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