3.3 Useful Energy and Efficiency

1. 3.3 Useful Energy and Efficiency

2. Review • What is the First Law of Thermodynamics? • What is the Second Law of Thermodynamics?

3. If machines or engines are to produce mechanical energy, they must have moving parts • Remember mechanical energy= kinetic energy+ potential energy • Moving parts will rub against each other and have friction, which in turn produces heat • Engineers try and reduce the amount of friction to reduce waste heat

4. Useful Energy • The purpose of a machine is to convert the initial energy added to it into the types of energy needed to do the work you want done. • Energy Input- the initial energy source • Useful Energy Output- the desired energy needed to do work • Useful Work Output- the work the machine is supposed to do • The efficiency of a system can never be 100%

5. Efficiency • A measure of how effectively a machine converts energy input into useful energy output • Efficiency= useful work output total work input

6. Conversion of Mechanical Energy to Mechanical Energy • Percent efficiency= useful mechanical energy or work output X100 total mechanical energy or work input = Em(useful output) X100 Em( total input) OR = W useful output X100 W total input

7. Practice • A crane lifts an elephant cage into a new pen on the third floor of a building. The crane does 2.30 x 104 J of mechanical energy input, while doing 8.00 x 103 J of mechanical energy output. What is the mechanical efficiency of the crane?

8. Thermal Energy to Thermal Energy • Percent efficiency= heat useful output X100 heat total input

9. Practice • Jane is heating a pot of water to cook KD and it took 2.00 x 103 J of heat from the element. If 5.00 x 102 J of energy was actually used by the water, what was the percent efficiency of the stove element?

10. Thermal to Mechanical or Vice Versa • Percent efficiency = Em(useful output) X100 heat total input OR Percent efficiency= heat useful output X100 Em (total input)

11. Practice • What is the output energy of a machine with 62% efficiency if it has an energy input of 2594 kJ?

12. To Do • 1-3 on page 216-217 • 8 and 9 on page 220