Lect04 - Current and Resistance Electric Charges in a...

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• Current • Resistance • Ohm’s Law • Simple Resistor Circuits – Series, Parallel September 11 Physics 104, Fal 2011 1 Electric Charges in a Circuit • When the switch is closed, E-field is established (e.m.f. due to battery) September 11 Physics 104, Fal 2011 2 Free charges every where in the conductor begin to move Electric shock wave moves down the wire to carry the signal ~ at the speed of light, 3 x 10 8 m/s! –Individual charges drift slowly (at V d ) V o + Electric Current • Whenever electric charges of like signs move, an electric current is said to exist • The current is the rate at which the charge flows through across a surface (such as the cross-section area of a wire) • The direction of the current is the direction positive charge would flow – This is known as conventional current direction • In a common conductor, such as copper, the current is due to the motion of the negatively charged electrons September 11 Physics 104, Fal 2011 3 I Δ Q Δ t Electric Current Current: Moving Charges – Symbol: I – Unit: Amp = Coulomb/second – Count number of charges which pass point/sec September 11 Physics 104, Fal 2011 4 Current & Resistance Current: Rate at which charge flows Resistance: That which impedes the free flow of charges - due to collisions in the material medium September 11 Physics 104, Fal 2011 5 Drift Velocity • Average velocity of free charges in a conductor • Characteristic of the material September 11 Physics 104, Fal 2011 6 V d ~ 0.4 mm/s - 10 12 times slower than the signal itself travels on a wire In a Cu conductor electrons drift; But, current is defined to be in opposite direction. I
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Lect04 - Current and Resistance Electric Charges in a...

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