Download Current, resistance, and electromotive force

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Chapter25 Current, resistance and EMF
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Current
• current is the amount of charge flowing through a specific area per unit
time.
dQ
I=
, units : 1A = 1C/s
(2.1)
dt
dQ
n|q|Avd dt
I=
=
= n|q|Avd , units : 1A = 1C/s
(2.2)
dt
dt
n, concentration
q, charge of charge carriers
vd , drift velocity, the constant average velocity of charge carriers
• current density
J=
I
= n|q|vd ,
A
(2.3)
Example 25.1
2.1
Resistance
• Ohm’s Law
∆V = IR
(2.4)
• Resistance, unit 1Ω = 1V /A
∆V
I
L
R=ρ
A
ρ = ρ0 (1 + α(T − T0 ))
R=
R = R0 (1 + α(T − T0 ))
(2.5)
(2.6)
(2.7)
(2.8)
ρ is electrical resistivity of materials
α is temperature coefficient of resistivity
R0 the resistance at reference temperature T0
Good conductor (small resistivity), superconducting
For metals, resistance and temperature coefficient of resistivity increases
as temperature increasing.
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2.2
Electrical energy and power
• Power unit: Watt, W
• Energy unit: 1kW h = 1000W ∗ 3600s = 3.6 × 106 J
P =
∆U
∆t
(2.9)
P = I∆V = I 2 R =
∆V 2
R
(2.10)
Example A circuit provides a maximum current of 20A at an operating voltage
of 120V. (a) how many 75W bulbs can operate with this voltage source? (b) at
12 cents per kilowatt-hour, how much does it cost to operate these bulbs for 8
hours?
2.3
Emf, Electromotive force, ε
• Emf is not a force. It describes the influence that makes current flow from
low voltage to high voltage.
• unit V (Volt)
• source of emf: batteries, electric generator, solar cellls, fuel cells
• Vab terminal voltage, r internal resistance
Vab = ε − Ir
Example 25.5
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(2.11)