Current and Resistance Electric Current Flow of electric

Current and Resistance

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 (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/Δt Electric Current is in the direction of flow of POSITIVE charges 1 A= 1 C/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 change in total charge of the system

Resistance Ohm’s Law For many materials, the ratio of the current density to the electric field is a constant that is independent of the electric field producing the current I/(volume) = σE Further derivations show that R=L/σA which is called resistance – the constant σ is called conductivity

Resistance (R) unit is ohm (Ω) R= ΔV/I 1Ω= 1 V/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 Unit 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 double, its resistance decrease by ½

Resistance and Temperature Variation of Resistance with temperature ρ=ρo[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 (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