# Help questions on electricity and magnetism

1. Jul 6, 2004

### supermenscher

help!!!! questions on electricity and magnetism

1. What is the frequency of the magnetic filed if it is know to be perpendicular to the electric field. The magnetic field is 0.00178T and the electric field is 8000V/m.

2. I just need the angle for this one. At a certain location, the Earth's magnetic field has a magnitude of 5.6E-5T and points in a direction that is 72 degrees below the horizontal. Find the magnitued of the magnetic flux through the top of a desk at this location that measures 110cmX73cm.

I know you used magnetic flux = BAcos theta....but what is the angle theta, I tried 72 degrees and 18 degrees, what angle do I use?

3. When the switch is closed in a RL circuit the current in the circuit increases from 0A to 0.22A in 0.15s. This current also has a resistance of 5.5 ohms. What is the inductance L, how long after the switch is closed does the current have a value of 0.41A, and what is the maximum current that flows in the cirucit.

I have tried this problem using every formula possible and still can't get it...can someone help me out, even to get me started? Thank you.

2. Jul 7, 2004

### wisky40

I want to know if in problem 3 you're forgetting the voltage (emf).

wisky40

3. Jul 7, 2004

### supermenscher

Yes, sorry the voltage is 9.0V

4. Jul 7, 2004

### Gza

I think your problems start from there. Stop thinking in formulas and start thinking physically. You are given the rate the current changes. Changing current causes a changing magnetic field (Hint: think faraday's law).