# Homework Help: Find magnetic dipole moment of coiled wire

1. Aug 9, 2015

### Jimbob999

1. The problem statement, all variables and given/known data
A 45-m long wire is coiled so that it makes a coil containing 100 circular loops, one on top of the other. If the wire carries a current of 13 A, what is the magnetic dipole moment of the coil?

21 A·m2

6.7 A·m2

3.3 A·m2

2.6 A·m2

1.2 A·m2

2. Relevant equations
B(z) = u0/2pi x u/z^3
Where u0 is the permeability constant and u is the magnetic dipole moment

3. The attempt at a solution

I can get as far as B(z) = (2 x 10^7 u)/z^3
I know I need to find u.
It doesn't seem to mention where exactly z is, thus can't work out magnetic field. Do you think there is enough information provided in this question to solve it?

Thanks.

2. Aug 9, 2015

### Jimbob999

Would this be a solenoid and thus use equation B = u0 x i x n ?

3. Aug 9, 2015

### Qwertywerty

I'm curious - how would finding B help ?

Use simply the formula - M = n*i*A , where n is no of loops , and i the current passing through the loops , and A the area of each loop .

Also , here's a hint : M due to n circular loops would be the same as due to one large , single loop of current - carrying wire . This might help .

4. Aug 9, 2015

### Jimbob999

Oh I couldn't find that equation.

But solving for this equation gives me 2.07 x 10^6.
I guess I need to convert this to A/m^2, but I am not sure the conversion and couldn't find it on Google anywhere?

5. Aug 10, 2015

### Jimbob999

Anyone got any idea?

6. Aug 10, 2015

### Qwertywerty

Firstly ,
Unfortunately , I have made a mistake here . Discard this .
Secondly ,
Try the working again now . You will get the answer . Also , I believe you mean Am2 .

Hope this helps .

7. Aug 10, 2015

### Jimbob999

Ok, let me show you my working.

n is 100
I is 13
A = Pi R^2, which is 506.3. However, I am inclined to take this as one big loop. Thus 45m divided 100 loops.
A = 0.159?

M = 206.7
or M = 2.07 if take n as 1.

So don't think any of my answers are right?

8. Aug 10, 2015

### Qwertywerty

I said I made a mistake - you shouldn't do this .

Find area of each individual loop , and use the formula M = n*i*A .

Hope this helps .

9. Aug 10, 2015

### Jimbob999

Thanks qwerty.