# Calculate the current in the coil

1. Jun 28, 2009

### mba444

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

A circular coil of 394 turns and area 0.78 m2
is in a uniform magnetic field of 0.8 T. The
maximum torque exerted on the coil by the
field is 0.0059 N · m.
Calculate the current in the coil. Answer in
units of A

2. Relevant equations
torque= i*A*B

3. The attempt at a solution

what i did is that i solved for the current using the above equation i= torque/(A*B) but i keep getting the wrong answer .. i think because of the number of turns but i dont know were to include it

i need ur help thanx in advance

2. Jun 28, 2009

### Staff: Mentor

Torque = μ X B, where μ = nIA.

3. Jun 28, 2009

### mba444

so is it
i = torque /(number of turns *A*B)

4. Jun 28, 2009

### Staff: Mentor

Yes.

5. Jun 28, 2009

### mba444

Assume the 394 turns of wire are used to form
a single-turn coil with the same shape but
much larger area.
What is the current if the maximum torque
exerted on the coil by the field is 0.0059 N · m?

this a second part of the question but i really didnt understand the difference

6. Jun 28, 2009

### Staff: Mentor

You need to figure out the area of this larger coil. Hint: What's the length of the wire?

7. Jun 28, 2009

### mba444

i tried to figure out the length where i came up with this equation but still i couldnt figure what to put for the F

length = F/(i*B)

im kind of lost

8. Jun 28, 2009

### Staff: Mentor

Figuring out the length of the wire is a geometry problem. You had a coil of a given area and number of turns. What was the circumference of that coil?

9. Jun 28, 2009

### mba444

C= 2*pi*r

10. Jun 28, 2009

### Staff: Mentor

Good. Use the known area to find the radius.

11. Jun 28, 2009

### mba444

ok i did so know
1 loop = 2*pi*r (i just found)
394= X
so do i do the cross multiplication to solve for X which is the new area

12. Jun 28, 2009

### Staff: Mentor

Do this:
Use the area of the original coil to solve for the radius, using A = pi*r².
Use the radius to find the length of each loop, using the circumference formula.
Find the total length of the wire by multiplying the length of each loop by the number of loops.
That becomes your new circumference of your big coil. Work backwards to find, its radius then area.

13. Jun 29, 2009

### mba444

ok thanx alot i got it know .. sorry if i wasted your time

14. Jun 29, 2009

### mba444

i got for the larger area = 121084 m^2

i plugged in this eqs i = (.0059)/(394*121084*0.8)

but i got the wrong answer =(

15. Jun 29, 2009

### Staff: Mentor

That looks OK.

When used to make the giant coil there's only one turn.

16. Jun 29, 2009

### mba444

thanx again i got the right answer