Electric Power

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					Giancoli 5th Ch 18 p. 538
      MHR Ch 15 p.734
Electric Power
 Electrical energy is used extensively in our lives since it
  is plentiful, relatively inexpensive, and easily
  transformed into other forms.
 In our homes, appliances such as ranges, dryers,
  toasters, and heaters transform electrical energy into
  thermal via a “heating element” i.e. a resistor
Electric Power
 Power is defined as energy transformed per unit time
 P = E/t In electrical devices, charge is moved in a
  circuit by a voltage so,
 P = qV/t but I = q/t so P = IV. From Ohm’s law,
 V=IR so P = I(IR) = I2R. Furthermore, I=V/R, so
 P = (V/R)V = V2/R
 P=IV applies to any device, but I2R & V2/R only apply
  to resistors
Examples
 A tail-light in my car is rated at 10. W and operates off
  the 12V battery. What resistance is offered by the tail-
  light?
 P = V2/R, so R = V2/P 12V2/(10.W)
 Shiva works in an office which has an electric heater
  connected to a 120 V line. The heater draws 15 A of
  current. How much power does it use and how much
  does it cost to run the heater per month if it runs for
  3.0 h a day and if NS Power charges 12 ¢ per kWh
 ( kilowatt-hour- a unit of energy)
Examples
 P =IV = (15A)(120V) = 1800 W = 1.8 kW
 Cost = (1.8kW)(3.0 h/day)(30. day/month)($0.12/kWh)
 Cost = $ 19.44
Power in household circuits
 The copper wiring in your home does offer some
  resistance to the flow of charge , but it is quite small,
  and copper is a good conductor. Because the wire has
  some resistance, if the current is large enough, there is
  a substantial heating effect in the wire i.e. P = i2R. If
  the wire gets hot enough, an electrical fire could start.
 A thicker wire (gauge) has a greater cross sectional
  area and can carry more current without overheating.
 When a circuit carries more current than is safe, we say
  it is “overloaded.”
Power in household circuits
 To prevent overloading and the likelihood of electrical
 fires, fuses and circuit breakers are installed in the
 circuits. These are wired in to your electrical panel or
 fuse box. Circuit breakers and fuses are really just
 switches designed to open the circuit when a certain
 amount of current in the circuit is exceeded. Fuses
 have a metal ribbon which melts and opens the circuit.
 Circuit breakers are spring loaded switches which
 “trip” when the current is exceeded. They must be
 reset after being tripped.
Power in household circuits-
example
 A 100 W lightbulb, an 1800 W electric heater, a 350 W
 stereo receiver and a 1200 W hair dryer are all
 connected in parallel to a 120 V line with a 20 A
 breaker. Will the breaker be tripped?

    Item          I=P/V          I (Amps)
    Lightbulb     100W/120 V     0.80 A
    Heater        1800 W/120 V   15.0 A
    Receiver      350 W/120 V    2.9 A
    Hair Dryer    1200 W/120 V   10.0 A
 The total amps in the circuits is
 0.80 + 15.0 + 2.9 + 10.0 = 28.7 A
 So the breaker will be tripped.

				
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