Question

how does the useful energy output from a system (making electricity by burning coal, for example) compare to amount of the energy input into a system?

useful energy is always less than energy input.

useful energy is always greater than energy input.

useful energy is always equal to energy input.

Explanation:

In energy conversion systems like burning coal to make electricity, some energy is lost as waste (e.g., heat, friction). So, useful energy output can't exceed or equal input.

Answer:

Useful energy is always less than energy input.