# Homework Help: Finding the diameter of a coil.

1. Oct 10, 2008

### cds5615

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

You have a 1.2-m-long copper wire. You want to make an N-turn current loop that generates a 0.60 mT magnetic field at the center when the current is 1.5 A. You must use the entire wire.

2. Relevant equations

What will be the diameter of your coil?

3. The attempt at a solution

I tried applying this formula d = sqrt ( (mu*L*I/pi*B)) and keep getting a really small number =0.000021 m

What am I missing here??

Last edited: Oct 10, 2008
2. Oct 10, 2008

### Rake-MC

What was the original formula? Name of it?

3. Oct 10, 2008

### cds5615

I actually just got it from a classmate, him and his friend were both able to get to the answer from that equation.

I tried using B= mu*(N/L)*I but you dont end up with a numerical value when solving for d. There must be a way to find the answer with another method...

4. Oct 10, 2008

### Rake-MC

Ahhh I see, well I'll s how you how to do it. Use Ampere's law. $$BL = IN\mu_0$$

It will yield the correct answer, and I can see whoever wrote the question planned it very well.

5. Oct 10, 2008

### cds5615

But I am solving for diameter. How can I apply that equation?

6. Oct 10, 2008

### Rake-MC

Well.. You know everything except for N (number of turns).
Once you've found how many turns you have divide the length of the coil by N. That will clearly give you the circumference of each turn (provided they are uniform).