Transcript Current and Resistance

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 (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
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