Key points
Liquids and gases are both Substances that can flow and change shape easily, like water. Liquids and gases are both examples of 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 The amount of force acting on a certain area. Pressure is measured in pascals (Pa) which are the same as newtons per metre squared (N/m虏). 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 鈥榖ounce鈥 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.
Pressure in gases
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 The amount of space that something occupies. The amount of space within closed surfaces. 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.
Try this experiment at home to find out about gas pressure.
Image caption, Partially inflate a balloon, so that it is slightly larger than a tennis ball, like the one in the picture.
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Why did the balloon in the freezer shrink?
When a gas is cooled, the particles have less kinetic (movement) energy and get closer together, this means that they take up less room. This also means that the pressure is lower. This is why the balloon in the freezer shrank.
Atmospheric pressure
Earth's An atmosphere is the relatively thin layers of gases surrounding a planet or other celestial body 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 The units used to measure pressure, represented using the letters Pa. A pressure of one pascal is the same as a force of 1 newton spread over a surface area of 1 square metre. 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 is an attraction force that pulls things toward one another. This size of the gravitational attraction is related to the mass of an object. , 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.
What is altitude sickness?
Altitude sickness is a set of symptoms caused by rapidly ascending to areas high above sea level. It can cause headaches, make you feel sick and short of breath.
This happens because there are fewer air particles at high altitudes and this includes fewer oxygen molecules. Altitude sickness is a reaction the body has to quickly reducing oxygen levels. People that live at high altitudes do not suffer from altitude sickness as their body is accustomed to the pressure and oxygen levels.
Pressure in liquids
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 All substances are made of particles. Density is a measure of how close together particles are. Closely packed particles have a higher density than particles that are spread out. 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 脳 10鈦 Pa (110,000,000 Pa).
Try this
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)
Watch the video to see how to do this experiment yourself
Image caption, Click to see a step-by-step slideshow.
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Why do soda drinks fizz when you open them?
The drink is made fizzy by having dissolved carbon dioxide in the liquid when it is under pressure in the closed bottle.When you open the bottle, you release the pressure and the liquid cannot hold all of the carbon dioxide at lower pressure, so the gas quickly bubbles to the top, causing your drink to fizz.
Pressure and buoyancy
An object in a liquid experiences a force called A force caused by pressure in liquids. A liquid pushing against something can cause objects to float if the upthrust equals the weight.. 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鈥檚 weight. An object will continue to sink if its weight is greater than the maximum upthrust.
Test your knowledge
Play the Atomic Labs game! game
Try out practical experiments in this KS3 science game.
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