zhang17-1

# zhang17-1 - Current and Resistance When charges move they...

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1/24/2010 1 Current and Resistance When charges move , they produce electrical “ current Direction of electrical current is defined as the direction positive charge would flow (for historical reasons) Motion of negative charges (e.g., electrons) produces current in the opposite direction What if we have both positive and negative charges move? I + + + + I - - - -

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1/24/2010 2 The magnitude of current depends - The amount of charges that are moving - How fast they move The (average) current is the rate at which the charge flows through a cross-sectional area The SI unit: Ampere (Amp, A); 1 A = 1 C/s t Q I av We have about 200 students in this class Suppose each students carries 0.5 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 s 100 C 5 . 0 200
3 Current in a wire may change 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 very fast They frequently bump into atoms and change their directions of motion abruptly In the absence of an external electric field, the average speed of electrons, called drift speed , , is zero; therefore no net

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zhang17-1 - Current and Resistance When charges move they...

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