Lecture 9 - Current • Definition I=dQ/dt through some...

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Unformatted text preview: Current • Definition: I=dQ/dt through some open surface. Units = C/s=Amperes=A. • I does not have a direction, but by convention we take positive I to correspond to the direction that positive charge carriers would move. • Of course, charge carriers are very often electrons; and so electrons move opposite the direction of I. This confusion can be blamed on Benjamin Franklin! Ooops! Current Density J • The current density J provides a more microscopic description of current flow. The relation of J to the actual moving charge is J =nq v , where n is the number density of charge carriers, q is the charge of each carrier, and v is the velocity. The units of J are A/m 2 . • We can derive a connection between J and an applied electric field E that is called Ohm’s Law. • The ideas is that we write F =m a =md v /dt . • F will have a contribution +q E from the electric field and a second contribution that can be written as -m v / τ that describes the collisions of charge carriers with the underlying molecules in the material. The number τ is called the mean collision time. Ohm’s Law– micro-version • What happens then is something we encountered in EP1 in discussing drag forces. E causes v to increase, but...
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This note was uploaded on 05/11/2010 for the course PHY 214 taught by Professor Timothybolton during the Spring '10 term at Kansas State University.

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Lecture 9 - Current • Definition I=dQ/dt through some...

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