How does a magnetic field "push" charges?

In summary: If there were no current, then the magnetic force would just push the charges around without doing any work.
  • #1
ghostfolk
59
1
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?
 
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  • #2
ghostfolk said:
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.
 
  • #3
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.
 
  • #4
UncertaintyAjay said:
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.
 
  • #5
mfb said:
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?
 
  • #6
ghostfolk said:
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.
 
  • #7
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.
 
  • #8
mfb said:
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"?
 
  • #9
ghostfolk said:
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.
ghostfolk said:
Also, what do you mean by "return line"?
The non-moving part of your circuit, to have a closed circuit.
 
  • #10
mfb said:
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?
 
  • #11
ghostfolk said:
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.
 
  • #12
ghostfolk said:
So the act of moving the circuit gives the electrons momentum and that is what allows the magnetic field to move charges?
Yes.
 

1. How does a magnetic field "push" charges?

The "pushing" effect of a magnetic field on charges is due to the Lorentz force, which is a combination of the electric and magnetic forces acting on a charged particle. When a charged particle moves through a magnetic field, it experiences a force perpendicular to both its velocity and the direction of the magnetic field. This force causes the particle to move in a circular path, with the direction of the circle determined by the polarity of the magnetic field.

2. What is the relationship between a magnetic field and the velocity of charges?

The strength of the magnetic force on a charged particle is directly proportional to its velocity. This means that as the velocity of a charged particle increases, so does the magnitude of the force it experiences in a magnetic field. This is why charged particles moving at high speeds, such as in particle accelerators, experience a much stronger magnetic force than particles moving at lower speeds.

3. Why do only charged particles experience a magnetic force?

Charged particles experience a magnetic force because they have an electric charge which interacts with the magnetic field. The strength of the magnetic force is dependent on the charge of the particle and its velocity. Neutral particles, such as neutrons, do not experience a magnetic force because they have no net charge.

4. Can a magnetic field change the direction of a charged particle's motion?

Yes, a magnetic field can change the direction of a charged particle's motion. As mentioned before, the Lorentz force causes charged particles to move in a circular path when they enter a magnetic field. This means that the direction of a charged particle's motion can be altered by changing the direction or strength of the magnetic field. This effect is utilized in devices such as particle accelerators and particle detectors.

5. How does the strength of a magnetic field affect the force on charges?

The strength of a magnetic field is directly proportional to the force on a charged particle. This means that as the strength of the magnetic field increases, so does the force experienced by the charged particle. The magnetic field strength is often measured in units of Tesla (T), where 1 T is equivalent to 1 Newton (N) of force acting on a 1-meter length of wire carrying 1 ampere of current. Therefore, a higher strength magnetic field will exert a stronger force on charged particles than a weaker magnetic field.

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