# I Magnetic field of a charge

1. Mar 9, 2016

### kent davidge

Let's imagine two frames of reference, one of wich is a imaginary room containing two charges. Now let's accelerate this room from 0 to v and keep that speed during some time interval t and then we "pause" the situation again. An observer inside the box would measure the electric force acting on the charges and the observer in the other frame would measure a greater electric force. But the final positions of the two charges after time t should be the same as seen by all observers. So, there must be a force acting on the opposite direction as seen by the observer outside the room, that decreases the net force, and that force is what we call magnetic force.

Is it correct to think so?

2. Mar 9, 2016

### davenn

Assuming you mean the 2 charges are just floating in the box ....
(ie not in physical contact with the inner walls of the box)

now it doesn't have to be charges ... it could be 2 of anything
what do you think will happen to those 2 objects when you accelerate the box ?

Dave

3. Mar 9, 2016

### kent davidge

yes
Okay. I just wanted to think of a box as being in one frame of reference, but let's forget it. Just think that the 2 charges start moving.

4. Mar 9, 2016

### davenn

I think you missed what I was trying to hint to you
They will only start moving relative to each other and the walls of the box if there is a mutual attraction or repulsion between them
That's even if the box isn't moving.

5. Mar 9, 2016