## Pressure and Depth

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
either because they fall outside the main syllabus and therefore would be distraction (however interesting) or for other reasons.

Moderator: msmod

Albin0Rhin0
Posts: 1
Joined: Tue Mar 11, 2014 5:49 pm

### Pressure and Depth

How deep is a lake if bubbles are rising from the bottom of the lake and expanding by three times by the time they reach the surface. The density of the water is 1000 Kg/m and the temperature of the water is the same as what it would be at sea level.

I've been given the formula P= pgh (Pressure = density x gravity x height)
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Pressure and Depth

That formula is for the change in pressure. At the surface, it is atmospheric pressure, about 100 kPa.
Because bubbles are small, they will be close to the temperature of the water and won't change much. So you can use the gas equation PV = nRT and the RHS is constant.
(There is a small effect due to surface tension which is probably negligible in this one-sig-fig problem, unless the bubbles are very small.)

Joe