An electric refrigerator rated 400 W operates 8 hours/day. Wha... | General Science MCQ

General SciencePrevious Year Paper 2024

An electric refrigerator rated 400 W operates 8 hours/day. What is the cost of the energy to operate it for 30 days at ₹3.00 per kW h?

₹400

₹36

₹288

₹9.6

Correct Answer

🔬 SC • Science Calculation & LawGeneral SciencePrevious Year Paper 2024

Option C

₹288

Quick Summary:

The correct answer is Option C ( $₹288$ ). The total electrical energy consumed by an appliance depends on its power rating, the duration of operation per day, and the total number of days.

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💡 Explanation

The correct answer is Option C ( $₹288$ ). The total electrical energy consumed by an appliance depends on its power rating, the duration of operation per day, and the total number of days.

💡 Everyday Analogy (Real-World Intuition)

If a light bulb consumes $1$ unit of electricity every day, in $30$ days it will consume $30$ units, and the total bill will simply be $30$ times the cost per unit.

🔢 Key Formulas

$E = P \times t$ — Electrical energy consumed

$1\text{ kWh} = 1\text{ Unit} = 3.6 \times 10^6\text{ J}$ — Commercial unit of energy

⚙️ Working Principle

Electrical energy ( $E$ ) consumed by an appliance is given by the product of its power rating ( $P$ ) and the total time ( $t$ ) for which it is used. Since commercial electrical energy is measured in kilowatt-hours ( $1\text{ kWh} = 1\text{ unit}$ ), power in watts is converted to kilowatts by dividing by $1000$ .

📌 Key Points

▸
Power rating of the refrigerator, $P = 400\text{ W} = \frac{400}{1000}\text{ kW} = 0.4\text{ kW}$ .
▸
Total operating time for $30$ days, $t = 8\text{ hours/day} \times 30\text{ days} = 240\text{ hours}$ .
▸
Total energy consumed, $E = 0.4\text{ kW} \times 240\text{ h} = 96\text{ kWh}$ .
▸
Total cost $= 96\text{ kWh} \times ₹3.00/\text{kWh} = ₹288$ .

🛠️ Applications / Uses

▸
Calculation of monthly electricity bills for household appliances.
▸
Designing energy-efficient electrical systems.

📄 Additional Information

▸
Option A ( $₹400$ ) is incorrect as it uses the power value directly without proper unit conversion.
▸
Option B ( $₹36$ ) is incorrect as it calculates energy for a single day instead of $30$ days ( $0.4 \times 8 \times 3 = ₹9.6$ is incorrect calculation error).
▸
Option D ( $₹9.6$ ) is incorrect because it represents the cost of energy consumed in just $1$ day ( $0.4 \text{ kW} \times 8 \text{ h} \times ₹3.00 = ₹9.6$ ).

📊 Diagram / Illustration

✅

C is correct — The total energy consumed in $30$ days is $96\text{ kWh}$ , costing $96 \times 3 = ₹288$ .

Core Concepts Used

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Electric Power Electrical Energy Consumption Commercial Unit of Energy (kWh)

💡 EXAM TIP

Always remember to convert power to kilowatts ( $\text{kW}$ ) and time to hours ( $\text{h}$ ) before calculating commercial energy units ( $\text{kWh}$ ).

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