How does a magnetic field "push" charges?

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Discussion Overview

The discussion revolves around the interaction between magnetic fields and electric charges, particularly in the context of a moving circuit within a magnetic field. Participants explore how magnetic fields influence charge flow, the nature of work done by magnetic forces, and the establishment of current in circuits.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that a magnetic field acts similarly to a normal force in mechanics, affecting the path of charges without doing work.
  • Others argue that while the magnetic field changes the direction of charge flow, it does not do work on the charges since the magnetic force is always perpendicular to their velocity.
  • A participant suggests that the magnetic field can do work, citing the attraction of steel to a magnet as an example.
  • There is a discussion about the role of external forces in establishing voltage and current in a circuit, with some asserting that both the magnetic field and the motion of the circuit are necessary.
  • One participant questions the concept of a "return line" in the context of a closed circuit and its relevance to the discussion.
  • Another participant clarifies that the force acting on charges is a result of their motion in the magnetic field, described by the equation ##q v \times B##.
  • Some participants express confusion about the nature of work done by magnetic fields, particularly in relation to magnetic dipoles versus isolated charges.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether magnetic fields can do work on charges or how exactly they influence current flow. Multiple competing views remain regarding the mechanics of charge movement in magnetic fields and the role of external forces in establishing voltage.

Contextual Notes

There are unresolved questions about the definitions of work in the context of magnetic fields, the assumptions underlying the discussion of charge flow, and the specific conditions under which magnetic forces operate.

ghostfolk
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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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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.
 
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.
 
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.
 
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?
 
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.
 
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.
 
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"?
 
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.
 

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