# Homework Help: Ampere's Law and Magnetic Fields

1. Oct 8, 2012

### zzyzz

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

The figure shows a zero-resistance rod sliding to the right on two zero-resistance rails separated by the distance L = 0.53 m. The rails are connected by a 10.9-Ω resistor, and the entire system is in a uniform magnetic field with a magnitude of 0.750 T.

(a) Find the force that must be exerted on the rod to maintain a constant current of 0.125 A in the resistor.

(b) What is the rate of energy dissipation in the resistor?

(c) What is the mechanical power delivered to the rod?

2. Relevant equations

a.) F=I x B x length
b.) P=I^2 x R
c.) P= F x velocity

3. The attempt at a solution

This sort of problem has not been covered in lecture yet and I am having difficulties with it. I found these formulas in the book and I think they apply for these situations but I am not sure. For part A of the problem I tried plugging in .125A for the current, .53m for the length, and .750T for the magnitude. Which gave me F=(.125)(.750)(.53)=.05 . This answer is off by a multiple of 10. I'm assuming my mistake is with the units of B because I have not worked with this before. Any help would be appreciated, thanks

2. Oct 8, 2012

### TSny

Hi, zzyzz. Your work looks correct to me. Be sure to include the proper unit for the force. Tesla (T) is the SI unit for B, so there is no need to do any unit conversion. The only way that I can see that your answer would be wrong is if you misread a number given in the problem.