EGEE3 - Energy Conversion Devices o Example: we put...

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Energy Conversion Devices o Example: we put electrical energy into a buld and the bulb produces light (which is the desired form of output) but we also get heat (undesired form of energy) o o When all forms of energy coming out of an energy conversion device they will equal the energy that is put into a device Energy output must be equal to the imput Energy can’t be destroyed or created, it can only change form Ex:The amount of electrical energy put into a bulb=the amount of light energy (desirable) and heat energy that comes out of the bulb (undesired) More ex: a lawn mower has desired energy of mechanical and undesired as heat, a car has desired of mechanical and undesired of heat, a computer and a tv have desired as radiation and undesired as heat. Efficiency=useful energy output/total energy input o Ex: an electric motor consumes 100 watts (J/s) of power to obtain 90 watts of mechanical power. Determine its efficiency. 90 watts/100watts=.9 or 90% efficient
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o Ex: the united states power plants consumed 39.5 quadrillion Btus of energy and produced 3.675 trillion kWh of electricity. What is the average efficiency of the power plants in the US? Total energy input=39.5x10^15 Btus Useful energy output=3.675x10^12 kWh (remember 1 kWh=3412 Btus) Efficiency=(3.675x10^12 kWh/39.5x10^15 Btus)x(3412 Btus/1kwh)=.3174 or 31.74% o About 40% of the US energy is used in power generation o About 27% of the US energy is used for transportation Yet the energy efficiency of a power plant is about 35% and efficiency of automobiles is about 25% Measuring thermal energy o Energy associated with random motions of molecules and indicated by
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EGEE3 - Energy Conversion Devices o Example: we put...

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