7.8 efficiency

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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. - PowerPoint PPT Presentation

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  • Think very carefully about commonly used electrical devices in your home such as a light bulbWhat type of energy conversions are taking place?

  • Inputted: electrical energyOutputted: LightHeat

  • If you think about the purpose of a light bulb, it is designed to produce light in order to illuminate a roomIs the heat energy being generated as well of any use to us?

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

  • 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 sameHowever, an incandesncent light bulb (IB) has a power rating of 60 W, whereas a CFB has a power rating of 15 W

  • For about of the electrical energy input, a CFB produces the same amount of light as a IBWhich 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 energyYou should realize this since no one wants to touch an IB that has been on for an extended period of time

  • CFBs are more efficient when left on they produce less heat energy which is not useful in the regular functioning of a light bulbSince they also last longer CFBs are a good choice for replacing IBs since they use less energy and they dont have to be replaced as often

  • 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 forA light bulb turns electrical energy into light energy But what about other machines? Are they as efficient?

  • Your stove takes electrical energy and converts it to similar formsBut what type of energy, when discussing a stove, is useful?

  • 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 energyWhereas in a light bulb, this is opposite

  • There are many interactions going on in any machine at the molecular or subatomic level that we cant seeRemember that most machines create movement: in a stove or a bulb, electrons are forced to move through resistorsThese 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

  • 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

  • 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 combustionThis creates motion in metal parts such as pistons inside an engineThis generates a great deal of heat think about how hot the hood of your car gets after a long drivehttp://www.grc.nasa.gov/WWW/K-12/airplane/engopt.html

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

  • Since there is no such thing as 100% efficient machine, the following must be true:Useful energy input > Useful energy outputWhy is it impossible to have a machine that has an efficiency?

  • Efficiency values of over 100% violate the Law of Conservation of energyIt 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

  • UEI = useful energy input in JUEO = useful energy output in J

    Efficiency = UEO x 100% UEI

    Where: UEI > UEO for a real machine