chapter27

chapter27 - Chapter 27 Current Resistance And Resistor...

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Chapter 27 Current Resistance  And  Resistor
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Electric Current, the definition Assume charges are moving perpendicular to a surface of area A If Δ Q is the amount of charge that passes through A in time t , then the average current is t Q I avg = or dt dQ I = + + + + + + I A Anyone notices a difference of this figure compared with the one in the book? If yes, and plain why I draw it this way. Correct answer = 1 bonus point How many of you preview, review, do homework within 2 days of class, 2 days toward due date?
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Electric Current, definition and unit Electric current is the rate of flow of charge through some region of space The SI unit of current is the ampere (A) 1 A = 1 C / s And ampere is a base unit. The unit for time, second, is also a base unit. The unit for charge is then defined as 1C=1A/1s The symbol for electric current is I
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Direction of Current The charges passing through the area could be positive or negative or both. This area may or may not be an actual physical surface and the charges can flow in a conductor or in vacuum. It is conventional to assign to the current the same direction as the flow of positive charges The direction of current flow is opposite the direction of the flow of electrons It is common to refer to any moving charge as a charge carrier
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Current in a conductor and the drift speed Charged particles move through a conductor under the drive of an electric field inside. Assume a cross-sectional area A that is perpendicular to and the number of charge carriers per unit volume n. Then nA Δ x is the total number of charge carriers Wait! There should be no electric field inside a conductor. Explanation please! E d V
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Current linearly proportional to drift speed The total charge is the number of carriers times the charge per carrier, q Δ Q = ( nA Δ x ) q The drift speed, v d , is the speed at which the carriers move v d = Δ x / Δ t and x = v d t Rewritten: Δ Q = ( nAv d Δ t ) q Finally, current, I ave = Δ Q / Δ t = nqv d A This is to say that current is linearly proportional to the drift speed v d
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A typical value of the drift speed Assume a copper wire, with one free electron per atom contributed to the current The drift speed for a 12-gauge copper wire carrying a current of 10.0 A is 2.23 x 10 -4 m/s This is a typical order of magnitude for drift speeds This is very slow, how come when a switch is close, the light comes on from a light bulb ~ 10m away? It should take ~10/ 2.23 x 10 -4 sec which is about 12 hours. What is wrong?
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Motion of the charge carrier, the electrons, in a conductor The actual charge carrier in conductor is the electron The zigzag black lines represents the motion of a charge carrier in a conductor The net drift speed is small The sharp changes in direction are due to collisions The net motion of electrons is opposite the direction of the electric field
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This note was uploaded on 05/06/2008 for the course PHYS 1304 taught by Professor Ye during the Spring '08 term at SMU.

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chapter27 - Chapter 27 Current Resistance And Resistor...

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