What is the engine's efficiency?
A heat engine absorbs 787 J of energy per cycle from a high-temperature source. The engine does 4.20 102 J of work during each cycle, expelling 3.67 102 J as heat. What is the engine's efficiency?
Efficiency is a reflection of how well an engine is doing the work it was designed to do. Combustion and heat engines are celebrated for doing the work they do, but a lot of the energy put into them is surrendered to the surrounding environment as heat energy. To calculate the efficiency of this engine, divide the 3.67 x 10 2 Joules converted to heat energy by the 4.2 x 10 2 Joules of energy put into the system to begin with, then multiply by 100%. This will give the amount of energy converted to heat energy, percentage wise.
3.67 x 10 2 J/4.2 x 10 2 J = .87 x 100% = 87%
This means 87% of the energy put into the system was converted to heat energy, while 13% was actually work output the engine was designed to do. Another way to look at this would be if you scored a "13" on a test you were expecting to make a "100". That would be looked upon as a rather bad efficiency rating, in terms of test-taking.