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Power and Energy Calculations

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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 ... – PowerPoint PPT presentation

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Title: Power and Energy Calculations


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 O

I V/R I 120V / 240O 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 O. Calculate the power of the
    TV.
  • I V/R
  • I 100V / 35O
  • 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 (Watthours) 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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