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Efficiency Powered By Docstoc
					Energy Input and
Unit D: Section 3.3
     Science 9
• 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
• I=?
                      I = (1000W)
• V = 120 V                (120V)

                      P = 8.33 A
• 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=?
• 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.
• Most often, the missing energy is lost as heat.
  Ex. Is a light bulb meant to give off heat or light??
 • 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
  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

          Efficiency =    5J         X 100
                         100 J

          Efficiency = 5 %

  Q: Where does the remaining 95% of the energy
  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

        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
• Pg. 338 #1-11

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