physics 213 general physics lecture 5. 2 last meeting: capacitance today: current and resistance
TRANSCRIPT
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Physics 213General Physics
Lecture 5
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Last Meeting: Capacitance
Today: Current and Resistance
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Energy Stored in a Capacitor
Energy stored = ½ Q ΔV
From the definition of capacitance, this can be rewritten in different forms
C2
QVC
2
1VQ
2
1Energy
22
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NC
C
CCC
eq
eq
...111
2
21
VCEnergy
NCC
CCC
eq
eq
...
C CC
C
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Quiz: The plates of a connected parallel-plate capacitor of capacitance C are brought together to one-third their original separation. The energy stored is now
(a) W/9(b) W/3(c) 3W(d) 9W
Answer: c
221 1,
2 2
QW CV W
C
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QUIZ
A fully charged parallel-plate capacitor remains connected to a battery while you slide a dielectric between the plates. Do the following quantities increase, decrease, or stay the same? (a) C; (b) Q; (c) E between the plates; (d) V.
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QUIZ
(a) C increases
(b) Q increases
(c) E stays the same
(d) V remains the same
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Charge Carrier Motion in a Conductor
The zig-zag black line represents the motion of a charge carrier in a conductorThe net drift speed is small
The sharp changes in direction are due to collisions
The net motion of electrons is opposite the direction of the electric field
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Charge Carrier Motion in a Conductor
The drift speed Vd is small (~1meter/hour)
The fermi speed Vf is very fast! Due to quantum mechanics of electrons. (~106 meters/second)
Charge rearrangements and signals travel near speed of light (~108 meters/second)
fv
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Variety of Resistors
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Temperature Variation of Resistivity
For most metals, resistivity increases with increasing temperature
With a higher temperature, the metal’s constituent atoms vibrate with increasing amplitude
The electrons find it more difficult to pass through the atoms
fv
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Temperature Variation of Resistivity, contFor most metals, resistivity increases
approximately linearly with temperature over a limited temperature range
ρ is the resistivity at some temperature T
ρo is the resistivity at some reference temperature To To is usually taken to be 20° C
is the temperature coefficient of resistivity
)]TT(1[ oo
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Energy is turned to heat (random motion)
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Electrical Energy and Power
The rate at which the energy is lost is the power
From Ohm’s Law, alternate forms of power are
QV I V
t
22 V
I RR
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Electrical Energy and Power, cont2
2 VI R
R
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Electrical Energy and Power, final
The SI unit of power is Watt (W)I must be in Amperes, R in ohms and V in
Volts
The unit of energy used by electric companies is the kilowatt-hourThis is defined in terms of the unit of power and
the amount of time it is supplied1 kWh = 3.60 x 106 J