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I have two magnetic fields question (high school level) which require clear answers...

  1. When a current carrying conductor, such as a copper wire, is placed in a magnetic field it will experience a force. Has the force produced anything to do with the wire itself? if not, what produces the force? (I don't understand the highlighted bit....)

  2. When a current carrying wire in a magnetic field experiences a force it is usually seen to move. Is this consistent with Newton's Third Law? Where is the equal and opposite force? Explain the observation.

My physics teach briefly explained it but I did not understand it fully..

Thank you.

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  • $\begingroup$ Re (1): write down the equation for the force per unit length on a wire carrying a current $I$ in a magnetic field $B$. Does the equation include anything to do with the wire e.g. it's diameter, the metal it's made of, etc? If the answer is no then the force does not have anything to do with the wire itself. Re (2): The force is between the wire and whatever is generating the magnetic field. For example if the field is from a permanent magnet the force is between the wire and the permanent magnet. $\endgroup$ – John Rennie May 30 '14 at 13:28
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  1. Assuming the wire is stationary, the wire has nothing to do with the force on the current. You can shoot electrons through empty space and a magnetic field will exert a force on the electrons. That said, if there is a wire, then the magnetic field will move those conduction electrons to one side, creating a charge imbalance, which then pulls the electrons in an opposite direction and pulls the wire in the direction the electrons were deflected. So the neutral wire does contribute to the whole wire feeling a force rather than just the moving conduction electrons. The Lorentz force due to the magnetic field and the moving conduction charges is ultimately responsible for initiating motion. Electric fields and forces are then responsible for transmitting and redistributing this impulse to the whole wire.

  2. Momentum transfer is from the field into the wire. Newton's third law is more general in the form of conservation of momentum, since forces usually act on masses.

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  1. Yes.
  2. Yes.

When the current-carrying conductor is placed in a magnetic field, it experiences a force(it might move in some cases). The direction of the force is defined by the Flemming's left hand rule.

Newton's third law states that action is always equal and opposite in direction to the reaction. The magnets producing the field also feel a force which is opposite to the direction of the motion of the current carrying conductor.

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  • $\begingroup$ For magnetic forces, Newton's third law does not necessarily apply. Momentum of matter need not be conserved. Only when EM momentum in space is accounted for, total momentum is conserved, but this cannot be described by the original Newton third law. $\endgroup$ – Ján Lalinský May 30 '14 at 16:59

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