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					Energy Input and
    Output
Unit D: Section 3.3
     Science 9
                   Power
• Power is the rate at which a device
  converts energy
  – The unit of power is a Watt (W)
  – 1 W = 1 Joule/second
  – The faster a device converts energy the
    greater its power rating (100W vs 60W bulb)
           Calculating Power
• Formula: P = I (current) x V (voltage)

• Units:    (W) = (Amperes) x (Volts)

• Analogy: The power of a waterfall is equal to
  the amount of water flowing (current) times the
  difference in potential energy (voltage) between
  the top and bottom of the falls.
             Calculating Power
• Example: A radio is plugged into a 120 V outlet
  and draws 1.5 A of current. What is its power
  rating of the radio?

• P=?                  P = IV

• I = 1.5A             P = (1.5A)(120V)

• V = 120 V            P = 180 W
          Calculating Power
• Example: A hair dryer has a power rating of
  1000 W. It is plugged into a 120 V outlet. What
  is the current flowing through the hair dryer?

• P = 1000W           P = IV rearrange I = P
                                           V
• I=?
                      I = (1000W)
• V = 120 V                (120V)

                      P = 8.33 A
                  Energy
• The energy consumption of an electrical
  device is its input power multiplied by the
  time the device is used

• Formula: E = P x t

• Units:   (Joules) = (Watts) x (seconds)
          Calculating Energy
• A microwave oven has a power rating of 800W.
  If you cook a roast in this oven for 30 min on
  high, how many joules of electrical energy are
  converted into heat by the microwave?
                        E=Pxt
• E=?
• P = 800 W             E = 800 W x 1800 s
• t = 30 min x 60s
               min      E = 1 440 000 J
                          = 1 440 kJ
  t = 1800 s              = 1.4 MJ
            Kilowatt Hours
• It doesn’t us long to consumer a large
  amount of energy
• For this reason, the kilowatt hour is often
  used as a unit for energy; this is what our
  electricity companies use on our bills
• The energy calculation is the same but the
  units are different:
          E (kWh) = P (kW) x t (h)
    Law of Conservation of Energy
• Energy cannot be created or destroyed,
   it can only be transformed from one
              form to another.
• But we usually find that the output energy of
   a device is smaller than the input energy.
                    WHY??
• Most often, the missing energy is lost as heat.
  Ex. Is a light bulb meant to give off heat or light??
    UNDERSTANDING EFFICIENCY
 • The ratio of the useful energy output to the
  total energy input in a device or system; given
                 as a percentage

• Efficiency can be calculated by:
      EFFICIENCY (in %) =      OUTPUT X 100
                                 INPUT
WHERE:
  OUTPUT is useful energy produced (in J)
  INPUT is the total energy put into the system (in J)

      Both are measured in Joules (J)
                                                         *
1. We get 5J of light energy from an incandescent
   light bulb although we put 100J of electrical energy
   into it. How efficient is the light bulb?

          EFF. = OUTPUT X 100
  INPUT

          Efficiency =    5J         X 100
                         100 J

          Efficiency = 5 %

  Q: Where does the remaining 95% of the energy
  go??
  It is lost as heat (think of an Easy Bake Oven)

  Q: Would you want this light bulb in your house??
2: A couple has to use 25 000 J of energy to heat some
water in their hot water tank, but only 4 000 J of energy
was absorbed by the water. How efficient is the heater?

        EFF. = OUTPUT X 100
               INPUT

        EFF. = 4 000 J X 100
              25 000 J
        EFF. = 16 %

• Efficiency WILL ALWAYS be less than 100% due to
  some “waste” energy
   – If the efficiency was 100%, we would have a
     perpetual motion machine – which is not a reality
                                                         *
Remember that:

• NO MACHINE IS 100% EFFICIENT as there is
  always a bit of friction or heat causing energy to
  be lost

• If you are doing a problem and get an efficiency
  greater than 100%, you have made a mistake
              Skill Practice
• Go to pg. 336

• For each device calculate the efficiency

• Which is the most efficient? Least efficient?

• What do you think causes the difference
  between the most and least efficient
              Assignment
• Pg. 338 #1-11

• Read pg. 339 – 342
  – Complete pg. 338 # 1-11
  – Complete pg. 342 # 1-7

				
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