Question

A 60 W light bulb is left on for 8.0 h.
(a) How much electrical energy will it use?

(b) If the light bulb is only 15% efficient, how much electrical energy will be converted into radiant energy (light)?

Answer 1

To calculate the electrical energy used by the light bulb and the energy converted into radiant energy, we can use the following formulas:

### (a) Electrical Energy Used

Electrical energy (in watt-hours) can be calculated using the formula:

\[
\text{Energy} = \text{Power} \times \text{Time}
\]

Where:
- Power = 60 W
- Time = 8.0 hours

Now substituting the values:

\[
\text{Energy} = 60 \, \text{W} \times 8.0 \, \text{h} = 480 \, \text{Wh}
\]

To convert watt-hours to joules (since 1 Wh = 3600 J):

\[
480 \, \text{Wh} = 480 \times 3600 = 1,728,000 \, \text{J}
\]

So, the electrical energy used is **1,728,000 J** or **1.728 MJ**.

### (b) Electrical Energy Converted into Radiant Energy

If the light bulb is only 15% efficient, we can calculate the radiant energy using the efficiency formula:

\[
\text{Radiant Energy} = \text{Efficiency} \times \text{Total Energy}
\]

Where:
- Efficiency = 0.15 (15%)
- Total Energy = 1,728,000 J (from part a)

Now substituting the values:

\[
\text{Radiant Energy} = 0.15 \times 1,728,000 \, \text{J} = 259,200 \, \text{J}
\]

Thus, the electrical energy converted into radiant energy (light) is **259,200 J** or **259.2 kJ**.