# Inertial charges

1. Jan 1, 2009

### Charlie G

I was wondering whether or not a charge must be accelerating to produce a magnetic field. If the charge is moving at a constant speed does it create a magnetic field? I f I took a van de graff generator onto an airplane moving at a constant rate, could I measure a magnetic force? Or are magnetic forces relative? Do I in the plane see an electric force while a person on the ground sees a magnetic?

2. Jan 1, 2009

Staff Emeritus
A charge at constant speed generates a magnetic field.

In your plane you would see an electric field and the people on the ground would see both an electric and a magnetic field.

3. Jan 1, 2009

### olgranpappy

Yeah. For example, I you are on a plane holding a point charge on your lap, you only see an electrostatic field. But, a person on the ground sees an electric and a magnetic field; the magnitude of the magnetic field is about v/c times the magnitude of the electric field (in gaussian units where electric and magnetic field have same units) and roughly "curls around" the charge a la the right-hand-rule.

4. Jan 1, 2009

### Charlie G

thx for the replies, of all the relative things, time, space and so on, this relative electromagnetsim seems to me the strangest.

5. Jan 1, 2009

### olgranpappy

It is pretty interesting, isn't it? This sort of thing is what got me hooked on physics in the first place. I.e., the impressive general principles (like relativity) and the neat effects that look almost magical (like moving magnets inducing currents, and so on). Cheers.

6. Jan 1, 2009

### tiny-tim

Welcome to PF!

Hi CharlieG! Welcome to PF!
Perhaps it makes more sense if you look at it from the point of view of the observer, rather than of the charge.

In other words, don't say "a stationary charge produces a pure electic field",

but "a charge produces an electromagnetic field which an observer stationary relative to the charge sees as a pure electic field"

oh â€¦ and happy new year!

7. Jan 2, 2009

### Charlie G

Yea, Ive really been trying to get my head around physics because for me it seems that when you understand something you appreciate a whole lot more. Ever since reading my first relativity book I cant help but imagine everything I see from another perspective. I think the best part is how when you learn the simple the simple principles behind small things, all the bigger ones fall right into place.