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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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Presentation Transcript
law of conservation
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.
slide3

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.
understanding efficiency
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

example
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

practice
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

slide7

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
assignment
Assignment:
  • Page 343
    • Numbers 2 - 8
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