Question

A 60 W light bulb is left on for 8.0 h.

(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 amount of electrical energy that is converted into radiant energy (light) by a 60 W light bulb that operates for 8 hours, you can follow these steps:

1. **Calculate the total electrical energy consumed** by the light bulb in kilowatt-hours (kWh).

The power of the light bulb is 60 W, which we need to convert to kilowatts:
\[
\text{Power in kW} = \frac{60 \, \text{W}}{1000} = 0.06 \, \text{kW}
\]

Next, calculate the total energy consumed over 8 hours:
\[
\text{Energy (kWh)} = \text{Power (kW)} \times \text{Time (h)} = 0.06 \, \text{kW} \times 8 \, \text{h} = 0.48 \, \text{kWh}
\]

2. **Calculate the amount of radiant energy produced**, considering that the bulb is only 15% efficient.

Efficiency is expressed as a fraction (or percentage). Given that the bulb is 15% efficient, the energy converted into light (radiant energy) can be calculated as:
\[
\text{Radiant Energy} = \text{Total Energy} \times \text{Efficiency}
\]
\[
\text{Radiant Energy} = 0.48 \, \text{kWh} \times 0.15 = 0.072 \, \text{kWh}
\]

3. **Convert the radiant energy into a more common unit, if needed**. In this case, we can keep it in kWh, or convert it to joules if necessary:
\[
1 \, \text{kWh} = 3.6 \times 10^6 \, \text{J}
\]
\[
\text{Radiant Energy in joules} = 0.072 \, \text{kWh} \times 3.6 \times 10^6 \, \text{J/kWh} = 259200 \, \text{J}
\]

Thus, the electrical energy converted into radiant energy is approximately **0.072 kWh** or **259,200 J**.