Week5_1 - Problem 20.54(a How much charge can be placed on a capacitor with air between the plates before it breaks down if the area of each of the

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1 Problem 20.54. ± (a) How much charge can be placed on a capacitor with air between the plates before it breaks down if the area of each of the plates is 5.00 cm2? ± (b) Find the maximum charge assuming polystyrene is used between the plates instead of air.
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Chapter 21 Current and Direct Current Circuits
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3 Electric Current ± Electric current is the rate of flow of charge through a surface ± The SI unit of current is the Ampere (A) ± 1 A = 1 C / s ± The symbol for electric current is I
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4 Average Electric Current ± Assume charges are moving perpendicular to a surface of area A ± If Δ Q is the amount of charge that passes through A in time Δ t, the average current is avg Q I t Δ = Δ
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5 Instantaneous Electric Current ± If the rate at which the charge flows varies with time, the instantaneous current, I, can be found 0 lim t Qd Q I td t Δ→ Δ == Δ
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6 Problem 21.2. ± A small sphere that carries a charge q is whirled in a circle at the end of an insulating string. The angular frequency of revolution is ω . What average current does this revolving charge represent?
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7 Direction of Current ± The charges passing through the area could be positive or negative or both ± It is conventional to assign to the current the same direction as the flow of positive charges ± The direction of current flow is opposite the direction of the flow of electrons ± It is common to refer to any moving charge as a charge carrier
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8 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 Δ x is the total number of charge carriers
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9 Current and Drift Speed, cont ± The total charge is the number of carriers times the charge per carrier, q ± Δ Q = (n A Δ x) q ± The drift speed, v d , is the speed at which the carriers move ± v d = Δ x/ Δ t ± Rewritten: Δ Q = (n A v d Δ t) q ± Finally, current, I = Δ Q/ Δ t = nqv d A
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10 Consider positive and negative charges moving horizontally through the four regions shown in Figure 21.3. Rank the currents in these four regions, from lowest to highest. (d ) , (b ) = (c), (a (b) , (c ), (d) = (b), (a , ( d 33% 33% 33% 1. (d), (b) = (c), (a) 2. (b), (c), (a), (d) 3. (c) = (b), (a), (d) 10
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11 Charge Carrier Motion in a Conductor ± The zig-zag black line represents the motion of 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
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12 Motion of Charge Carriers , cont ± When the potential difference is applied, an electric field is established in the conductor ± The electric field exerts a force on the electrons ± The force accelerates the electrons and produces a current
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13 Motion of Charge Carriers, final ± The changes in the electric field that drives the free electrons travel through the conductor with a speed near that of light ± This is why the effect of flipping a switch is effectively instantaneous ± Electrons do not have to travel from the light switch to the light bulb in order for the light to operate ± The electrons are already in the light filament ± They respond to the electric field set up by the battery
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This note was uploaded on 06/02/2008 for the course PHYS 102 taught by Professor N/a during the Spring '08 term at Drexel.

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Week5_1 - Problem 20.54(a How much charge can be placed on a capacitor with air between the plates before it breaks down if the area of each of the

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