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7.8 Efficiency

7.8 Efficiency. Think very carefully about commonly used electrical devices in your home such as a light bulb What type of energy conversions are taking place?. Is complete energy conservation possible?. Inputted: electrical energy Outputted: Light Heat. LIGHT BULB.

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7.8 Efficiency

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  1. 7.8 Efficiency

  2. Think very carefully about commonly used electrical devices in your home such as a light bulb What type of energy conversions are taking place? Is complete energy conservation possible?

  3. Inputted: • electrical energy • Outputted: • Light • Heat LIGHT BULB

  4. If you think about the purpose of a light bulb, it is designed to produce light in order to illuminate a room Is the heat energy being generated as well of any use to us? How much of that is “useful”?

  5. Recently, the Ontario government has been pushing homeowners to change their old incandescent bulbs to the new compact flourescent bulbs (CFB) Flourescent vs. incandescent

  6. And incandescent light bulb and a flourescent light bulb both produce the same amount of light – which means that the USEFUL ENERGY OUTPUT generated by the bulbs is about the same However, an incandesncent light bulb (IB) has a power rating of 60 W, whereas a CFB has a power rating of 15 W Is this worth it?

  7. For about ¼ of the electrical energy input, a CFB produces the same amount of light as a IB • Which means that it is 4 times more efficient • This suggests that compared to the CFB, the IB wastes about 75% of its energy – it gets converted to heat energy • You should realize this since no one wants to touch an IB that has been on for an extended period of time What does that mean?

  8. CFB’s are more efficient when left on – they produce less heat energy which is not useful in the regular functioning of a light bulb Since they also last longer – CFB’s are a good choice for replacing IB’s since they use less energy and they don’t have to be replaced as often Efficiency

  9. A machine is a device that converts energy or force into a form that can be used to complete a job that the machine is designed for A light bulb turns electrical energy into light energy But what about other machines? Are they as efficient? Is every machine this efficient?

  10. Your stove takes electrical energy and converts it to similar forms But what type of energy, when discussing a stove, is useful? Think about other devices

  11. Remember that the type of device will determine what is considered to be USEFUL ENERGY OUTPUT In a stove, heat is the USEFUL ENERGY OUTPUT whereas the light produced would be wasted energy Whereas in a light bulb, this is opposite One machine’s trash is another machine’s treasure…

  12. There are many interactions going on in any machine at the molecular or subatomic level that we can’t see • Remember that most machines create movement: in a stove or a bulb, electrons are forced to move through resistors • These small particles carry the inputted energy and in order to convert it to another type, it has to undergo some interaction that saps energy from it What makes a machine inefficient?

  13. Electrons slam into the molecules of resistors, increasing the motion of these particles and thus creating energy in the form of heat In mechanical devices like cars, moving metal parts rub up against each other creating heat through friction Moving particles

  14. That most cars are less than 50% efficient? • Most of the gasoline you pump into a car (chemical energy) is converted to mechanical energy through combustion • This creates motion in metal parts such as pistons inside an engine • This generates a great deal of heat – think about how hot the hood of your car gets after a long drive • http://www.grc.nasa.gov/WWW/K-12/airplane/engopt.html Did you know…

  15. The amount of energy put into the machine to complete a job is the useful energy input For example, for both the light bulb and stove, this is the amount of electrical energy inputted as defined by E = Vit It is important to note that for all machines, useful energy input is always GREATER than useful energy output Useful energy input

  16. Since there is no such thing as 100% efficient machine, the following must be true: Useful energy input > Useful energy output Why is it impossible to have a machine that has an efficiency? NO MACHINE IS 100% EFFICIENT

  17. Efficiency values of over 100% violate the Law of Conservation of energy It essentially means that the work being completed by the machine is being completed for free – since you are getting more out of the machine than you are putting in Violation of the law

  18. UEI = useful energy input in J UEO = useful energy output in J Efficiency = UEO x 100% UEI Where: UEI > UEO for a real machine Efficiency

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