A motor of a corn mill machine operates on electrical energy of 1.44 × 10^ 5 J. If one-quarter of the energy…

Analyze the question: The motor takes in 1.44 × 10⁵ J of electrical energy. One-quarter of this is wasted as sound and heat. We need the efficiency.

Concept: Efficiency = (useful energy output ÷ total energy input) × 100%

Solution:

1. Total input energy = 1.44 × 10⁵ J
2. Wasted energy = ¼ × 1.44 × 10⁵ = 0.36 × 10⁵ = 3.6 × 10⁴ J
3. Useful output energy = 1.44 × 10⁵ − 0.36 × 10⁵ = 1.08 × 10⁵ J
4. Efficiency = (1.08 × 10⁵ ÷ 1.44 × 10⁵) × 100% = 0.75 × 100% = 75%

Shortcut: if ¼ is wasted, then ¾ is useful, and ¾ = 75%.

Answer: The efficiency is 75%, which is option D.

Tip: Don't be distracted by the big number 1.44 × 10⁵ J. It cancels out when you work out efficiency. Option A (25%) is the trap: it is the percentage wasted, not the efficiency. Always ask yourself, "Is this the useful part or the wasted part?"