1. Nov 11, 2007

### kissatlips

1. The problem statement, all variables and given/known data
Suppose you drop an aluminum rod of length 1.0 m out of a window at a
equator, where the magnetic field of the Earth is 2.0x10-5 T. The rod stays oriented
horizontally as it falls.

2. Relevant equations

What is the induced EMF between the ends of the rod after 5
seconds?

3. The attempt at a solution

NO idea !

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1. The problem statement, all variables and given/known data
A rectangular coil of 150 loops forming a closed circuit measures 0.2 m x 0.1
m. The resistance of the coil is 5 Ohm. The coil is placed between the poles of an
electromagnet, face on to the magnetic field. Suppose that when we switch the
electromagnet off, the magnitude of magnetic field decreases at the rate of 20 T/s.

2. Relevant equations

What is the induced EMF in the coil? What is the magnitude and direction of the induced
current?

3. The attempt at a solution

NO idea !

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1. The problem statement, all variables and given/known data
The windings of a large electromagnet have an inductance of 10.0 H and a
resistance of 8.00 Ohm. The electromagnet is connected to the external EMF of 230 V.

2. Relevant equations

After electromagnet is switched on, how long does it take for the current to build up to
50% of its final value? What is the final, steady value of the current that is attained after
fairly long time?

3. The attempt at a solution

NO idea !

2. Nov 11, 2007

### dynamicsolo

For future reference, be aware that you will get help a lot more quickly if you:

1) place only one problem in a thread -- this should really be three threads here

2) use a descriptive title for each problem thread, so potential readers will know what you want help with; "I need help" doesn't tell anyone very much -- most of the people posting in the Homework forum are asking for help...

Here's the picture you want to think about with this one. You appear to be covering Faraday's Law and you have perhaps had the topic "motional EMF". In any event, imagine the rod as it falls horizontal: its length makes the width of a rectangle and its ends mark the height of a growing rectangle as it falls. So you have a rectangular area through which the Earth's magnetic field passes, which means you have a magnetic flux through the rectangle. What does Faraday have to say about inducing EMF? How fast would the flux through this imaginary rectangle be changing five seconds after the rod began falling?

This time, the area through which the magnetic field passes is constant, but the field itself is changing. So how fast is the flux changing? How does the number of turns in the coil comes into this? Find the EMF first, then use the resistance to find the magnitude of the current?

How do you find the direction of the current? Have you had Lenz' Law?

Have you talked about RL circuits in your course yet? If so, and you've had equations describing the behavior of the current in such a circuit, you have the information you need to make the calculations...