## Homework Statement

A 102-turn, 3.8-cm-diameter coil is made of 0.59-mm diameter copper wire. A magnetic field is perpendicular to the coil. At what rate must B increase to induce a 3.4 A current in the coil? The resistivity of copper is 1.7·10-8 Ω·m.

## The Attempt at a Solution

A = (0.00059/2)^2 * pi = 2.734 x 10^-7
I = 3.4 A
I don't what to do???

Is there an answer to compare to at the back of the book?

no because its an even number :(

B=µoNI/2r, B is magnetic force, µo is magnetic constant, N is number of turns, I is current, r is radius

I think that's it but that R is unused in the problem, and it concerns me that I missed something.

is r the radius of the coil or th wire??

well i did the radius of the coil and its wrong but thanks for trying

How do you know its wrong?

its my homework problem and our homework is on lon-capa....so we have like 4 tries for each problem and tells you when you enter the answer.

oh my gosh this problem is driving me nuts I can't figure it out!!! I tried using all sorts of formulas like...
R = pL/A to get the resistance but I don't have L
So then I tried...
B = U*I*(N/L) to get L but I don't have B!!

hage567
Homework Helper
Yes you do have L because you know the diameter of the coil and how many turns there are.

Isn't the L for B=UIN/L the length of the solenoid not the length of the wire?

oh yea...ugh I hate this problem!!!

hage567
Homework Helper
Isn't the L for B=UIN/L the length of the solenoid not the length of the wire?

I was referring to the L in the resistivity equation that Bubbles was asking about.

I was referring to the L in the resistivity equation that Bubbles was asking about.

Oh I see. What's the step after finding R?

How do I find L with having the diameter and number of turns??

I was going to find E using I = E/R then use that in the equation...
E = A * B/t to get the rate

How do I find L with having the diameter and number of turns??

Well, circumference=pi*diameter, lol 102 turns = 102 circumferences

oh lol....ok so my length is 9.8696 the when I use R = pL/A is it the area of the wire or coil??

hage567
Homework Helper
I was going to find E using I = E/R then use that in the equation...
E = A * B/t to get the rate

This is what I would try.

hage567
Homework Helper
oh lol....ok so my length is 9.8696 the when I use R = pL/A is it the area of the wire or coil??

The wire, since that is what you are finding the resistance of.

oh lol....ok so my length is 9.8696 the when I use R = pL/A is it the area of the wire or coil??

Area of wire, cross section area.

hage567
Homework Helper
Isn't the L for B=UIN/L the length of the solenoid not the length of the wire?

The question did not actually state that this was a solenoid. It said it was a loop of 102 turns with a magnetic field perpendicular to it (so going through it). The magnetic field is not being produced by the coil. So I don't think the B = U*I*(N/L) equation is what you are looking for here, IMO.

and the length would be the length of the wire not the coil right

hage567
Homework Helper
Yes, that's right.

The question did not actually state that this was a solenoid. It said it was a loop of 102 turns with a magnetic field perpendicular to it (so going through it). The magnetic field is not being produced by the coil. So I don't think the B = U*I*(N/L) equation is what you are looking for here, IMO.

Oh I see. Thanks for setting me straight. I'm still learning and your opinion is very helpful to me.