Charges moving parallel to magnetic fields and direction of magnetic force

In summary, the conversation discusses several questions related to the behavior of magnetic forces. The first question is why moving a charge parallel to a magnetic field results in zero magnetic force. This can be explained by the cross product in the magnetic force equation, but it is also a consequence of relativity. In a frame of reference where the magnetic field is moving, the charge will experience a velocity-dependent force. The second question is about the different directions of magnetic forces between opposite charges of equal magnitude. This is a property of the electric fields from the charges. From the perspective of a moving frame of reference, the charges will experience a velocity-dependent force in different directions.
  • #1
nateja
33
0
I hope this isn't in the wrong forum, it's not a problem, just a conceptual question for physics 2.

I have a few questions that I cannot find answer for in my physics book or online.
1) Why does moving a charge parallel to the magnetic field result in zero magnetic force?
Is this quality based on experimental data like the magnetic force equation (something that the book mentions: says the mag-force equation was not developed theoretically, but experimentally)? F = qv x B. I know that due to the cross product, the force will just be 0, but is there another explanation besides just the math?

Why do opposite charges of equal magnitude result in different directions of magnetic forces? Is this a property of the electric fields from the oppositely charged masses (positive are isotropic and negative all point toward the negative charge)?
 
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  • #2
hi nateja! :smile:
nateja said:
1) Why does moving a charge parallel to the magnetic field result in zero magnetic force? … I know that due to the cross product, the force will just be 0, but is there another explanation besides just the math?

here's a conceptual answer …

a velocity-dependent force is an inevitable consequence of relativity (good old-fashioned Newtonian relativity, not the einsteinian sort) …

imagine a stationary line of negative charge, and a negatively-charged particle moving perpendicularly towards it

obviously, it is repelled, so it slows down, is momentarily stationary, then speeds up again, moving away, all on the same perpendicular line

now look at it from a frame of reference in which the line is moving along itself at speed v …

what happens to the moving charge? :wink:​
 
  • #3
Thanks! my professor explained this in class with the same frame of reference example. Pretty cool stuff!
 

1. What is the relationship between charges moving parallel to magnetic fields and the direction of magnetic force?

When a charged particle moves parallel to a magnetic field, it experiences a force perpendicular to both the direction of motion and the magnetic field. The direction of this force is determined by the right-hand rule, where the thumb points in the direction of the particle's velocity, the index finger points in the direction of the magnetic field, and the middle finger points in the direction of the force.

2. How does the strength of the magnetic field affect the force on a moving charge?

The strength of the magnetic field directly affects the force on a moving charge. A stronger magnetic field will result in a greater force on the particle, while a weaker magnetic field will result in a smaller force. This can be seen in the equation F = qvB sinθ, where B is the strength of the magnetic field.

3. What happens to the direction of the force on a charged particle when the magnetic field is reversed?

When the magnetic field is reversed, the direction of the force on a charged particle will also reverse. This means that the particle will experience a force in the opposite direction as before. This can be seen in the right-hand rule, where reversing the direction of the magnetic field will also reverse the direction of the force.

4. Can a charged particle ever move parallel to a magnetic field without experiencing a force?

No, a charged particle will always experience a force when moving parallel to a magnetic field. This is because the particle's velocity and the magnetic field are always perpendicular to each other, resulting in a non-zero angle in the equation F = qvB sinθ.

5. How does the mass of a charged particle affect its motion in a magnetic field?

The mass of a charged particle does not directly affect its motion in a magnetic field. However, since the force on the particle is determined by its velocity, a heavier particle with the same velocity will experience a smaller acceleration and will therefore move less in the same amount of time compared to a lighter particle. This can be seen in the equation F = ma, where a is directly proportional to m.

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