Pressure in fluids

• Liquids and gases are both fluids. Fluids can flow or be poured, because the particles in them are able to move around freely and can change positions.

• Liquids and gases exert pressure on any objects immersed in them and on surfaces in contact with them, for example the walls of their containers. This is because the particles in a fluid are free to move around and can therefore expand and ‘bounce’ off surfaces.

• The movement and space between particles in a fluid affects the amount of pressure the fluid exerts. This means that the pressure in a fluid changes depending on its height, depth and temperature.

The particles in a gas move quickly in random directions. Therefore the particles regularly bump into each other and the walls of their container.

These collisions exert pressure on the walls of the container and on any objects surrounded by the gas.

If the temperature of a gas is increased, the particles move faster, so they hit the walls of the container more often. This causes the pressure to increase.

Decreasing the volume of the container also increases the pressure exerted by the gas. This is because the rate at which the particles collide with the surfaces increases because there are more particles in a smaller space.

Earth's atmosphere is a mixture of gases, and is comprised mainly of nitrogen and oxygen. The nitrogen and oxygen particles in the atmosphere are constantly colliding with us and exert a pressure of around 100,000 pascals at sea level.

Imagine someone climbing Mount Everest. As the climber moves up through the atmosphere, the pressure decreases. This is because particles in the atmosphere are attracted to Earth's gravity , so there are fewer particles higher up.

At higher altitudes the weight of the air particles above the climber is less and the air is thinner, meaning the particles in the air are further apart. This means that the particles collide with the climber less frequently.

By the time the climber reaches the highest point of Mount Everest, the air pressure is only around 33,700 Pa approximately one third of the atmospheric pressure at sea level.

Just like gases, liquids exert pressure on objects due to collisions between the liquid particles and the object.

The amount of pressure exerted depends on both the density of the liquid and the depth of the liquid. The deeper you go:

• the greater the weight of liquid above the object
• the greater the liquid pressure

The Mariana Trench in the western Pacific is the deepest part of the ocean and is nearly 11 km below sea level. The pressure at that depth is estimated to be around 1.1 × 10⁸ Pa (110,000,000 Pa).

Try this experiment at home to find out about pressure in liquids.

You will need the following items:

• Large plastic bottle with lid
• 1 small pin
• A pencil
• Sticky tape
• Water
• A large container (sink/bath)

An object in a liquid experiences a force called upthrust. This is due to the particles in the liquid colliding with the surface of the object, which exerts pressure.

An object placed in a liquid will begin to sink. As it sinks, the liquid pressure on it increases and so the upthrust increases.

For a floating object, the upthrust is equal and opposite to the object’s weight. An object will continue to sink if its weight is greater than the maximum upthrust.