not commenting on the thought-provoking responses to my question nearly THREE years ago. Thanks to all who took the time to respond. I hadn't for a moment thought about magnetic fields and spin possibly having an effect. Even if I am still not certain of the answer, I now understand there was much more to the question than I had originally imagined. Thanks again to all. – D. Martin Aug 30 '23 at 00:37

2 Answers2

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Yes, conservation of angular momentum applies and the system will not rotate. I assume that the bulb radiates isotropically and also that any other radiation effects are isotropic. The electrons cannot be set into motion without a reaction force on the battery. However the magnetic field will polarise electron spins, free or located in ferromagnetic atoms. This will align their spin angular momentum possibly causing an undoubtedly tiny opposite angular momentum of the setup.

More importantly any orbital angular momentum of the electrons will be transferred (back) to the lattice. There should be a transient effect on the angular orientation when the current is changing.

my2cts
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Newton's laws do apply. The overall system will not rotate. While the electrons are rotating in one direction, the rest of the gear will indeed be rotating imperceptibly slowly in the other. But the rotation is constant, as the propulsive EMF is being exactly opposed by collision forces between electrons and metal atoms.

I would suggest that these collisions would also destroy any alignment of electron spins, negating any cumulative effect there (But I am not that much the physicist, so I can't really argue that case). A shame, as an experiment, to see if (mass) electron spin alignment is reflected in a counteracting macroscopic material angular momentum, would be an interesting one.

Guy Inchbald
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