Transcript Power and AC - Lompoc Unified School District / Home

Power and AC
Energy Transformation
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Electric energy can be transformed into other
more useful forms
Power is the rate of energy transformation
P = ΔPE/t
ΔPE = QV
P = QV/ t
I = Q/ t
P = IV units – Watt (W)
Example
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What is the resistance of a 40 W car
headlight designed for a 12V car
battery?
KiloWatt - Hour
Electric companies charge by the
amount of energy used
 Larger unit than Joule is used – called
the kilowatt-hour (kW•hr)
 1 kW•hr = 3.6 x 106 J
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Example
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A heater draws 15.0A on a 120V line.
How much power does it use and how
much does it cost per month (30 days)
to operate if it runs 3.0 hours per day
and the electric company charges 10.5
cents per kW•hr?
Example
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A lightning bolt transfers 109 J of energy
across a potential difference of 5 x 107V
in .2 s. Find the charge transferred, the
current, and the power of the bolt.
Overload
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All wires have resistance and some of the
electric energy is changed to heat
 Overload is when wire carries more than the
safe amount of current
 Fuses and circuit breakers protect from
overload
 Causes of overload: too many devices
drawing too much current, “short” which is
crossed wiring
Example
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A 100W lightbulb, 1800W heater, 350W
stereo, and 1200W television are all
connected to the same circuit which has
a 20A fuse. Can all of these be
operating at the same time?
Alternating Current (AC)
Direct current (DC) is produced from a
battery – flows steadily in one direction
 Electric generating plants produce AC
current – this reverses direction many
times per second
AC
DC
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Sinusoidal Nature
Current and voltage produced in AC are
sinusoidal
 V = Vosin 2πft
 I = Io sin 2πft
 Vo and Io are called peak voltage and
current
 F is the frequency of oscillation (60 Hz)
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Power
Even though average current is zero,
power is always delivered
 P = Io2R
 Since Io is squared, power is always +
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Average Current, Voltage, Power
Root mean square averages
 These are the average current, voltage,
and power delivered by an ac line
 Iavg = .5 Io2 = .707 Io
 Vavg = .707 Vo
 Pavg = (Vavg)2/ R
 Pavg = (Iavg)2R
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Examples
What is the peak voltage of a 120V ac
line?
 A stereo receiver is capable of average
power output of 100W into an 8Ω
loudspeaker. What is the rms voltage
and rms current fed to the speaker a) at
max power of 100W and b) at 1.0 W
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