# Find the velocity and strength of magnetic field from the given info

1. Apr 28, 2013

### rocapp

1. The problem statement, all variables and given/known data

A 7.00-cm-long wire is pulled along a U-shaped conducting rail in a perpendicular magnetic field. The total resistance of the wire and rail is 0.320Ω . Pulling the wire with a force of 1.30N causes 4.50W of power to be dissipated in the circuit.

What is the speed of the wire when pulled with 1.30N ?

What is the strength of the magnetic field?

2. Relevant equations

ε=vBL
P=Fv
I=ε/R

I=sqrt(P/R)

3. The attempt at a solution

I=sqrt(P/R)

I=sqrt(4.5/0.32)=3.75 amps

I=ε/R

3.75 amps = ε/0.32

ε=1.2 V

ε=vBL

(1.2 V)=v(???)(0.07 m)

I'm not sure how to find the magnetic field, which I obviously need for both parts. Please help! Thanks!

2. Apr 28, 2013

### TSny

You wrote P = Fv as a relevant equation. Maybe it really is relevant.

3. Apr 28, 2013

### rocapp

Thanks!

But for the second part, I'm not sure of what to do.

F=qvB
1.3=q(3.46)B
But where does q come from?

4. Apr 28, 2013

### TSny

The equation F = qvB is not relevant here. Now that you have a value for v, why not go back to your earlier equation ε = vBL?

5. Apr 28, 2013

### rocapp

Doh! Thanks a bunch!