# Homework Help: Faraday's law problem

1. May 3, 2007

### iamtrojan3

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

Take a loop of wire with a radius of 2.0 cm. A B field is perpendicular to the area enclosed by the wire. If the field is reduced to zero from 0.55 T in 0.25 s, what is the induced emf between the ends of the wire?

2. Relevant equations
B=Magnetic Field
A=Area
Flux= BA cos(Theta)
EMF = Change in Flux/ Change in time

3. The attempt at a solution

Trying to find Flux but since its perpendicular to the area, cos90 = 0 and this makes no sense. The answer is suppose to be 0.0028V. Any help would be great, Thanks

2. May 3, 2007

### heafnerj

Your geometry is wrong. You're using the wrong angle.

3. May 3, 2007

### iamtrojan3

ok my bad its suppose to be cos0 which is 1
with that i found Flux to be = (0.55T)(4pi) which is around 6.9T/m^2
With that 6.9(T/m^2)/0.25(Secs)= 27.6 Volts?
Like i said answer is suppose to be 0.0027V. Is there something wrong or did i just screw up the units some where?
Thanks again...

4. May 3, 2007

### heafnerj

Again the issue is geometry. Look carefully as how you're calculating the area.

5. May 3, 2007

### iamtrojan3

The area is just( pi)r^2 right?

6. May 3, 2007

### heafnerj

Yes, but be careful what you use for "r".

7. May 3, 2007

### iamtrojan3

Ohh. I see. its in cm. that should change things abit...Thanks everyone.

8. May 3, 2007

### heafnerj

Success! I have to ask this question: What about physics did you learn from doing this problem?

9. May 3, 2007

### iamtrojan3

... wat did i learn? I hate physics Tests. =D and UNITS.

10. May 3, 2007

### heafnerj

Your answer is more or less correct. My point is that there wasn't much physics in this problem.