Current
Resistivity
Current – any motion of ___________ from one region to another.
For some materials, especially metals, the current density (J) is
directly proportional to electric field (E) - _______ “Law”
____ is the standard symbol used for current.
ρ=
dQ
definition of current
I=
dt
I
definition of current density
J=
A
Current units: ___________ 1 A = 1 C/s
E
J
definition of resistivity
Units: (V/m)/(A/m2) = Ω·m
(Ω =V/A,
Ω - Ohm)
_______________ is the inverse of resistivity
Current density units: ___________
Current ‘flows’ from _________ potential to _________ potential.
DC - ______________ current – always in the same direction
AC - ______________ current – continuously changes directions
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EF 152 Fall, 2009 Lecture 4-5
Resistivity and temperature
2
EF 152 Fall, 2009 Lecture 4-5
Resistance
R=
R=
V
I
ρL
definition of resistance
relationship between resistivity and resistance
A
R – units of Ω (Ohms)
ρ (T ) = ρ 0 [1 + α (T − T0 )]
EF 152 Fall, 2009 Lecture 4-5
temperature dependence of resistivity
ρ0 and T0 are reference values
α – temperature coefficient of resistivity
100 m of 12-gauge copper wire
100 W light bulb R=140 Ω
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EF 152 Fall, 2009 Lecture 4-5
R=0.5 Ω
4
a
b
EF 152 Fall, 2009 Lecture 4-5
Find: Vad, Vbc, rate of energy use
Example
EF 152 Fall, 2009 Lecture 4-5
Sum of potential changes for a complete circuit is ________
Potential changes around a circuit
EF 152 Fall, 2009 Lecture 4-5
Terminal voltage: Vab = E – Ir
Internal resistance: r
Sources of emf: _______________________________________
emf is not really a force – it is an energy/charge quantity like
potential.
Electromotive force (emf, E) causes current flow from lower to
higher potential.
To have a steady current you must have a ___________ circuit.
Electromotive Force
9
7
5
Measures
Internal
Connect
EF 152 Fall, 2009 Lecture 4-5
Vab
R
2
Power delivered to a resistor
Power - Rate at which energy is delivered to or
extracted from a circuit element.
Units of Watts (W) 1 W = 1V·A = (1 J/C)(1 C/s) = 1 J/s
P = Vab I = ( IR) I =
P = Vab I
Energy and power in circuits
EF 152 Fall, 2009 Lecture 4-5
Ohmmeter
Ammeter
Voltmeter
Device
Symbols
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6