# Electromagnetism, finding current.

1. Jun 1, 2013

### physics604

1. A 35 loop square coil 0.12 m on a side is positioned in a 0.050 T magnetic field. A 0.20 kg
mass is suspended from one side of the coil as shown in the diagram attached. How much current must pass through the coil in order for the coil to remain horizontal?

A. 2.3 A
B. 4.7 A
C. 9.3 A
D. 330 A

2. Relevant equations

τ=BAIN
F=IlB

3. The attempt at a solution

τ=BAIN
so I=τ/BAN

I know that BAN is 0.050 x 0.122 x 35 (the information is given). However, I'm having a little trouble finding τ (torque).

I know that I need to find the forces on the square coil. But since I don't know I (current), I won't be able to find the force with the equation.

Any help would be greatly appreciated.

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2. Jun 1, 2013

### Staff: Mentor

The required force is given by the mass. Which torque do you get from the mass?
The coil has to compensate this with its own torque.

3. Jun 1, 2013

### physics604

That's alright, I finally understand it now.

τ = Fl = 0.20x9.8x0.06 = 0.1176

4. Jun 2, 2013

### Staff: Mentor

Right.
It would be useful to keep the units everywhere, this gives a direct cross-check for your measurements.

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