# E & M

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Quiz 4 Notes:

1. Almost everyone got this right: Current is measured in amps, voltage in volts, resistance in ohms, and power in watts.

2. You had to make sure that your magnetic field lines were closed loops that went all the way through the center of the magnet, that they formed a torus (doughnut) shape, and that they had arrows pointing from the N pole to the S pole (although if the only thing was that you didn't have arrows I allowed it).

3. Review this handout if you are confused about this question.

5. You had to use the 'left hand' rule: point the thumb of your left hand along the wire in the direction that the negative charges flow (so from negative towards positive). The curl of your fingers around your thumnb (the wire) indicates the direction of the magnetic field from N to S. So the arrow points in the direction that your fingers point.

6. About half of you got this question right. If the current is KEPT constant while resistance increases, we are asked to relate POWER to RESISTANCE, which can only be done by using both our equations (Ohm's law and Joule's law) which are:

V = I x R and

P = I x V.

If current, "I" stays the same, but R gets bigger, then V must also get bigger to keep the equation true (we can't modify one side of the equation without compensating on the other side equally, that's what the equal sign means!)

This means that Voltage goes up when the current is constant but resistance increases. This should make sense; what it means is that if I have a device that puts up more of a resistance to the flow of my charges, in order to keep the same number of charges flowing past a point in a given amount of time I will have to provide those charges with more energy to exactly overcome the increased resistance. In other words, a device with more resistance requires a higher voltage if we are to have the same amount of current.

Then we have to use the second equation, P = I x V. We have just figured out that the voltage increased... and since the current is being kept constant, it must mean that the power goes up because we can't modify the value of the right side of the equation without equally compensating on the left. So the answer is that if current is kept constant but the resistance is increased, then the power must also increase (i.e. the device uses more power, more energy per time, in order to get over the increased resistance). There WILL be a question similar to this one on the final exam.

7. Many of you gave vague answers to this one, but physics is NOT vague, it is exact, so the more exact your answer, the better. In this case, the answer was that all materials are not attracted to magnets because in order for a material to be attracted to a magnet it needs to have magnetic domains which are free to align in the presence of an external magnetic field. Only iron, nickel, and a few other common materials have this property.

8. (Extra credit): The bird is NOT fried when landing on a bare hgh-voltage power line because the charges flowing in the wire find it much much more easy to flow along the low-resistance wire than through the bird. The bird will have the same high voltage as the wire, but because the WHOLE bird has the same voltage, there is no reason for the charges in the bird to MOVE (and we know that the damaging part of electrical shocks is generally their CURRENT). If the bird touches some object of a vastly different voltage (which would be pretty much anything in this case, like another wire, a tree, or anything that is grounded), then there will be a voltage difference ACROSS the bird's body, and so charges will flow and the bird would get zapped.

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