Efficiency


 * __Efficiency__**

Whenever a body does work, not all of the energy that it takes in is transfered as work done, some of it is wasted. The amount of energy that is transfered in a useful way compared to the amount that is wasted is called the efficency. In pther words the efficency is the proprtion of the input that is transfered usefully. The equation for efficency of energy transfer is:

efficency = useful output ÷ input

When you do work, for instance runing up stairs, you body converts glucose from your food into energy by combining it with oxygen, however not all the energy is transfered to work, some is lost, especially as heat.



This is the same for any machine, engine or mechanical device, the useful output is always less than the input. The more efficent the device the less of the input is wasted; a bigger proportion of it is a useful output.

A heat engine transfers energy from a hot source to work done but some of the energy is lost to cold sink (its surroundings).

The law of convervation of energy can be applied to the heat engine: energy from hot source = Work done + energy given to cold sink Q1 = W + Q2 which can be rewritten as W = Q1 - Q2 the efficency is the ratio of work done to Q1, the energy taken by heating from the high temperature source. efficency = W ÷ Q1 = (Q1 - Q2) ÷ Q1 = 1 - Q2 ÷ Q1 There is always some energy flow to the cold sink so a heat engine is never 100% efficient.



It can be shown that the maximum efficiency of a heat engine depends on the Kelvin temperatures of the hot source (T1)and the cold sink (T2).

Maximum efficency = 1 - T2 ÷ T2

Only if the temperature of the sink T2 was 0K could the efficency be 100%. For thermal power stations the sink temperature has a minimum os 373K ( =100°C, boiling point of water). Turbine designers are constantly looking for higher temperature materials so they can rasie T1 source temperature to increase efficency. Todays turbines operate at around 873K (600°C)

Therefore for them theorically:

Maximum efficiency = 1 - 373K ÷ 873K = 57%

in practice most efficiencies are about 40%

Josie Wall 12N