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General SciencePhysics: Electricity

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A 100 W bulb is used for 5 hours daily. How many units of electricity (kWh) are consumed in 30 days?

15 kWh

30 kWh

45 kWh

60 kWh

Correct Answer

⚙️ TE • Technical Concept & PrincipleGeneral SciencePhysics: Electricity

Option A

15 kWh

Quick Summary:

Electrical energy consumption is calculated as the product of power (in kilowatts) and time (in hours). A unit of electricity, commonly known as 1 kWh, represents the energy consumed by a 1000 W appliance operating for 1 hour.

🔬SCScience SolutionConcept & Principle

💡 Explanation

💡 Everyday Analogy (Real-World Intuition)

Think of 1 kWh as a 'bucket' of energy; using a 1000 W bulb for one hour fills one bucket, so using a 100 W bulb for 10 hours fills the same bucket.

🧠 Memory Mnemonic / Shortcut Aid

P-H-D: Power (kW) $\times$ Hours $\times$ Days = Total Units.

🔢 Key Formulas

$P_{\text{kW}} = \frac{P_{\text{W}}}{1000}$ — conversion of Watts to Kilowatts

$E = P_{\text{kW}} \times h \times d$ — calculation of total energy in kWh

⚙️ Working Principle

The total energy consumed is determined by the formula $E = P \times t$ . Since the power is given in Watts (W), it must be converted to Kilowatts (kW) by dividing by 1000. Over a period of $n$ days, the total energy is calculated as $E = \frac{P \times h \times n}{1000}$ , where $P$ is power in W, $h$ is hours per day, and $n$ is the number of days.

📌 Key Points

▸
1 kWh is equivalent to 1 unit of electricity on your utility bill.
▸
Always ensure time is in hours and power is in kilowatts before multiplying.
▸
The 30-day period effectively increases the total operational hours.

✅ Advantages

▸
Standardized unit for billing by utility companies globally.

❌ Disadvantages / Limitations

▸
Does not account for reactive power in AC circuits (power factor).

🛠️ Applications / Uses

▸
Domestic electricity meter billing.
▸
Energy auditing of household appliances.

📄 Additional Information

▸
Calculation: $E = \frac{100 \text{ W}}{1000} \times 5 \text{ hours/day} \times 30 \text{ days} = 0.1 \text{ kW} \times 150 \text{ hours} = 15 \text{ kWh}$ .
▸
Option B (30 kWh) results from neglecting the division by 1000 or using incorrect days/hours.

📊 Diagram / Illustration

✅

A is correct — The total electricity consumption is 15 kWh based on the formula $E = \frac{P \times t}{1000}$ .

Core Concepts Used

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Power consumption Energy conversion Commercial unit of energy

💡 EXAM TIP

In numerical physics problems involving billing, always check if the power is in Watts (W) or Kilowatts (kW) before you start calculating!

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