Efficiency of the heat engine?
The energy provided each hour as heat to the turbine in an electric power plant is 8.4 1012 J. If 6.3 1012 J of energy is exhausted each hour from the engine as heat, what is the efficiency of the heat engine?
When you talk about the efficiency of something, you are talking about how much of the energy you put in is put to useful work, in terms of product realized. If you input 8.4 x 10 12 Joules of energy and 6.3 Joules of the energy is dispensed as heat, simply divide the 6.3 by the 8.4, and multiply by 100% to get the percentage of energy turned into heat energy.
6.3 x 10 12/8.3 x 10 12 = .75 x 100% = 75%
This means 75% of the energy we put into the system is despensed in the form of heat energy. That leaves 25% that is actually put into use to turn the turbine to produce electricity. Another way to look at this is to look at it in terms of test performance; how would it be to study and prepare for a test, only to receive a grade of 25%, indicating 75% of the points available were not secured? Among most schools, a 25 on a test reflects dismal performance. Combustion and steam engines are notorious in their lack of efficent work performance. Most of the energy is surrendered to the environment in the form of heat energy, rather than producing the work they were designed for.