An electric iron rated 250 V has a coil of resistance 125Ω. What is the cost of using it for I hour at 10k per kWh?

Analyze the question: We need the electrical energy used by the iron in 1 hour, then multiply by the cost per kWh.

Concept:

• Power: P = V²/R
• Energy (kWh) = Power (kW) × time (h)
• Cost = Energy × rate

Solution:

1. Power of the iron: P = V²/R = (250)² / 125 = 62,500 / 125 = 500 W = 0.5 kW

2. Energy used in 1 hour: E = 0.5 kW × 1 h = 0.5 kWh

3. Cost at 10k per kWh: Cost = 0.5 × 10k = 5.00k

Tip: When you see a voltage and a resistance, go straight to P = V²/R. Then convert watts to kilowatts (divide by 1000) before multiplying by the time in hours and the rate. Most mistakes in these questions come from forgetting that conversion.