# Magnet Fields dilemma

1. Mar 7, 2009

### Mentallic

A moving electric charge (moving relative to another object) will create a magnetic field around it. So if this charge moves passed a permanent magnet, their fields will interact and cause a force (basic principle of the motor effect).
However, if the electric charge is standing still in relation to the magnet, there wont be any force because the charge isn't emitting a magnetic field.

But if a 2nd magnet moves passed the still magnet and charge, neglecting the fact that the 2 magnets will affect each other, the charge and 2nd magnet now are moving relative to each other so the charge emits a magnetic field, causing a force between them. This means the magnetic field must have been there the whole time. How is it possible that the magnetic field can always be there, but not interact with the 1st magnet?

2. Mar 7, 2009

### Phrak

There will be a force. The magnet is now moving.

The magnetic field is not there the whole time. It's somewhat artificial. It's values depend on the velocity of those measuring it.

3. Mar 7, 2009

### Mentallic

Yes, if they are moving relative to each other. Else, there is no force acting on the charge or magnet.

ok, lets say the magnetic field isn't always there. When the 2nd magnet begins to move, a force will be acted upon the charge and the magnet since they're moving relative to each other. At this point, it is obvious the magnetic field is emitted from the electric charge, because there needs to be 2 fields to interact to create the force. But the 1st magnet would also be in this magnetic field emitted from the charge, but isn't affected...?

4. Mar 7, 2009

### Staff: Mentor

No, the charge does not emit a magnetic field if the charge is stationary, the magnet emits an electric field when it moves. Remember Faraday's law, a changing magnetic field induces an electric field. It is this induced electric field which exerts the force on the stationary charge.

As Phrak mentioned, the distinction between electric and magnetic field is somewhat arbitrary and depends on the coordinate system used to measure them.

5. Mar 7, 2009

### Mentallic

Ahh. this was never mentioned to us in class. Only that moving charges create magnetic fields, but not the other way around.

Ok this clears up my problem then. But what if the situation were to change such that there is now a 2nd charge moving constantly with the 2nd magnet? The 1st charge and 1st magnet are stationary relative to each other, so no force is being distributed upon them, but now they are both moving relative to the 2nd charge and 2nd magnet. There will be an interaction between the 1st magnet and 2nd charge, but not the 2nd magnet and 2nd charge.

I don't completely understand this. Electric and magnetic fields are only distinguishable between each other by coordinate measuring systems?

6. Mar 7, 2009

### Staff: Mentor

OK, the 2nd magnet is moving, so there is both a magnetic field and an induced electric field. The electric field obviously exerts a force on the 2nd charge, but since the 2nd charge is moving the magnetic field also exerts a force. It so happens that those two forces cancel each other out so that there is no net force from the 2nd magnet on the 2nd charge.

This is a fairly advanced topic of special relativity and I don't want to confuse you. Don't worry about it too much for now. Just be aware that an electric and a magnetic field are not really separate things, they are two sides of the same coin as it were.

7. Mar 7, 2009

### Mentallic

Ahha! This explains it all
Thanks DaleSpam, I very much appreciate your help.

I'll take your word for it. Hopefully I'll be able to handle a more in-depth analysis on the topic in due time.

8. Mar 8, 2009

### Phrak

It's not too far a reach to see should be true. Take a simple beam of electrons. You know there's a magnetic field surrounding the wire from your studies. But if you could move along at the same speed as the electrons, they are now a stationary line of charge.

It might be good to remind you at this point that fields don't move, but the value of the field at any given point can change. This is by definition. Otherwise you might think that perhaps a moving magnetic field may somehow act differently than a stationary one.

9. Mar 8, 2009

### Staff: Mentor

You are very welcome! I am sure you will be able to easily grasp it when you want to, but it seemed like you were just learning electromagnetism, so I didn't want to stray from the main topic. Please feel free to ask again when you get to that point and have questions.