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Question

a 100W light bulb is 20% efficient. That means 20% of the electric energy is converted to light energy.

a. how many joules does the light bulb convert into light each min it is in operation?

b. how many joules of thermal energy does the light bulb produce each min?

Answer 1

a. E = P*t
E = 100W * 60sec
E = 6000 Joules of energy
But it's only 20% efficient, so:
6000 * .2 = ___1200 Joules ___

b. The thermal energy will be whatever energy that doesn't turn into light, so 80% of the energy.
6000J was the total energy, so 6000 * .8 = ___4800 J ___

Answer 2

1 watt = 1 joule per second

a) 6000 joules per min * .20 eff = 1200 joules of light
b) the rest is thermal = 4800 joules