Amount of water evaporated by laser, questionable results.

In summary, a laser with 30kW output energy is pointed at a boiling pot, and it evaporates 13-14 grams of water per second.
  • #1
Jarfi
384
12
Assume a laser with 30kW output energy is pointed at a boiling pot, how much water would it evaporate per second?

30kW=30kJ/s.

I assume the water is at boiling point and get:

Water heat of vaporization (40,65 kJ·mol−1).

Thus we get:

(30kJ/s)/(40,65kJ/mol)=0,74mol/s evaporated.

This gives us(using the molecular mass of water)

0,74mol/s*18g/mol=13,2 g/s.

Results are: 13 grams of water are evaporated per second if a 30kW laser is heating the pot.

I find this very little and have assumed more water would have been evaporated, are any of these calculations wrong?
 
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  • #2
That looks correct. The problem is you thinking that 13,2 g/s is not much. It is an incredibly fast evaporation: it would take 76 s to completely boil away one liter of water. Your pasta won't have time to cook!
 
  • #3
Your calculations are correct.
The problem is you still don't have a good sense about units you're using.
Try to "understand" what is a joule, a second, a watt!
 
  • #4
I always keep in mind the order of magnitude to evaporate water: 2.5 GJ/ton = 2.5 kJ/g.
So, you are right.
 
  • #5
The latent heat of water is about 2.2 kJ/g.
Your laser provides 30 kJ in one second. So it may evaporate about 13-14 g /s.
Your calculation seems OK.

But you assume that all the laser light is absorbed by water, which is questionable.
The fraction absorbed depends on the wavelength of the laser. But ti is not very likely to be 100%.
If it's visible light, won't be too much absorption.

edit. So many people answering at the same time. :smile:
 
  • #6
nasu said:
The latent heat of water is about 2.2 kJ/g.
Your laser provides 30 kJ in one second. So it may evaporate about 13-14 g /s.
Your calculation seems OK.

But you assume that all the laser light is absorbed by water, which is questionable.
The fraction absorbed depends on the wavelength of the laser. But ti is not very likely to be 100%.
If it's visible light, won't be too much absorption.

edit. So many people answering at the same time. :smile:

Thanks for the help guys,

It is light in the infrared to ultraviolet spektrum.

I am glad my calculations are ok and fully aware that I was making assumptions,

not including: Loss of power through distance traveled through air, amount of EM-waves reflected and amount of the beam that would miss the pot(depending on the distance from the laser.)

Which would make even less water boil !
 

Related to Amount of water evaporated by laser, questionable results.

1. How does a laser evaporate water?

Laser evaporation uses a high-powered laser beam to heat the water molecules, causing them to become excited and escape into the air as vapor.

2. Can a laser really evaporate water?

Yes, a laser can be used to evaporate water, but the amount of water that can be evaporated depends on the power of the laser, the duration of exposure, and the properties of the water.

3. What are some possible reasons for questionable results in water evaporation experiments using a laser?

Possible reasons for questionable results in water evaporation experiments using a laser include insufficient power or duration of exposure, variations in the water's temperature or composition, and interference from external factors such as air flow or humidity.

4. Is laser evaporation a reliable method for measuring the amount of water evaporation?

Laser evaporation can be a reliable method for measuring the amount of water evaporation, but it is important to carefully control and consider all variables and potential sources of error in the experiment.

5. What are some potential real-world applications of using a laser to evaporate water?

Laser evaporation can be used in various industrial and scientific applications, such as in desalination processes, in the production of solar panels, and in research studies on the effects of water evaporation on different surfaces and materials.

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