Efficiency Formula for Energy Transfer

Steps:

• Recall that efficiency measures how much input energy becomes useful output.
• Identify the standard formula: efficiency equals useful energy transferred out divided by total energy supplied in.
• Compare choices to this formula, noting that "useful" specifies the effective portion.
• Eliminate options that include total output or power, as they mismatch the energy transfer definition.

Why A is correct:

• It matches the physics definition of efficiency as the ratio of useful energy output to total energy input, often expressed as a percentage.

Why the others are wrong:

• B includes all output, ignoring wasted energy, so it overestimates efficiency.
• C divides by total output, which doesn't relate input to useful work.
• D uses power instead of energy and reverses input/output, misapplying the concept.

Final answer: A