# How does a magnetic field "push" charges?

ghostfolk
I was under the assumption that a magnetic field acts similar to that of the normal force in mechanics; both affect the path of the object, but do no work. So now suppose that we have a rectangular circuit with the left side in an uniform magnetic field that is pointing towards the computer screen and the right side being moved at a velocity ##\vec{v}##. Since the whole circuit is moving with a velocity ##v## there is a magnetic force perpendicular to ##\vec{v}##, but still in the direction of the wire. Therefore the magnetic field pushes the charges along. If we assume the charges to be that of protons and electrons, the magnetic field will separate electrons from protons and cause current to flow. My confusion is on the very fact that the magnetic field is causing the current to flow. I simply thought the magnetic field would change the direction of current flow. How can this be?

## Answers and Replies

Mentor
I simply thought the magnetic field would change the direction of current flow.
It changes the direction of charge flow - from "purely to the right" to "to the right and up" and "to the right and down", respectively. You just don't care about the motion to the right for currents because that happens to both positive and negative charges in the same way.

UncertaintyAjay
The magnetic field can do work. Say you have a piece of steel and you hold a magnet close to it. The steel is attracted to the magnet. It moves under the influence of the force, towards the magnet.

ghostfolk
The magnetic field can do work. Say you have a piece of steel and you hold a magnet close to it. The steel is attracted to the magnet. It moves under the influence of the force, towards the magnet.
How? I thought the magnetic force does no work since it is always perpendicular to the velocity.

ghostfolk
It changes the direction of charge flow - from "purely to the right" to "to the right and up" and "to the right and down", respectively. You just don't care about the motion to the right for currents because that happens to both positive and negative charges in the same way.
All right. So in the example I stated, what would be causing the current to flow?

Mentor
So in the example I stated, what would be causing the current to flow?
The magnetic field with the moving cable plus some return line for a circular current flow.

@UncertaintyAjay: That's not the influence of magnetic fields on free charges, that is mainly the influence of spins on other spins.

UncertaintyAjay
No I know that it isn't an effect of the magnetic field on charges, but it nevertheless is a demonstration if the magnetic force doing work? Correct me if I'm wrong, please.

ghostfolk
The magnetic field with the moving cable plus some return line for a circular current flow.
If there is a current, then that means a voltage was set up by an external force. How is it that both the magnetic force and thenforce moving the circuit is needed to establish the voltage? Also, what do you mean by "return line"?

Mentor
If there is a current, then that means a voltage was set up by an external force.
Why external? The force comes from the motion of electrons in the magnetic field. The force is ##q v \times B##, so you need both motion and the magnetic field.
Also, what do you mean by "return line"?
The non-moving part of your circuit, to have a closed circuit.

ghostfolk
Why external? The force comes from the motion of electrons in the magnetic field. The force is ##q v \times B##, so you need both motion and the magnetic field.
The non-moving part of your circuit, to have a closed circuit.
So the act of moving the circuit gives the electrons momentum and that is what allows the magnetic field to move charges?

cabraham
How? I thought the magnetic force does no work since it is always perpendicular to the velocity.
Velocity of what? The mag field is perpendicular to velocity of charge carriers, but the mag Lorentz force acts in the same direction as the steel object's speed. Mag fields do work on magnetic dipoles, not on isolated charges.

Mentor
So the act of moving the circuit gives the electrons momentum and that is what allows the magnetic field to move charges?
Yes.