# What pressure to take in surface tension question?

Pressure inside a water droplet is $0.32 N/cm^2$ if surface tension of liquid is $0.095 N/m$. We have to find the radius of the bubble

since it's a water droplet the formula used is $$P_i-P_o=\frac{2T}{R}$$ The $P_i$ is internal pressure wehreas $P_0$ is external pressure so the $P_0$ should be taken in consideration but the solution of the question directly puts $0.32$ in place of pressure difference. where I am doing wrong in the question.

• How does 0.32 N/cm2 compare to atmospheric pressure? – Jeffrey J Weimer Sep 17 '18 at 12:55
• @JeffreyJWeimer sorry - you and me both mate! – Solar Mike Sep 17 '18 at 12:56
• @Wasabi answer is already in the question, OP just needed a pointer not a 5 page essay. – Solar Mike Sep 17 '18 at 14:45

Have you considered that the solution is assuming that the pressure is gauge pressure ie above atmospheric...

So that means if you have the bubble pressure as an absolute pressure reading and you subtract the external atmospheric pressure you end up with the difference which is the value given in the solution.

• Please edit your answer to flesh it out. For inspiration, read this discussion on short answers (in the context of comments, but much of what is discussed applies to very short answers such as this one). – Wasabi Sep 17 '18 at 14:39

I think , a good hint is, the given value is the gauge pressure value.

• Let's see how long it takes @Wasabi to give you a comment.... Let us hope he does not discriminate... – Solar Mike Sep 17 '18 at 18:54
• Please edit your answer to flesh it out. For inspiration, read this discussion on short answers (in the context of comments, but much of what is discussed applies to very short answers such as this one). And @SolarMike, let's keep comments on-topic, please. – Wasabi Sep 17 '18 at 19:11
• Both answers are succinct and on-topic.... and correct – Solar Mike Sep 17 '18 at 19:23