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Power and Energy Calculations

Power and Energy Calculations. Power Ratings. Power is the rate that an electrical device converts electrical energy into another form (ex. light or heat) At the first lamp… V = 120 V R = 240 Ω. I = V/R I = 120V / 240 Ω I = 0.5A. What is the power of the bulb?

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Power and Energy Calculations

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  1. Power and Energy Calculations

  2. Power Ratings • Power is the rate that an electrical device converts electrical energy into another form (ex. light or heat) • At the first lamp… V = 120 V R = 240 Ω I = V/R I = 120V / 240Ω I = 0.5A

  3. What is the power of the bulb? Power (Watt) = Voltage (Volts) x Current (Amperage) Or P = V x I P = 120V x 0.5A P = 60 W (watts)

  4. Example A TV uses 100 V of electricity and has a resistance of 35 Ω. Calculate the power of the TV. I = V/R I = 100V / 35Ω I = 2.9A P = VI P = 100V x 2.9A P = 286 W

  5. Energy Used If the same TV is watched for 3 hours, how much energy does it use? Energy (Watt*hours) = Power (Watt) x Time (hours) E = Pt E = 286W x 3h E = 858Wh or 0.86kWh 1000Wh = 1kWh

  6. Cost of Operating Devices How much energy does a 120 W bulb use in 1 week, if it is left on 12 hours a day? E = Pt E = (120W)(12 hours)(7 days) E = 10080Wh E = 10.08kWh

  7. If electricity costs $0.08 per kWh, how much would it have cost to keep the light bulb on? Cost = Rate x Energy Cost = $0.08 x 10.08kWh Cost = $0.81

  8. How much energy does a 0.56kW fridge use in one year? How much does it cost to operate the fridge if energy costs $0.12 per kWh? E = Pt E = (0.56kW)(24 hours)(365 days) E = 4905.6 kWh Cost = Rate x Energy Cost = ($0.12)(4905.6kWh) Cost = $588.67

  9. Calculating Efficiency of a Device Efficiency of a device is a comparison (or a ratio) of the useful energy that comes out of the device to the total energy that went in. Percent Efficiency = (energy out / energy in) x 100%

  10. A oven uses 490J of input energy to produce 59J of heat energy. What is its percent efficiency? Input energy = 490J Output energy = 59J Percent Efficiency = ? Percent Efficiency = (energy out / energy in) x 100% Percent Efficiency = (59J / 490J) x 100% Percent Efficiency = 12% Therefore, the efficiency of the oven is 12%

  11. Practice Problems • Page 492 #1 • Page 493 # 1 – 3 • Worksheet • If you finish this, read pages 490 -491, 494 – 495 and then do: • Page 498 # 1 – 12, 15

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