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A single-phase energy meter has a rating of 1200 revolutions per kilowatt-hour (kWh). If an electric appliance rated at 500 W operates continuously for 4 hours, how many revolutions will the meter complete?

Difficulty: Medium

Correct Answer: 2400 revolutions

Explanation:


Introduction / Context:
Energy meters (also called watt-hour meters) measure the energy consumed by electrical loads. They are calibrated such that a fixed number of revolutions of the meter disc corresponds to one kilowatt-hour of electrical energy. This question involves computing the number of revolutions for a given power, time, and meter constant.


Given Data / Assumptions:

  • Meter constant: 1200 revolutions per 1 kWh.
  • Load power: 500 W = 0.5 kW.
  • Time of operation: 4 hours.


Concept / Approach:
Energy consumed (kWh) = Power (kW) × Time (hours). Number of revolutions = Energy consumed × meter constant. By substituting the given data, we can directly calculate the revolutions.


Step-by-Step Solution:

Step 1: Power = 500 W = 0.5 kW.Step 2: Time = 4 h.Step 3: Energy consumed = 0.5 × 4 = 2 kWh.Step 4: Revolutions = 2 × 1200 = 2400 revolutions.


Verification / Alternative check:

If load was 1 kW for 1 hour, energy = 1 kWh → 1200 revolutions. Our case is 2 kWh, so 2 × 1200 = 2400. This matches.


Why Other Options Are Wrong:

1200 corresponds to 1 kWh, but here energy is 2 kWh.1800 and 2100 are arbitrary mismatches.1500 is not consistent with the meter constant.


Common Pitfalls:

Forgetting to convert watts to kilowatts or ignoring operating hours.


Final Answer:

2400 revolutions
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