## 2008-04-09

### Understanding Pressure in Liquids

1. For a liquid at rest, the pressure at a certain point in the liquid is the same in all directions.

2. The pressure in a liquid is due to

a) Density of the liquid, p.
b) Depth of the liquid, h, below the surface liquid.
c) Acceleration of the gravity, g.

3. The pressure on a liquid is proportional to the density of the liquid, p and the depth, h, at which the liquid is measured.

4. The pressure in a liquid at rest (static liquid) is independent of the shape (area and slope) of the container.

5. The applications of pressure in liquids are:
i) Dams
ii) Domestic Water supplies

i) Dams

Dams are very much thicker at the bottom than at the top, since the pressure at the bottom is the greatest.

Large dams are built for the hydroelectric generation of electricity.

The high pressure on the deep-water side of the dam causes water to flow through these holes at great speed turning the turbines in the holes and generate the electricity.

ii) Domestic Water Supplies

The main water comes from a reservoir but in order to maintain a constant high pressure to the consumer, it is pumped to the top of a water tower located on high ground.

The main pressure is determined by the height, h.

Hope this helps!

#### 3 comments:

Student 2 said...

In my recent mid year exam the Q in paper 2 asked " what is meant by pressure in a liquid ?" How to answer this question rightly? I didnt get any marks when I answered " pressure in a liquid is the pressure acting on the surface of an object or the container in which the liquid is placed."

Dean James said...

Dear student 2,

to answer the question rightly you have to answer like this

Pressure in a liquid is the pressure exerted on all surfaces of an object by the liquid at a particular depth in which the object is placed. It is equal to P=hpg.

you have to add more elaboration and details to your answer. Thanks.

By the way, not all teachers have the same way of giving marks. It depends to their jurisdiction and decision whether your answer satisfies their scheme or not. Therefore, it is always nice to give as much details as possible in your answer and avoid vauge answers. Answers should be directly to the point.

student 2 said...

Thanx to Mr Dean James for the response. I'll try to include more elaboration in case similar questions or almost similar questions come out again.