0
$\begingroup$

I have the following question

enter image description here

And I have the formula for finding the pressure difference which is as follows:
$$P(below) = P(above) + ρgh $$ And since they provide the information that it is vacuum, then Patm > Pabs.

Here is the solution:
enter image description here

I don't really understand why it is minus ρgh and I'm confused how to use the fact that it is vacuum. Also, what can we usually consider P(below) and P(above)?

Improve this question
$\endgroup$

1 Answer 1

Reset to default
1
$\begingroup$

You're right, the question is confusing because there is no vacuum here.

The minus sign is because the pressure in the pipe + the pressure due to 3cm of liquid = atmospheric pressure. The pressure in the pipe is below atmospheric pressure.

The pressure relative to atmospheric pressure is called gauge pressure. So the gauge pressure is $-\rho g h$. Pressure relative to zero is absolute pressure, $P_0-\rho gh$.

In this case $P(below)$ is the pressure at the lower side of the manometer, ie atmopheric pressure $P_0$. $P(above)$ is the pressure on the higher side of the manometer, ie the pressure in the pipe.

Improve this answer
$\endgroup$
1
  • $\begingroup$ but what if you don't have a drawing, how would you assume what is the lower side of the manometer and what is the higher side? $\endgroup$
    – Jack
    Commented Oct 27, 2016 at 4:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.