Transcript Electric Energy

Electric
Energy
Electric Energy
Using Electric Energy
Energy can be neither created nor
destroyed,…but it can be transformed
from one kind to another
Objectives
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Explain how electric energy is converted
into thermal energy
Determine why high-voltage
transmission lines are used to carry
electric energy over long distances
Define kilowatt-hour
Perform energy and power calculations
Power and Energy
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Power is the time rate of doing work or
expending energy
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Unit of work or energy is the joule
Unit of power is the watt (1 joule per
second)
Energy
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Work is equal to the product of the
power consumed and the time of
charge flow
W
P   W  Pt
t
W
P
t
W  qV
q
q
1 I
I
t 
 
t
I
t q
W
I
 P   qV 
t
q
P V I
W  qV
q
I
 q  I  t
t
W  V I  t
bg
V t
W Vb
I  tg
 I Rt 
R
2
2
Equivalent Expressions
bg
2
V t
W  V I  t  I Rt 
R
2
W
V
2
 P   VI  I R 
t
R
2
Thermal Energy
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Current through a resistor generates heat
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Light bulbs
Space heaters
Stoves, etc.
Voltage “drops” across a resistor
Energy converted to heat is lost
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“Joule heating” or I2R loss
W  q  V  I Rt
2
Example
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A heater has a resistance of 10 Ω
It operates on 120.0 V
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What is the current through the resistor?
What thermal energy is supplied by the
heater in 10.0 seconds?
Example
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See solution on Page 521 of textbook
Sketch the problem
List the knowns and
unknowns
Strategy
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Use I = V/R to find
the current
Use W=I2Rt to find
the energy*
*The textbook uses “E” for energy
Knowns
R  10.0
V  120.0V
t  10.0s
Unknowns
I  ?( A)
W  ?( J )
Where Does Electric Energy
Come From?
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Chemical energy (battery)
Mechanical energy (generator)
Gravitational energy (water falls)
Steam turbines & wind turbines (heat)
Solar panels (light)
Nuclear energy
Energy can be neither created nor destroyed,…but it
can be transformed from one kind to another
Where Does Electric Energy
Go?
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Chemical energy (making batteries)
Mechanical energy (motor)
Heat (intentional or byproduct)
LEDs (light)
Nuclear energy (particle accelerators)
Energy can be neither created nor destroyed,…but it
can be transformed from one kind to another
Transmission of Electricity
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Power is lost as heat in transmission line
Reduce the loss
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Low resistance wire
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High conductivity
Large diameter
Ploss  I R
Lower current
Why high-voltage?
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Increase voltage and reduce current
Ptransmitted  V  I
2
Units of Electrical Energy
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Your electric bill is for energy, not power
(even though we call the utility, “the power
company”)
Electric meters measure the power
consumption in kilowatts and the time in
hours
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1 kWh = (1000 J/s)(3600 s) = 3.6 x 106 J
W
P   W  Pt
t
Example
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The resistance of a stove element is
11Ω at operating temperature with a
voltage of 220V.
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What is the current through the stove
element?
How much energy is used in 30 minutes?
What would the cost be at 12 cents per
kilowatt-hour?
Extra Credit
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Find the name of each person for whom
the following units are named
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Watt
Joule
Coulomb
Ampere
Volt
Practice Problems
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Textbook, Page 522 #14 – 17
Textbook, Page 525 #18 – 20
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Two people do #20