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Published bySharon Blakely Modified over 2 years ago

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A lot of devices are used to convert electrical energy into some other useable form ◦ Light bulb, hair dryer, stove 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 To determine how much total energy is converted to thermal energy…. ◦ P = E/t E = Pt ◦ P = IV V = IR ◦ So… ◦ E = I 2 Rt

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What is the current through the resistance KnownUnknown ◦ R = 10.0 ΩI = ??? ◦ V = V I = V / R = V / 10.0 Ω = 12.0 A

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What thermal Energy is supplied by the heater in 10.0 s KnownUnknown ◦ I = 12.0 AE = ??? ◦ R = 10.0 Ω ◦ t = 10.0 s E = I 2 Rt (12.0 A) 2 (10.0 Ω)(10.0 s) =14.4 x 10 3 J =14.4 kJ

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Niagara Falls & Hoover dam produce a lot of electricity w/o much pollution ◦ Has to transmitted long distances ◦ Thermal energy lost is P=I 2 R ◦ 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 P = I 2 R = (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 10 6 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? KnownUnkown I = 2.0 AE = ??? V = VTotal 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 10 4 Wh = 5.0 x 10 1 kWh

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Cost = (5.0 x 10 1 kWh)($0.11/kWh) = $5.50

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