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?

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?

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bandmanjoe's profile pic

bandmanjoe | Middle School Teacher | (Level 2) Senior Educator

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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.

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