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

Question

(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)?

(c) What happens to the energy that is not converted electrical energy?

Answer 1

To calculate the electrical energy used by the light bulb, we can use the formula:

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

(a) Electrical Energy Used

1. Power of the light bulb: 60 W (watts)
2. Time: 8.0 hours = 8.0 × 3600 seconds = 28800 seconds

Now, we can calculate the energy used:

\[ \text{Energy} = 60 , \text{W} \times 28800 , \text{s} \]

\[ \text{Energy} = 1728000 , \text{J} \text{ (joules)} \quad \text{or} \quad 1728 , \text{kJ} \text{ (kilojoules)} \]

(b) Electrical Energy Converted into Radiant Energy

If the light bulb is 15% efficient, the energy converted into radiant energy (light) can be calculated as follows:

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

1. Efficiency: 15% = 0.15
2. Total Energy: From part (a), we calculated it as 1728000 J.

Now we can compute the radiant energy:

\[ \text{Radiant Energy} = 0.15 \times 1728000 , \text{J} \]

\[ \text{Radiant Energy} = 259200 , \text{J} \quad \text{or} \quad 259.2 , \text{kJ} \]

(c) What Happens to the Remaining Energy?

The energy that is not converted into radiant energy is lost primarily as heat. In the case of a light bulb, most of the electrical energy is converted into heat due to resistance in the filament and other components, which is why incandescent bulbs are known for being inefficient. This heat energy typically dissipates into the surrounding environment, contributing to an increase in ambient temperature.