Efficiency of a Light Bulb: Calculating Energy Transfer

[Molly1235]

Homework Statement

"A light bulb is 85% efficient. If it is supplied with 60J of electrical energy, how much of this energy is transferred to its surroundings as light energy?


The attempt at a solution

So I calculated that 85% of 60 is 51J, however I don't know if being "efficient" refers to energy that remains in the circuit, or energy that is released as light energy, seeing as this is the main purpose of a light bulb? I'm thinking it's the first, which would mean 9J is lost as light energy?

 

[Millacol88]

I think it's safe to assume that the useful energy in this case is the light being emitted from the bulb. Which would mean that 51 J of light energy is emitted. The other energy doesn't "remain in the circuit" it is lost to heat.

 

[Molly1235]

I think it's safe to assume that the useful energy in this case is the light being emitted from the bulb. Which would mean that 51 J of light energy is emitted. The other energy doesn't "remain in the circuit" it is lost to heat.

Ah ok! Thank you very much :)