Efficiency Elliott

Energy Efficiency No device is ever 100 % efficient.  You always have to put more energy in than you get out.  This is due to: Friction in the moving parts; Thermodynamic losses.

Sankey Diagrams Sankey diagrams can also show the quantitative nature of the losses by having a scale. In this diagram we can see that a scale has been used, and the diagram has been plotted on graph paper.

This diagram shows that for a car, 1000 J is put in (from the chemical energy in the petrol).  Only 300 J of energy are converted into kinetic energy that moves the car along the road. 700 J are wasted as how grade heat to warm up the surroundings.  While it is possible to recover heat, for example in the interior heater or a turbo-charger, the heat is eventually wasted.

Calculating Efficiency

Check Your Progress An electric winch takes 500 A from a 12 V source.  It is 25 % efficient.  What is its useful power?

Answer Total Power = 500 A × 12 V = 6000 W Useful power = 25 × 6000 W = 1500 W                        100

Perpetual Motion If a machine had an efficiency of 100 %, it would be a perpetual motion machine.  People have been trying to make these for centuries, without success The picture shows a possible perpetual motion machine.  The motor shafts are joined by glue. The idea of this device is that one of the motors acts as a generator, while the other motor drives the generator.  And what the generator produces drives the motor. Explain why the perpetual motion machine will not work. 

Answer In this device both motors will act as generators and will oppose each other.Even if it were possible to get one of the motors to act as a generator, and the other to drive it, energy will be lost as heat due to: resistance in wires; friction in the bearings. resistance in coils of both motors.

Energy Chains Nuclear energy is converted into heat. Heat boils water to steam Heat in the steam is converted into kinetic energy in the turbines Which is converted into electrical energy in the generator.

Kinetic Energy

Potential Energy If an object falls, the potential energy is turned into kinetic energy.  Then we combine the equations for Ep and Ek (conservation of energy):

Check Your Progress Calculate the kinetic energy of a 4 kg shot-put thrown by an athlete at a speed of 15 m s-1. 2 A coin is dropped from the viewing platform of an observation tower 80 m high.   How fast will it travel just before it hits the ground? Explain why you do not need the mass of the coin in the question above.

Answer Kinetic energy = 1/2 mv2 = 1/2 × 4 kg × (15 m s-1)2 = 1/2 × 4 kg × 225 m2 s-2 = 450 J Potential Energy = Kinetic Energy mgΔh = 1/2 mv2 m terms cancel out  Rearranging: v2 = 2gΔh v2 = 2 × 10 m s-1 × 80 m = 1600 m2 s-2 v = 40 m s-1 We don't need the mass as the masses cancel out.