# Magnetic field in conductor

1. Mar 17, 2009

### cataldo

I have two conductors in no magnetic material such as copper. One has diameter 20mm and other has diameter 50mm and both through by same current about 100A. Where do i have the maxi magnetic field on surface of conductor...I tinks on conductor with diameter 20mm..Is exactly ???

2. Mar 17, 2009

### clem

For the same current, the magnetic field at the surface is proportional to 1/R.

3. Mar 18, 2009

### cataldo

Tanks cleam.. Excuse me for the stupid question but i had one discussion with my friend which sayed that the magnetic field at the surface was higher on conductor with diemater 50mm ...
Bye

4. Mar 18, 2009

### Bob S

If the two conductors are isolated from one another, the higher field is at the surface of the 20 mm conductor. The B field outside an infinite straight conductor of radius R is

B = 2 x 10-7 I/a tesla (a>R)

where a is the distance from the center of the conductor. The field inside the wire is

B = 2 x 10-7 r I/R2 tesla

where r is a point inside a conductor of radius R (r<=R)

So the field scales as r inside the conductor, and 1/a outside. If the current is an AC current, Eddy currents force the currents toward the surface of the conductor.