# Does a magnetic field sustain a current in a loop?

A particular current produces a constant particular magnetic field. If the fact holds under scrutiny, why doesn't a magnet held within a loop sustain a particular current?

Are the electron spins responsible for the current creating the magnetic field within a magnet pushed back and turned off? So that, that is, the current stops?

Magnetic objects are never used as power supplies it seems, so I'm quite at a loss as to where it is I'm off here if at all.

There are a couple different viewpoints that can be used to address your question.

• It may be that activity A causes activity B, but activity B does not cause activity A. For example, lighting a piece of paper on fire gives off light, but shining light on a piece of paper doesn't necessarily make it catch fire. Additionally, if I bring two like charges near each other, they'll repel. But if I manually move two object apart, that doesn't mean that there is an electrostatic repulsion between the two. To say it in a confusing but possibly more accurate terms: Causes can cause effects, but those effects don't have to cause those causes.

• Magnetic fields are vector fields. They're more complicated than "there is or isn't a magnetic field." They have direction and strength. So, you might expect that holding a magnet near a wire produces a different type of magnetic field than what electric current produces on its own. Different magnetic fields will cause different behaviors.

• The effect of magnetic fields on charged particles is to change their direction only. They can't be used to speed up charged particles. But moving electrons in a wire slow down naturally unless something keeps them going. Magnetic fields are unable to do this since they can only cause a change in direction.

• I'm saying, apply a magnetic field equal to what would be produced by a wire at a certain current. This is in theory indistinguishable from the magnetic field produced by the wire. Statement one makes sense, but I need to know why this situation is one of those effects that don't cause the cause. – Andres Salas Aug 4 '14 at 21:40
• And actually I thoughty the possibly more accurate terms were better as a summary. – Andres Salas Aug 4 '14 at 21:47

It helps here to be a bit more specific. Are you talking about an external magnetic field applied to a conducting loop?

Are you referring to a wire of current $I$ that creates a magnetic field $B$?

If you hook up a loop of wire with a potential $V$ so that it has a current $I$ and hence a magnetic field $B(I)$. You then you pull out the potential. The resistance in the circuit will slow down the charges, the current will dec. and the magnetic field will start to decrease as well.

What I think you are proposing would violate the conservation of energy. A B field being produced by a loop of wire needs an external source of energy to actually maintain that situation.

An external time-varying magnetic field would induce an emf and current in a loop of wire, but again, we're pumping energy into the system. Static magnetic fields on their own do not linearly accelerate charges (they do no work on a system) therefore they cannot be used as a power source directly.

• Yes, I'm talking about an external magnetic field (i.e. a magnet) – Andres Salas Aug 4 '14 at 21:31