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chapter27notes - Chapter 27 Current and Resistance Electric...

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Chapter 27 Current and Resistance
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Electric Current square6 Electric current is the rate of flow of charge through some region of space square6 The SI unit of current is the ampere (A) square6 1 A = 1 C / s square6 The symbol for electric current is I
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Average Electric Current square6 Assume charges are moving perpendicular to a surface of area A square6 If Δ Q is the amount of charge that passes through A in time Δ t , then the average current is av Q t Δ = Δ I
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Instantaneous Electric Current square6 If the rate at which the charge flows varies with time, the instantaneous current, I , can be found I dQ dt =
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Direction of Current square6 The charges passing through the area could be positive or negative or both square6 It is conventional to assign to the current the same direction as the flow of positive charges square6 The direction of current flow is opposite the direction of the flow of electrons square6 It is common to refer to any moving charge as a charge carrier
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Current and Drift Speed square6 Charged particles move through a conductor of cross- sectional area A square6 n is the number of charge carriers per unit volume square6 nA Δ x is the total number of charge carriers
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Current and Drift Speed, cont square6 The total charge is the number of carriers times the charge per carrier, q square6 Δ Q = ( nA Δ x ) q square6 The drift speed, v d , is the speed at which the carriers move square6 v d = Δ x / Δ t square6 Rewritten: Δ Q = ( nAv d Δ t ) q square6 Finally, current, I av = Δ Q t = nqv d A
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Charge Carrier Motion in a Conductor square6 The zigzag black line represents the motion of a charge carrier in a conductor square6 The net drift speed is small square6 The sharp changes in direction are due to collisions square6 The net motion of electrons is opposite the direction of the electric field
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Motion of Charge Carriers, cont. square6 In spite of all the collisions, the charge carriers slowly move along the conductor with a drift velocity, v d square6 Changes in the electric field that drives the free electrons travel through the conductor with a speed near that of light square6 This is why the effect of flipping a switch is effectively instantaneous
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Motion of Charge Carriers, final square6 Electrons do not have to travel from the light switch to the light bulb in order for the light to operate square6 The electrons are already in the light filament square6 They respond to the electric field set up by the battery square6 The battery does not supply the electrons, it only establishes the electric field
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Current Density square6 J is the current density of a conductor square6 It is defined as the current per unit area square6 J = I / A = nq v d square6 This expression is valid only if the current density is uniform and A is perpendicular to the direction of the current square6 J has SI units of A/m 2 square6 The current density is in the direction of the positive charge carriers
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Conductivity square6 A current density J and an electric field E are established in a conductor
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