Power and Electricity

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Power and Electricity

• In a circuit, power is the rate that electricity
  is used
• P E/t
• The unit for power is a watt (joules per sec)

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Power equations

• There are other equations where power can be
  found these are
• PVIt (since EVI)
• If given resistance use the following equations
• PI2R or
• PV2/R

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Sample problems

• What is the resistance of a 600W kettle that
  draws a current of 5.00 A?

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Power Rating

• The power rating shown on electrical appliances
  gives a comparative indication of the cost of
  operating these appliances
• The energy consumed by an electrical appliance
  depends on its power rating and length of time
  operating.

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EnerGuide labels

• The EnerGuide label is a tool to help you make an
  energy-wise choice when buying a new appliance.
  It shows how much energy appliances consume in a
  year of normal service and makes it easy to
  compare the energy efficiency of each model to
  others of the same size and class.

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EnerGuide labels

• EnerGuide ratings for major household Appliances
  range from the most energy-efficient (i.e with
  the lowest energy consumption) to the least
  energy-efficient (i.e with the highest energy
  consumption) in a given size and type category.
• For major appliances, an indicator arrow on the
  left side of the EnerGuide scale means lower
  operating costs and long-term savings.

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EnerGuide

• Check your electricity bill to find out what
  you're paying per kWh of electricity. Multiply
  the price (in /kWh) with the big, bold kWh per
  year number on the EnerGuide label. The result
  will tell you how much you should budget on an
  annual basis to operate the appliance once it is
  installed in your home.

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The cost of electricity

• We pay for all of the electricity that we use.
• We usually pay a few cents for a kilowatt hour
• 1 kilowatt-hour (1-kW-h) equals the energy used
  to operate a device rated at 1 kilowatt for one
  hour (or the energy dissipated in 1 hour by a
  load with a power of 1 kW)
• 1 kW-h 3.6 x 106 J
• 1 kW-h 3.6 MJ

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Energy used

• The energy consumed can be calculated using
• EPt
• E energy consumed (in kW-h)
• P amount of Power (in kW)
• Ttime (in hours)

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Sample problems

• Calculate the electric energy (in kW-h) used in
  operating a 40 W bulb for 30 mins?

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Sample problem

• Estimate the cost of watching TV for 3 hrs. A
  typical color TV has a power of 150W. Use a unit
  cost of 6 cents per kilowatt-hour.

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Assignment

• Power and cost of electricity handout