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 equal to energy input.
useful energy is always greater than energy input.

Explanation:

According to the law of energy conservation and the concept of energy efficiency in systems (such as power - generation systems like burning coal for electricity), energy is conserved overall (total energy input equals total energy output in a closed system considering all forms of energy). But useful energy (the energy we intend to use, e.g., electrical energy in this case) is less than the total energy input. This is because some energy is lost as waste heat (due to inefficiencies in the conversion process, like heat loss in a power plant), friction (in moving parts of machinery in the plant), etc.

Answer:

Useful energy is always less than energy input.