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Electric Power and Energy

Electric Power and Energy. MHR pg. 652-664. Key Question: How much does electricity cost and what do you pay for?. The watt (W) is a unit of power. Power is the rate at which energy moves or is used. Since energy is measured in joules , power is measured in joules per second.

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Electric Power and Energy

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  1. Electric Power and Energy MHR pg. 652-664

  2. Key Question: • How much does electricity cost and what do you pay for?

  3. The watt (W) is a unit of power. • Power is the rate at which energy moves or is used. • Since energy is measured in joules, power is measured in joules per second. • One joule per second is equal to one watt.

  4. One watt is a pretty small amount of power. • In everyday use, larger units are more convenient to use. • A kilowatt (kW) is equal to 1,000 watts. • The other common unit of power often seen on electric motors is the horsepower. • One horsepower is 746 watts.

  5. Power (watts) P = VI Voltage (volts) Current (amps)

  6. A light bulb with a resistance of 1.5Ω is connected to a 1.5-volt battery in the circuit shown at right. • Calculate the power used by the light bulb.

  7. Alternative equation for Power • P = V2/R • Or • P = I2R

  8. Problem #2: Kettle • An electric kettle is rated at 1500 W for 120V potential difference. • What is the resistance of the heating element of the kettle? • What will be the power output if the potential difference falls to 108V? Answer: • R = 9.60Ώ • P = 1.22 x 103 W

  9. Paying for electricity • Electric companies charge for the number of kilowatt-hours used during a set period of time, often a month. • One kilowatt-hour (kWh) means that a kilowatt of power has been used for one hour. • Since power multiplied by time is energy, a kilowatt-hour is a unit of energy. • One kilowatt-hour is 3.6 x 106 joules.

  10. Calculate power • Your electric company charges 14 cents per kilowatt-hour. Your coffee maker has a power rating of 1,050 watts. • How much does it cost to use the coffee maker one hour per day for a month?

  11. Your Turn… • MHR pg. 655 # 40 – 42 • MHR pg. 658 # 43-45 • MHR pg.662 # 46-50 • MHR pg. 664 # 51-53 • We are done…. • No more lessons…. • Last one for the year 

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