# Calculating the diameter of a bubble

1. Jan 16, 2016

### jdawg

1. The problem statement, all variables and given/known data
Suppose the temperature is 80℃ and we have a soap bubble that has a pressure increase of 40 [Pa]. What is the diameter of the bubble?

2. Relevant equations
ΔP=4Y/R

3. The attempt at a solution
I'm very confused about how to find the coefficient of surface tension (Y).
In an example my professor gave us in class he gave us Y(20°C)= 0.0728 N/m. So I'm guessing the coefficient changes with temperature? How would I calculate Y(80°C)? Is there a formula I'm missing?

Also I saw a different version of the above formula where they had ΔP=4T/R. Under what condition would you be able to just neglect the coefficient of surface tension and plug the temperature straight in? Or maybe is that formula completely incorrect? Thanks!

2. Jan 16, 2016

### haruspex

3. Jan 17, 2016

### jdawg

Ok! That makes more sense. So how do you find the coefficient of surface tension?

4. Jan 17, 2016

### haruspex

Did you try the link I posted?

5. Jan 17, 2016

### jdawg

Does that formula only work for water though? Is there a formula like that for soap?

6. Jan 17, 2016

### haruspex

What if you assume that the difference between water and a soap solution is due to a difference in the critical temperature?

7. Jan 17, 2016

### jdawg

Then you could use that formula?

8. Jan 17, 2016

### haruspex

Yes. You could use the given datapoint for the soap solution and the formula to estimate the critical temperature for that soap solution. Then you could find the water temperature that should have the same coefficient as the soap solution at 80C.
Mind you, this is just guesswork. I have not been able to find any info online to confirm this approach.

9. Jan 17, 2016

### jdawg

I'll give it a shot, thanks for your help!