chapter17_updated

chapter17_updated - 1/30/2009 Current and Resistance When...

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1/30/2009 1 Current and Resistance When charges move , they produce electrical current How do we define/measure/calculate the current? The amount of charges that are moving How fast they move
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1/30/2009 2 The (average) current is the rate at which the charge flows through a surface Look at the charges flowing perpendicularly through a surface of area A The SI unit of current is Ampere (Amp, A); 1 A = 1 C/s t Q I av We have about 200 students in this class Suppose each students carries 1 C charge After the class, it takes 100 seconds for all students to exit the classroom What is the average current flowing out of the door during this 100 sec interval? t Q I av A 2 100 200
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1/30/2009 3 The current passing through the filament of a light bulb is 2 A. What is the amount of charge that passes in 4 sec? (1) 0.5 C (2) 2 C (3) 4 C (4) 8 C 1 A = 1 C/s t Q I av The direction of the current is defined as the direction positive charge would flow (for historical reasons) In common conductors , such as copper, aluminum, etc, the current is due to the motion of the negatively charged electrons When electrons move, they produce a current in the opposite direction What happens if both positive and negative charges move in a device? - - - - I + + + + I
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1/30/2009 4 The magnitude of the current (arbitrary units) (a) 3 + 2=5 (b) 3 (c) 2+2=4 (d) 2 Note that in this problem, the negative and the positive charges move in opposite directions , therefore their currents add up. If they move in the same direction , the currents cancel (partially or completely). Which one on the right has the same current as that in the above? (Assume the speeds of all charges are the same) (1) A (2) B (3) C + + + + + + + + + + + - + + + + - - + + + + - - A B C
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1/30/2009 5 When students exit the classroom, some may move faster, some may move slower, the rate of flow varies with time How do we define the current at a given time , say t ? The instantaneous current is the limit of the average current as the time interval goes to zero: If there is a steady current, the average and instantaneous currents will be the same t Q I I t av t inst 0 0 lim lim There are some 10 22 /cm 3 electrons in a typical metal (copper, aluminum, etc) Electrons in a conductor move like crazy: They frequently bump into atoms and change their direction of motion abruptly In the absence of an external electric field, the average speed of electrons, called drift speed , , is zero; therefore no net current in the absence of an external field light) of speed the 1% about ( / 10 ~ 6 s m v d v
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1/30/2009 6 When an external field is applied to a conductor, there is a non vanishing average speed (drift speed, ) of electrons The net motion of electrons is opposite the direction of the electric field The direction of electric field is the same as the direction of current
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This note was uploaded on 03/30/2009 for the course PHY 102 taught by Professor Luo during the Spring '08 term at SUNY Buffalo.

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chapter17_updated - 1/30/2009 Current and Resistance When...

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