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# Magnetic Field in a Toroid

1. ### sfdevil

7
1. The problem statement, all variables and given/known data

Use Ampere’s law to calculate the magnetic field inside of a toroid with 850 turns. The inner radius of the rectangular coils is 3.00 cm, the outer radius is 7.00 cm.

2. Relevant equations

ampere's law: path integral(B*ds) over a closed path is equal to the enclosed current times mu_0

3. The attempt at a solution

by ampere's law for toroids, I get B= mu_0*N*I/(2*pi*R)
Im confused as to what the inner and outer radius have to do with this problem,
hence the cry for help :(
anyone ?

2. ### Kurdt

4,941
Staff Emeritus
The two radii are there so you can determine the axis of toroidal coils. This is chosen as it simplifies Amperes circuital law to the equation you have stated.

3. ### sfdevil

7
so basically these numbers dont really matter, right ?
since Im finding the magnetic field inside the toroid, its just gonna be r as an arbitrary length from the center to some point inside (between 3 and 7 cms) ....correct ?

if so, then the final solution is mu_0*850*I/(2*pi*R), right ?

4. ### Kurdt

4,941
Staff Emeritus
5. ### sfdevil

7
so from that explanation, I got that since inner radius is 3 and outer is 7, then radius of the rectangular cross-section is 4/2=2
so r=3+2=5

thats about as much as I could get from that page, please let me know If Im still missing something...

6. ### Kurdt

4,941
Staff Emeritus
That should be ok.