Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 27 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 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 Iavg= ΔQ/ΔI Electric Current I= dQ/dt 1A= 1C/s 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 Resistance Current density (J) units of ampere/meter2 J=I/A I= nqVdA then J= nqVd 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 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 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 Resistance Resistivity is the inverse of conductivity ρ=1/σ Unity is Ohm meters (Ωm) 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 Resistance and Temperature Variation of Resistance with temperature ρ=po[1+α(T-To)] ρ=resistivity at some temp. T (°C) α is the temperature coefficient of resistivity Temperature Coefficient of Resistivity α= (1/ρo)x(Δρ/ΔT) Δρ= ρ - ρo Superconductors Class of metals and compounds whose resistance decreases to zero when they are below a certain temp. (Tc) 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=I2R = (ΔV)2/R Unit of power= Watt