Physics 213 General Physics Lecture 5. 2 Last Meeting: Capacitance Today: Current and Resistance.
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Transcript of Physics 213 General Physics Lecture 5. 2 Last Meeting: Capacitance Today: Current and Resistance.
Physics 213General Physics
Lecture 5
2
Last Meeting: Capacitance
Today: Current and Resistance
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
4
NC
C
CCC
eq
eq
...111
2
21
VCEnergy
NCC
CCC
eq
eq
...
C CC
C
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
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.
QUIZ
(a) C increases
(b) Q increases
(c) E stays the same
(d) V remains the same
8
9
10
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
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
13
14
15
16
17
18
19
Variety of Resistors
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
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
22
Energy is turned to heat (random motion)
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
Electrical Energy and Power, cont2
2 VI R
R
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