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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 s Electric current is the rate of flow of charge through some region of space s The SI unit of current is the ampere (A) s 1 A = 1 C / s s The symbol for electric current is I
Average Electric Current s Assume charges are moving perpendicular to a surface of area A s 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 s If the rate at which the charge flows varies with time, the instantaneous current, I , can be found I dQ dt =
Direction of Current s The charges passing through the area could be positive or negative or both s It is conventional to assign to the current the same direction as the flow of positive charges s The direction of current flow is opposite the direction of the flow of electrons s It is common to refer to any moving charge as a charge carrier

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Current and Drift Speed s Charged particles move through a conductor of cross- sectional area A s n is the number of charge carriers per unit volume s nA Δ x is the total number of charge carriers
Current and Drift Speed, cont s The total charge is the number of carriers times the charge per carrier, q s Δ Q = ( nA Δ x ) q s The drift speed, v d , is the speed at which the carriers move s v d = Δ x / Δ t s Rewritten: Δ Q = ( nAv d Δ t ) q s Finally, current, I av = Δ Q t = nqv d A

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Charge Carrier Motion in a Conductor s The zigzag black line represents the motion of a charge carrier in a conductor s The net drift speed is small s The sharp changes in direction are due to collisions s The net motion of electrons is opposite the direction of the electric field
Motion of Charge Carriers, cont. s In spite of all the collisions, the charge carriers slowly move along the conductor with a drift velocity, v d s Changes in the electric field that drives the free electrons travel through the conductor with a speed near that of light s This is why the effect of flipping a switch is effectively instantaneous

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Motion of Charge Carriers, final s Electrons do not have to travel from the light switch to the light bulb in order for the light to operate s The electrons are already in the light filament s They respond to the electric field set up by the battery s The battery does not supply the electrons, it only establishes the electric field
Current Density s J is the current density of a conductor s It is defined as the current per unit area s J = I / A = nq v d s This expression is valid only if the current density is uniform and A is perpendicular to the direction of the current s J has SI units of A/m 2 s The current density is in the direction of the positive charge carriers

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Conductivity s A current density J and an electric field E are established in a conductor whenever a potential difference is maintained across the conductor s J = σ E s The constant of proportionality, σ , is called the conductivity of the conductor
Ohm’s Law s Ohm’s law

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## This note was uploaded on 11/01/2009 for the course PHY phy2049 taught by Professor Wilfredngwa during the Fall '09 term at University of Central Florida.

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chapter27notes - Chapter 27 Current and Resistance Electric...

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