Question

Ten 100 W light bulb burns on an average of 10 hours a day for one week. The weekly consumption of energy will be ____ units.

• A. 7
• B. 70
• C. 0.7
• D. 0.07

Hint:

Think of a "unit" as keeping a 1000-Watt appliance (like a small heater) running for exactly one hour. Here, your 10 bulbs combined act like that 1000-Watt appliance.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Electrical energy consumption is measured in "units." In commercial and domestic billing, 1 unit is equivalent to 1 kilo-Watt-hour (kWh).

Step 2: Key Formula or Approach:
\[ \text{Energy (units)} = \frac{\text{Total Power (Watts)} \times \text{Time (Hours)}}{1000} \]

Step 3: Detailed Explanation:
1. Total Power: There are 10 bulbs of 100 W each. \[ \text{Total Power} = 10 \times 100 = 1000\text{ W} = 1\text{ kW} \] 2. Total Time in a week: 10 hours a day for 7 days. \[ \text{Total Hours} = 10 \times 7 = 70\text{ hours} \] 3. Energy Calculation: \[ \text{Energy} = 1\text{ kW} \times 70\text{ hours} = 70\text{ kWh} \] Since $1\text{ kWh} = 1\text{ unit}$, the consumption is 70 units.

Step 4: Final Answer:
The weekly consumption of energy is 70 units.

Answer 2

The question asks for the weekly electrical energy consumption of ten 100 W bulbs run for 10 hours a day, expressed in "units" (kWh). Let's work this out using a per-day approach and check each option.

1. 7: This value would only result from a serious under-calculation, such as treating just one bulb's daily consumption in kWh without accounting for all ten bulbs or the full week, so it is far too small.
2. 70: First, find the energy used by all ten bulbs in a single day: total power is \( 10 \times 100\text{ W} = 1000\text{ W} = 1\text{ kW} \), and running for 10 hours gives \( 1\text{ kW} \times 10\text{ h} = 10\text{ kWh} \) per day. Over a full week of 7 days, the total becomes \( 10\text{ kWh/day} \times 7\text{ days} = 70\text{ kWh} \), and since 1 kWh equals 1 unit, this is 70 units.
3. 0.7: This figure is off by two orders of magnitude from the correct value, likely arising from a decimal or unit-conversion slip, so it is not correct.
4. 0.07: This is even smaller and far too low to represent the energy used by ten 100 W bulbs running for 70 combined hours, ruling it out as well.

Calculating the daily consumption first and then scaling up to a full week confirms the total weekly energy usage.

Therefore, the correct answer is 70 units.