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ch17_review

# ch17_review - their resistance would not be the same So we...

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A. Manalaysay – PHY2054, Fall 2004 CHAPTER 17 Current —whenever charge is in motion. If you were standing next to a flow of charge and counted, each second, how much current passed you, that would be a measure of the current. Obviously, the more charged that passed you in that one second, the more current there would be flowing. Current is charge per time: If charges are moving, that means something is applying a force to them. How do you apply a force to a charged particle? You place it in an electric field! So, since the charge moves, that means there is an electric field existing over some distance. Look back at how E and distance combine: . So obviously there is a potential difference involved. Typically, the amount of current that flows is directly proportional to the voltage difference: V=IR --- Ohm’s Law . The constant of proportionality is the resistance. Keep in mind this is specific to a particular object. If I were to take two un-identical chunks of copper,
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Unformatted text preview: their resistance would not be the same. So we like to talk about a quantity that is specific to the material only: This is where resistivity comes in. Resistivity is related to the resistance like so: , where is the resistivity. A common type of resistor is a normal, incandescent light bulb. What happens to a light bulb when current passes through it? It lights up and gives off heat. Compare this to the action of capacitors: capacitors store energy, while resistors give off energy. All resistors give off energy when there is current flowing through them, but some give off more than others. So, we are interested in how fast the resistor is dissipating energy, or the rate. Remember this concept from PHY2054? This is power = energy/time. The amount of power that a resistor gives off is:-- Power from a resistor (the three forms are equivalent and found by combining any one with Ohm’s Law)....
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