electric energy using electric energy energy can be neither created nor destroyed,…but it can be...
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Electric Energy
Electric Energy
Using Electric EnergyEnergy can be neither created nor destroyed,…but it can be transformed from one kind to another
Objectives
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
Power is the time rate of doing work or expending energy Unit of work or energy is the joule Unit of power is the watt (1 joule per
second)
Energy
Work is equal to the product of the power consumed and the time of charge flow
PW
tW P t
PW
tW qV
Iq
tt
q
I t
I
q
PW
tqV
I
q
P V I
1
W qV
Iq
tq I t
W V I t
W V I t I RtV t
R
b gb g 2
2
Equivalent Expressions
W V I t I RtV t
R
PW
tVI I R
V
R
b g 22
22
Thermal Energy Current through a resistor generates
heat Light bulbs Space heaters Stoves, etc.
Voltage “drops” across a resistor Energy converted to heat is lost
“Joule heating” or I2R lossW q V I Rt 2
Example
A heater has a resistance of 10 Ω It operates on 120.0 V
What is the current through the resistor?
What thermal energy is supplied by the heater in 10.0 seconds?
Example Sketch the
problem List the knowns
and unknowns Strategy
Use I = V/R to find the current
Use W=I2Rt to find the energy*
Knowns
R
V V
t s
Unknowns
I A
W J
10 0
120 0
10 0
.
.
.
?( )
?( )
*The textbook uses “E” for energy
See solution on Page 521 of textbook
Where Does Electric Energy Come From?
Chemical energy (battery) Mechanical energy (generator) Gravitational energy (water falls) Steam turbines & wind turbines
(heat) Solar panels (light) Nuclear energyEnergy can be neither created nor destroyed,…but it can be transformed from one kind to another
Where Does Electric Energy Go?
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 Power is lost as heat in transmission
line Reduce the loss
Low resistance wire High conductivity Large diameter
Lower current Why high-voltage?
Increase voltage and reduce current
P I Rloss 2
P V Itransmitted
Units of Electrical Energy 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 1 kWh = (1000 J/s)(3600 s) = 3.6 x 106 J
PW
tW P t
Example
The resistance of a stove element is 11Ω at operating temperature with a voltage of 220V. 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
Find the name of each person for whom the following units are named Watt Joule Coulomb Ampere Volt
Practice Problems
Textbook, Page 522 #14 – 17 Textbook, Page 525 #18 – 20
Two people do #20
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