Physics for Scientists and Engineers 8ed - ch27 - PowerPoint Slides

Physics for Scientists and Engineers 8ed - ch27 - PowerPoint Slides

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Chapter 27 Current  And  Resistance
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Electric Current Electric current is the rate of flow of charge through some region of space 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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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 , then the average current is avg Q t = I
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Instantaneous Electric Current If the rate at which the charge flows varies with time, the instantaneous current, I , can be found dQ dt I
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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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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 nA Δ x 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 Δ Q = ( nA Δ x ) q The drift speed, v d , is the speed at which the carriers move v d = Δ x / Δ t and x = v d t Rewritten: Δ Q = ( nAv d Δ t ) q Finally, current, I ave = Δ Q / Δ t = nqv d A
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Charge Carrier Motion in a Conductor The zigzag black lines 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 Use the active figure to change the field and observe the effect PLAY ACTIVE FIGURE
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Motion of Charge Carriers, cont. In the presence of an electric field, in spite of all the collisions, the charge carriers slowly move along the conductor with a drift velocity, The electric field exerts forces on the conduction electrons in the wire These forces cause the electrons to move in the wire and create a current d v r
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Motion of Charge Carriers, final The electrons are already in the wire They respond to the electric field set up by the battery The battery does not supply the electrons, it only establishes the electric field
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Drift Velocity, Example Assume a copper wire, with one free electron per atom contributed to the current The drift velocity for a 12-gauge copper wire carrying a current of 10.0 A is 2.23 x 10 -4 m/s This is a typical order of magnitude for drift velocities
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Current Density J is the current density of a conductor It is defined as the current per unit area J = I / A = nq v d This expression is valid only if the current density is uniform and A is perpendicular to the direction of the current J has SI units of A/m 2 The current density is in the direction of the positive charge carriers
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Physics for Scientists and Engineers 8ed - ch27 - PowerPoint Slides

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