# Homework Help: Electromagnetism, current in coil

1. Jun 2, 2013

### physics604

1. A coil of 25 turns of wire is suspended by a thread. When a current flows through the coil, the tension in the thread is reduced by 4.0 x 10-2 N. What are the magnitude and direction of the current? {magnitude of current; direction of current}

a) 4.1 A; clockwise
b) 0.16 A; clockwise
c) 0.16 A; counter-clockwise
d) 4.1 A; counter-clockwise

2. Relevant equations

F=IlB
B= $\frac{\mu NI}{l}$

3. The attempt at a solution

B=$\mu$NI/l
F= IlB = Il($\mu$NI/l) = I2$\mu$N
I = $\sqrt{ \frac{F}{\mu N} }$

so I = √4.0x10-2 / 4∏x10-7 x25 = 35.48A

My answer doesn't match with any of theirs.

Also, how would I find the direction of current? I don't know the direction of B (magnetic field).

Any help would be greatly appreciated.

2. Jun 3, 2013

### Simon Bridge

Show your reasoning - why would you expect that a current through the wire would decrease the tension in the thread?

3. Jun 3, 2013

### rude man

Just posting to get on the thread. Seems like there's a lot of info missing. Seems like this problem gives a whole new meaning to "pulling yourself up by the bootstraps"!

4. Jun 3, 2013

### physics604

Is there uniform circular motion involved?

5. Jun 4, 2013

### Simon Bridge

You are the one with the whole problem statement in front of you, you tell me: is their uniform circular motion involved?