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Q & A: What is the engine efficiency?

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Most recent answer: 10/10/2013
Q:
My teacher didn't explain very good and he is giving us problems like an engine transforms 1000J of chemical potential energy to 300J of kinetic energy, and 700 J into wasted heat and sound energy. What is the efficiency? I don't know what to do with the wasted energy number. Sorry, I submitted with the wrong email address the first time.
- Luke Vizcarra (age 13)
Montevideo, Uruguay
A:

In general, we don't answer school exercise questions. Perhaps we should modify that policy to help with a maximum of one per country, in honor of receiving our first question from Uruguay.

By engine "efficiency" your teacher just meant this ratio: (useful energy output)/(total energy input).
It sounds here that only the part called "kinetic energy" was in the form of usable mechanical energy. In more standard terms, that might be called "work", since the "heat and sound" also is largely kinetic energy, on a microscopic scale.

Mike W.


(published on 10/10/2013)

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