# Homework Help: Find magnetic field at a distance from wire

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1. Oct 30, 2016

### ooohffff

1. The problem statement, all variables and given/known data
A wire with radius R= .01 m carries a current with uniform density. At a distance r1 =1.25m from the wire, the magnetic field has a magnitude of 2.55 μT. Calculate the magnitude of the magnetic field r2=.0065m from the wire.

2. Relevant equations
B= (μ0I)/ 2πr

3. The attempt at a solution
So first I did:
B1=(μ0I)/( 2πr1)
Solving for I:
I = (B12πr1)/μ0
Plugging it into the B2 equation:
B2=(μ0B12πr1)/(2πr2μ0)
= (B1r1)/r2
=4.9 x 10^-4 T

I'm not sure if I did this right since other people in my class got something different. Can someone corroborate this? Thanks.

2. Oct 30, 2016

### TSny

Hello. Keep in mind that the equation B= (μ0I)/ (2πr) is only valid for points outside the wire.

3. Oct 30, 2016

### ooohffff

Oh I see, I would have to use the equation for the magnetic field inside the wire for B2:

B2= (μ0Ir2)/ (2πR2)

Plugging I into this equation:
B2= (μ0B12πr1r2)/ (2πR2μ0)
=(B1r1r2)/ R2

4. Oct 30, 2016

### TSny

That looks correct to me.