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Published byBeverly Parrish Modified over 9 years ago
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Chapter 27
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Electric Current Flow of electric charges through a piece of material Amount of flow depends on material and the potential difference across the material Whenever there is a net flow of charge through a region= there is an electric current
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Electric Current Current (I): the rate at which charge flows through a surface Unit of current is ampere= A If ΔQ is the amount of charge that passes though the surface in time (Δt) the average current I avg = ΔQ/ΔI Electric Current I= dQ/dt 1A= 1C/s
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Electric Current The direction of the current is opposite the direction of the flow of electrons Refer to a moving charge as charge carrier If the ends of a conducting wire are connected the electric field is zero within the conductor No net transport of charge Also no current
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Resistance Current density (J) units of ampere/meter 2 J=I/A I= nqV d A then J= nqV d This above equation is ONLY VALID if current density is UNIFORM and only if the cross-sectional area A is PERPENDICULAR to the direction of the current
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Resistance Ohm’s Law For many materials, the ratio of the current density to the electric field is a constant σ that is independent of the electric field producing the current J=σE Further derivations show that R=L/σA which is called resistance
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Resistance Resistance (R) unit is ohm (Ω) R= ΔV/I 1Ω= 1V/A circuits use elements called resistors to control the current in the circuit at different places: Two types are: Composition resistor Wire-wound resistor
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Resistance Resistivity is the inverse of conductivity ρ=1/σ Unity is Ohm meters (Ω m) Resistance of a uniform material of length(L) R= ρ x L/A If the length of wire is doubled, its resistance doubles If its area is doubled, its resistance decreases by ½
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Resistance and Temperature Variation of Resistance with temperature ρ=p o [1+α(T-T o )] ρ=resistivity at some temp. T (°C) α is the temperature coefficient of resistivity Temperature Coefficient of Resistivity α= (1/ρ o )x(Δρ/ΔT) Δρ= ρ - ρ o
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Superconductors Class of metals and compounds whose resistance decreases to zero when they are below a certain temp. (T c )
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Electrical Power Power (P) is the rate at which energy is delivered to the resistor by a battery P= IΔV Unit= Watt Power delivered by voltage source to any device P=I 2 R = (ΔV) 2 /R Unit of power= Watt
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