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A lot of devices are used to convert electrical
energy into some other useable form
◦ Light bulb, hair dryer, stove
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A lot of energy lost in heat
◦ Light bulb & engines get warm
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What are some appliances used to convert
almost all energy into thermal energy
◦ Hair dryer, space heater, hot plate
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To determine how much total energy is
converted to thermal energy….
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P = E/t  E = Pt
P = IV  V = IR
So…
E = I2Rt
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What is the current through the resistance
Known
Unknown
◦ R = 10.0 Ω
◦ V = 120.0 V
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I=V/R
= 120.0 V / 10.0 Ω
= 12.0 A
I = ???
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What thermal Energy is supplied by the heater
in 10.0 s
Known
Unknown
◦ I = 12.0 A
◦ R = 10.0 Ω
◦ t = 10.0 s
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E = I2Rt
(12.0 A)2(10.0 Ω)(10.0 s)
=14.4 x 103 J
=14.4 kJ
E = ???
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Niagara Falls & Hoover dam produce a lot of
electricity w/o much pollution
◦
◦
◦
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Has to transmitted long distances
Thermal energy lost is P=I2R
Engineers want to reduce I or R
**1km of wire = 0.20 Ω
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Supposed a farm house is 3.5 km away and has a
41 A electric stove.
The power dissipated (loss) in the wires =
2(3.5 km)(0.20 Ω/km)
◦ = 1.4 Ω
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P = I2R
= (41 A)2 x 1.4Ω = 2400 W
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Electric Companies charge you for energy
Energy used by any device is its rate of
energy consumption
◦ Joules/second (J/s)(sec)
◦ Too small for commercial use so companies use the
Kilowatt-Hour or kWh
 = 1000 watts delivered continuously for 3600 sec
 = 3.6 x 106 J
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Other than heating appliances, most don’t
need more than 1000-W of power
◦ Evident in your lab yesterday
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A television set draws 2.0 A when operated on 120 V.
A) How much power does the TV use?
B) If the set operated for an average of 7.0 h/day,
what energy in kWh does it consume/month(30 days)
C) At 11 cents per kWh, what is the cost of operating
the set per month?
Known
Unkown
I = 2.0 A
E = ???
V = 120.0 V
Total Cost = ???
t = (7.0 h/day)(30 days)
Cost = 11 cents / kWh
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P = IV
= (2.0 A)(120.0 V)
= 240 W
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E = Pt
(240 W)(7.0 h/day)(30 d)
= 5.0 x 104 Wh
= 5.0 x 101 kWh
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Cost = (5.0 x 101 kWh)($0.11/kWh)
= $5.50