Magnetic field due to a moving charge.

[eem2am]

Hello,

There are two men.....

One is sitting on a moving wagon and is holding a Coulomb of charge in his lap.

The other is stood by the roadside, watching the wagon go past him.

Both men have Magnetic field detectors with them.

The man on the wagon detects no magnetic field with his detector.......sounds about right -since to him, the charge is NOT moving.

The man stood by the roadside DOES detect a magnetic field.....again, sounds about right because to him, the charge IS moving.

They CANNOT both be right.

Either the magnetic field is there or it is not.

So does any reader know which man's magnetic field detector is lying?

 

[subharpe]

Magnetic Field measurement is totally a relative mesurement. So both are right from their own reference frame.

A similar kind of problem has been discussed in Griffith Electrodynamics, in introduction of special theory of relativity. This can be referred to.

 

[eem2am]

thankyou subharpe but surely this cannot be so.....a magnetic field has energy associated with it and so is either there or it is not.

It cannot be there for one and not there for another


A magnetic field could do work like turn the needle of a small compass......

 

[subharpe]

Yes, there is definitely magnetic field. But measuring it is different from it's existance. It depends on the reference frame from where the observer is trying to mesure, isn't it?

For example, you have something like a kinetic energy measuring meter and you run at the same speed the sample. What do you measure? 0. But it has Kinetic energy.