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# chapter17 - Chapter 17 Current and Resistance Electric...

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Chapter 17 Current and Resistance

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Electric Current The 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 (A) 1 A = 1 C/s av Q I t
Instantaneous Current 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 00 lim lim t av t Q II t

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Electric Current, cont 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 It is common to refer to a moving charge as a mobile charge carrier A charge carrier can be positive or negative
Current and Drift Speed Charged particles move through a conductor of cross- sectional area A n is the number of charge carriers per unit volume n A Δx is the total number of charge carriers

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Current and Drift Speed, cont The total charge is the number of carriers times the charge per carrier, q ΔQ = (n A Δx) q The drift speed, v d , is the speed at which the carriers move v d = Δx/ Δt Rewritten: ΔQ = (n A v d Δt) q Finally, current, I = ΔQ/Δt = nqv d A
Current and Drift Speed, final If the conductor is isolated, the electrons undergo random motion When an electric field is set up in the conductor, it creates an electric force on the electrons and hence a current

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Charge Carrier Motion in a Conductor The zig-zag black line 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
Electrons in a Circuit Assume you close a switch to turn on a light The electrons do not travel from the switch to the bulb The electrons already in the bulb move in response to the electric field set up in the completed circuit A battery in a circuit supplies energy (not charges) to the circuit

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Electrons in a Circuit, cont The drift speed is much smaller than the average speed between collisions When a circuit is completed, the electric field travels with a speed close to the speed of light Although the drift speed is on the order of 10 -4 m/s, the effect of the electric field is felt on the order of 10 8 m/s
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chapter17 - Chapter 17 Current and Resistance Electric...

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