If a 60 W light bulb in a bedside lamp is only 20% efficient and converting electrical energy into light energy, how many joules of light energy does this light bulb produce every minute?
The formula to calculate the total number of Joules of energy by the light bulb is as follows:
E (Joules) = P (Watts) X t (seconds)
E = 60 W X 60 seconds
E = 3600 Joules of energy
The total amount of energy generated by the light bulb every minute is 3600 Joules. If the light bulb is only twenty (20) percent efficient in converting that energy to visible light energy, that would be twenty percent of the total.
3600 Joules times .2 = 720 Joules
Of the total energy, 3600 Joules, only 720 Joules is converted to visible light energy. Old fashioned, incandescent bulbs with filaments were notoriously inefficient, converting much of their energy in the form of heat energy. Only a small portion of their energy was spent for what it was designed for, visible light. Newer bulb designs use more energy for visible light and spend less energy on heat, making their use more efficient.