# EMF problem - Current in Coil

1. Apr 1, 2008

### ttiger2k7

[SOLVED] EMF problem - Current in Coil

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

A small square coil is located inside an ideal solenoid at the center with its plane oriented perpendicular to the axis of the solenoid. The resistance of this coil is 2.00 $$\Omega$$ and each side is 4.00 cm long. The solenoid has 125 windings per centimeter of length. If the current in the solenoid is increasing at a constant rate of 1.50 A/s, the current in the square coil is:

a) steady at 18.8 $$\mu A$$
b) initially equal at 18.8 $$\mu A$$ but is increasing
c) increasing at 1.50 A/s
d) decreasing at 1.50 A/s
e) zero

2. Relevant equations
$$\epsilon=\frac{d\Phi_{B}}{dt}$$

$$\Phi_{B}=BA$$

3. The attempt at a solution

First, I plugged in what I know for magnetic flux:

$$\Phi_{B}=BA=B(.04 m^{2})$$

Then I used that information to plug into the induced emf formula:

And since

$$B=\frac{N}{L}*i*\mu_{0}$$

Then

$$\epsilon=\left|.04 m^{2}*\frac{N}{L}*\mu_{0}*\frac{di}{dt}\right|$$

where
$$\frac{di}{dt}$$ is 1.5 A/S
N = (125 *.04 m) = 5

**

My question is, am I going about this the right way? And if so, How do I find L and how can I use that to eventually get to the induced current?

2. Apr 1, 2008

### rock.freak667

Well $\frac{N}{L}$ is the number of turns per unit length. Which is given in the question. (Just convert the units)

When you get the induced emf, use V=IR to find the induced current...

3. Apr 1, 2008

### ttiger2k7

Okay. (I thought L was inductance). I got a value, but how do I know if its increasing/decreasing, or steady?

Last edited: Apr 1, 2008