Efficiency as Useful Output Relative to Total Input

Steps:

• Recall the standard definition of efficiency in physics: it quantifies how much of the input energy becomes useful work.
• Examine each option to identify which matches the formula η = (useful energy output / total energy input) × 100%.
• Eliminate options describing differences (subtraction) rather than ratios (division).
• Confirm the ratio-based option aligns with energy conservation principles.

Why C is correct:

• Efficiency is defined as the ratio of useful energy output to total energy input, per the first law of thermodynamics, ensuring no energy creation or destruction.

Why the others are wrong:

• A: Describes net energy gain, not efficiency, which ignores the proportional conversion.
• B: Compares output to waste, not to input, violating the input-output ratio standard.
• D: Ratios output to waste only, excluding total input and misrepresenting overall performance.

Final answer: C