B3.1 Laws of Thermodynamics. 1 st Law of Thermodynamics Energy cannot be created nor destroyed, only converted from one form to another Example: Flashlight.

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Presentation transcript:

B3.1 Laws of Thermodynamics

1 st Law of Thermodynamics Energy cannot be created nor destroyed, only converted from one form to another Example: Flashlight chemical electrical light + heat energy The total energy of a system is always conserved 2 nd Law of Thermodynamics Heat energy always flows from hot to cold objects In every type of energy conversion, some energy is converted to thermal energy We say energy is “lost” because it is not useful to do work (example: heat produced by engine) Since some energy is always lost, no machine is 100% efficient at converting energy to useful forms

"It may be perpetual motion, but it will take forever to test it." People have attempted to create the perfect machine called a perpetual motion machine in which 100% of the input energy is converted to mechanical energy Once energy was added to the machine, it could operate indefinitely. Since no machine is 100% efficient, it is useful to calculate how efficient a machine is The formula % Efficiency = useful energy output X 100% total energy input

B3.3 Useful Energy and Efficiency Pages

Useful Energy The purpose of any machine is to convert an energy input into a useful energy output which is used to do work –Example: a flashlight converts potential chemical energy from a battery into light and heat energy No machine is 100% efficient. Some energy will be converted to wasted energy (eg friction of pistons of engine, heat in above example)

Efficiency Measures how effectively a machine converts energy input to useful energy output To Calculate Efficiency: Note: Work may be substituted for Energy % Efficiency = Useful Energy Output X 100% Total Energy Input

Examples: 1) It takes 820 J of work to drag a 15 kg box to the top of a 2.5 m high ramp. Calculate the efficiency of the ramp. 2.5 m

Example 2 A 100 kg load was pulled at a constant speed up a ramp to a height of 15 m by a force of 1120 N. Calculate the % efficiency of the ramp. 15 m

Example 3 A small motor has an efficiency of 85%. In lifting the load, it produces 15 J of mechanical energy input. Calculate the useful mechanical energy output of the motor. (13J)