ITPEC FE Morning October 2021 Question 22
A device, which operates using a 100V household power supply and through which a current of 10 A flows during operation, is operated from 0 minutes to 120 minutes as shown in the figure below. How much electrical energy, in watt-hour, is consumed during this time? Here, the value of the voltage and current is the effective value, no current flows when the device stops, and the power factor is 1.
Explanation
1,500 Wh — power times the hours the device actually runs.
- •Power: P = V × I = 100 × 10 = 1,000 W (power factor 1)
- •Operating time from the chart: 0–60 minutes and 90–120 minutes, 60 + 30 = 90 minutes = 1.5 h; from 60 to 90 minutes it is stopped and draws no current
- •Energy: 1,000 × 1.5 = 1,500 Wh
Answer (c)
Why not others:
- •(a) 1,000 — counts only the first hour, 0–60 minutes, and misses the second run from 90 to 120 minutes
- •(b) 1,200 — would need 72 minutes of operation, which no reading of the chart gives
- •(d) 2,000 — charges the full 120 minutes, ignoring the 30 minutes the device is stopped
Key rule: Energy in watt-hours is power in watts times time in hours: add up only the intervals when current flows and convert minutes to hours before multiplying.
Hardware · AI-written, may contain errors
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