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:

This is based on the second law of thermodynamics, which states that energy conversions are never 100% efficient. In any energy system (like burning coal for electricity), some energy is always lost as waste heat or other unusable forms, so the useful energy output cannot exceed or equal the total energy input.

Answer:

Useful energy is always less than energy input.