# Electric charges moving and magnetic field

1. Mar 6, 2007

### haiha

I know that a current creates a magnetic field and so do moving electric charges. My question is how fast a single charge moves so that it will create a magnetic field. Should its velocity be around c to make that?

Thks

2. Mar 6, 2007

### Meir Achuz

A moving electron creates a magnetic field B~vXE/c.

3. Mar 6, 2007

### haiha

Oh, cool, I can calculate the magnetic field of a current by this formula as well. Thanks.

4. Mar 6, 2007

### arunma

This is the magnetic field generated by a moving electric charge.

$$\vec{B} =\frac{q \vec{v} \times \vec{r}}{r^3}$$

5. Mar 7, 2007

### griefchan

I think that no matter how much of the charge are given, there should have a magnetic made. But whether or not it is easy to found .
is it ?

6. Mar 7, 2007

### kendr_pind

to answer haiha's question, typically in an electrical ckt, the electrons dont move around with speed of light..infact surprisingly it moves with a drift velocity. this is typically around 1cm/sec ! yes, electrons' drift velocity is very slow...it is the velocity of the signal that moves with speeds comparable to light( well it depends upon the dielectric co-effs etc). "signal" here roughly means the electro-magnetic field through space...

7. Mar 7, 2007

### Meir Achuz

The magnetic field of a single moving charge is a tricky calculation using Special Relativity, especially if the charge is accelerating.