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Liquid Pressure

# Our Objective

To show that liquid pressure depends only upon the height of the liquid column and not the volume of the liquid.

# The Theory

The force acting on a unit area of a surface is called pressure. Pressure is exerted by solids, liquids and gases. The pressure exerted by liquid is called liquid pressure.

The liquid pressure at a point is due to weight of the liquid above it. As the liquid at lower depth has to bear the entire weight of the water above it, the liquid pressure increases with depth.

In addition, the particles or molecules in a liquid can move in all direction. As they move, they collide with each other as well as the walls of the container. This collision causes pressure which is exerted in all directions. When kept in a container, the liquid exerts pressure on the walls of the container. Hence if we make a hole on the wall of the container the liquid flows out due to this pressure.

If the hole in the container is near the bottom, then due to greater pressure, the liquid will be pushed out with greater force and they will cover a larger distance before falling down. On the other hand, when the liquid column above the position of hole is less, liquid stream gushing out will fall closer to the container.

Consider the case where water is taken up to the same height in containers of different size. What do you think will happen if we make a hole in these container at same height from the bottom? Since at the same depth, the pressure is exerted equally in all direction, the water will come out and fall at same distance. This distance does not depend on the shape or volume of the container only on the vertical depth of water column. In other words, the liquid pressure depends only upon the height of the liquid column and not the volume of the water in the container.

Increase in pressure of liquid with depth can be felt as a painful or unpleasant feeling in your ears when you try to dive deeper in a river or an ocean. The deeper you go under the sea, the greater the pressure of the water pushing down on you. The submarines are thus designed to withstand such high pressures underwater. Similarly, the sea creatures living at such depth have special features which help them survive deep below the surface.

In fact the pressure of water is used to generate electricity in hydroelectric projects. Huge dams are built on rivers with turbines kept at the bottom. The huge pressure of water due to several meters of water column present above it creates enough pressure to rotate the turbine and generate electricity. The increase in pressure with depth is also the reason why dams have thicker walls at the bottom.

# Learning Outcomes

• Students learn about liquid pressure.
• Students learn to find application of liquid pressure in daily life.