NECO 2002 Physics · Question 15

A transformer supplies 12V when connected to a 240V mains. If the transformer takes 0.120A from the mains when…

Question

A transformer supplies 12V when connected to a 240V mains. If the transformer takes 0.120A from the mains when used to light a 12V, 24W lamp, what is its efficiency?

A.57.20%
B.66.60%
C.83.30%
D.92.50%
E.100%

Answer: C. 83.30%

Explanation written with AI and checked against the answer key

Analyze the question: A transformer steps 240 V down to 12 V. It feeds a 12 V, 24 W lamp and draws 0.120 A from the mains. We need its efficiency.

Concept: Efficiency = (output power ÷ input power) × 100%

Output power = power delivered to the lamp = 24 W
Input power = primary voltage × primary current

Solution:

Input power = 240 V × 0.120 A = 28.8 W

Efficiency = (24 ÷ 28.8) × 100% = 83.33%

Answer: This matches option C (83.

Tip: In transformer efficiency questions, always compute the input power as V_p × I_p and compare it with the useful output power, which is often given as the lamp's wattage. The efficiency can never exceed 100%, so if you get a value above 100%, you have swapped input and output. Also, 24/28.8 simplifies to 5/6, which is about 83.3%. Spotting that fraction saves time in the exam.

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