When a current-carrying rectangular loop is placed in a magnetic field, the forces acting on either side of the loop which is perpendicular to the field provides a torque which rotates the loop. enter image description here

However, according to Newton's 3rd Law, there should be some equal and opposite force acting on the object exerting the force. But in fields, it is the field that exerts forces on objects. Does that mean that the loop exerts an equal and opposite force on the magnet or on the magnet's field? How do these forces play out and what effect do they have on the magnet?

  • $\begingroup$ Interesting! As far as I know, Newton's laws were never meant for force-fields - they define force itself and acceleration as its consequence. Hail Oersted, for he unleashed the tormenting truth of fields (magnetic fields, though it was later generalized to all kinds of forces). $\endgroup$ Commented Feb 20, 2021 at 5:51
  • $\begingroup$ I would rather suggest to read Faraday's or Maxwell's on the idea of lines of force. $\endgroup$ Commented Feb 20, 2021 at 5:53
  • $\begingroup$ This is a great answer physics.stackexchange.com/questions/114466/… $\endgroup$ Commented Feb 20, 2021 at 6:07
  • $\begingroup$ this wikipedia is pretty good as well en.wikipedia.org/wiki/Momentum#Electromagnetic $\endgroup$ Commented Feb 20, 2021 at 6:21
  • $\begingroup$ Fleming's right hand rule described that flow of electrical current in the loop (flipping the bird sign) will induce a magnetic field acting on the magnetic field from the permanent magnet (reprimanding sign) which then result in a magnetic force (compliment sign) so it is 2 magnetic fields battle royale ;D $\endgroup$
    – user6760
    Commented Feb 20, 2021 at 6:34

1 Answer 1


Newton's third law does cause magnets to have equal and opposite effects on each other, whether electromagnets or permanent magnets. The equal and opposite forces are transferred by the magnetic field. In an electric motor or generator, just as the field magnets are pushing /pulling on the armature via their magnetic fields, the armature is also pushing / pulling the field magnets. If you hold a small electric motor in your hand when it is started you can feel the anti spin ward torque of the motor housing as it applies spinward torque on the armature.


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