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Sec 3.3 Energy and Efficiency 2

Sec 3.3 Energy and Efficiency 2. P. 335 - 342. Law of Conservation. Energy cannot be created or destroyed . It does not appear or disappear, can only be converted from one form to another .

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Sec 3.3 Energy and Efficiency 2

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  1. Sec 3.3 Energy and Efficiency2 P. 335 - 342

  2. Law of Conservation • Energy cannot be created or destroyed. • It does not appear or disappear, can only be converted from one form to another. • However, we usually find that the outputenergy of a device or system is smaller than the input energy, sometimes much smaller.

  3. Most often the energy is lost to heat. • Example: when heating a pot, not all the heat goes into the pot, some is lost to the surroundings. • Mechanical systems also lose a lot of energy. They lose some energy to sound, heat from friction. These are non-usable forms of energy. • In fact, all mechanical systems dissipate or lose some energy, so their usable output energy is always less than their input energy.

  4. Understanding Efficiency • The efficiency of a device is the ration of useful energy that comes out of a device to the total energy that went in. • The more input energy that a device converts to useable output energy, the more efficient the device is. • Efficiency is usually calculated as a percentage: • % Efficiency = Joules of useful output x 100% Joules of input energy

  5. Example 5 J of light energy • The efficiency of this light bulb is 5 J/100 J x 100% = 5 %. In other words only 5% of the energy used is converted to light, and the rest is lost to heat. 100 J electric energy going in 95 J lost to heat energy

  6. Practice: • A diesel truck produces 47.5kJ of useful energy from 125 kJ of diesel fuel. What is the trucks efficiency? • % Efficiency = Joules of useful output x 100% Joules of input energy • % Efficiency = 47.5 kJ x 100% 125 kJ • % Efficiency = 0.38 x 100% = 38% efficient

  7. A lawn mower consumed 8 MJ of chemical energy. The efficiency of this mower is 35% efficient. How much useful output energy did the lawn mower produce? • The lawn mower used 35% of 8 MJ. • 0.35 x 8 = 2.8 MJ • % Efficiency = Joules of useful output x 100% Joules of input energy • 35% = Joules of useful output x 100% 8 MJ • 8 MJ x 0.35 = 2.8 MJ

  8. Section 3.4 Reducing the Energy Wasted by Devices • Read pgs 339 - 342

  9. Assignment: • Page 343 • Numbers 2 - 8

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