# Energy conversion efficiency (Light -> heat)

1. Feb 12, 2014

### Tah

Hi, all

I'm seeking advice on how to calculate the energy conversion efficiency, in case of heat from light energy.

I think that one of the general formulas 'energy output/energy input' can not be applied in this problem.

Assuming that light(or laser) has 10W power output and material 1 exposed to the light source generates heat from 20C' to 30'C for a second, while material 2 generates heat from 20'C to 40'C for a second.

In this case, we can say that the energy conversion efficiency is better for material 2, but I want to describe this as a quantitative expression.

Please can anyone help me?

2. Feb 12, 2014

### Simon Bridge

Note: temperature and heat are different things. You need the material heat capacity so you can relate the power delivered by the laser to the rate heat-energy increases.

3. Feb 12, 2014

### Tah

Sorry for that, I didn't realize that.

Heat capacity can be 3600 J/(kg*K)

4. Feb 12, 2014

### Staff: Mentor

All energy expended by a laser (and most other devices) eventually becomes heat (thermal energy). The only real question is how much is absorbed and how much is reflected.

If the laser is the same and only the material is different, then the difference can either be in how much is reflected or what the heat capacity is (or both).

5. Feb 12, 2014

### Simon Bridge

OK - so can you work out the energy needed to get the temperature changes that were observed?
What is the definition of "energy conversion efficiency"?