Efficiency of a transformer is equal to

Analysis

The question asks for the definition of the efficiency of a transformer. In physics, efficiency is a measure of how well a device converts input energy into useful output energy.

Concept Explanation

A transformer is an electrical device that changes voltage levels. Ideally, a transformer would have 100% efficiency, meaning no energy is lost. However, in reality, energy is lost due to factors like heat (resistance in the coils), eddy currents, and magnetic flux leakage.

The efficiency ($\eta$) of any machine or electrical device is defined as the ratio of the useful output power to the total input power, expressed as a percentage:

In a transformer:

• Input Power is the power supplied to the primary coil ($P_p = V_p I_p$).
• Output Power is the power developed in the secondary coil ($P_s = V_s I_s$).

Step-by-Step Solution

1. Evaluate Option A: This is vague and does not specify what property of the coils is being measured. It is incorrect.
2. Evaluate Option B: This is the formula for the efficiency of a simple machine (like a pulley or lever), not an electrical transformer.
3. Evaluate Option C: This is the formula for the turns ratio of a transformer, which relates voltage and current, but it does not define efficiency.
4. Evaluate Option D: This correctly identifies the ratio of output power (secondary) to input power (primary) as the definition of efficiency.

Conclusion

The efficiency of a transformer is the ratio of the power output from the secondary coil to the power input at the primary coil. This matches the definition provided in option D.

Exam Tip: Always remember that "Efficiency" in physics is almost always (Output / Input) × 100%. If you are ever confused in an exam, look for the option that represents "what you get" divided by "what you put in."