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)?

1 answer

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.