Definition and Molecular Basis
Definition of gas pressure: Gas pressure is the force that is created when gas molecules move around quickly and hit the walls of the container they are in. These molecules are constantly moving and bouncing around, and every time they hit the wall, they push against it a little bit. All of these tiny pushes add up to create pressure.
Molecular motion: Gas molecules never stop moving. They zoom around in all directions, and because they are so small and light, they bounce off everything — each other and the sides of their container. This random and constant movement causes them to collide with the container walls, and those collisions are what we feel as gas pressure.
Collision frequency and force: If the molecules move faster or there are more of them, they will hit the walls more often and with more force. The more collisions there are, and the harder they hit, the higher the pressure becomes. So if you heat the gas or add more gas into the same space, the pressure will go up.
Measurement of Gas Pressure
Use of manometer: A manometer is a special instrument used to measure the pressure of gas. It is usually shaped like a U-tube and filled with liquid, such as mercury or water. One end is connected to the gas container, and the other is either open to the air or sealed off.
Manometer operation: When gas is connected to the manometer, it pushes on the liquid inside. If the gas pressure is higher than the outside pressure, it will push the liquid down on its side and up on the other. The difference in height between the two sides tells us how strong the gas pressure is.
Height difference and pressure: The more the liquid moves, the greater the difference in height between the two columns. A bigger height difference means the gas is pushing harder, so the pressure is higher.
Liquid support by gas: The gas must push hard enough to move the liquid against gravity and any outside pressure. The higher the liquid rises on one side, the stronger the gas is pushing.
Proportional relationship: The pressure from the gas is directly related to the height difference in the liquid. This means that if the liquid height goes up by a certain amount, the pressure also goes up by a related amount.
Choice of liquid: Mercury is a very dense (heavy) liquid, so it doesn’t need to move much to show high pressure. Water is lighter, so it has to rise a lot more to show the same pressure, which is why mercury is used for strong gases.
Types of manometers:
- Open-ended manometer: This has one side open to the air. It compares gas pressure to atmospheric pressure.
- Closed-ended manometer: This has one side sealed off (like a vacuum). It only shows the pressure inside the gas, without air pressure involved.
Formula for Gas Pressure
Gas pressure equation: We can calculate gas pressure using a simple equation: P = Patm + hρg. This tells us the total pressure of the gas based on how high it pushes the liquid.
Formula variables: Each letter in the formula stands for something:
- P is the total pressure from the gas.
- Patm is the atmospheric pressure (air pressure around us).
- h is how much the liquid levels differ between the two sides of the manometer.
- ρ (rho) is how dense the liquid is.
- g is the force of gravity, which is about 9.8 m/s² on Earth.
Absolute pressure: Absolute pressure is the total pressure inside the gas container, including the air pressure on the outside. It’s what the formula gives us.
Open manometer reading: If you are using an open-ended manometer, the gas pressure is found by adding the height pressure to the atmospheric pressure. That’s because the gas is pushing against the air.
Closed manometer reading: If the manometer is closed at one end, then we don’t have to add anything. The pressure is just from the gas, which is P = hρg.
Gauge Pressure and Relationships
Gauge pressure definition: Gauge pressure is the extra pressure the gas has compared to the air. It doesn’t count the atmospheric pressure. We find it using just the height of the liquid, the liquid’s density, and gravity.
Height and pressure relation: If the gas pushes the liquid to a higher height, that means the pressure is stronger. So the taller the liquid column, the higher the gas pressure.
Density effect on height: If we use a heavier liquid like mercury, we don’t need a tall column to show the same pressure. A lighter liquid like water needs to go higher to show the same gas pressure.
Applications of Gas Pressure
Balloons: Balloons stay puffed up because the gas inside them pushes outward on the balloon walls. This pressure keeps the balloon round and floating.
Vehicle tyres: Tyres are filled with gas, usually air. The pressure inside helps the tyre hold its shape and carry the weight of the car, making it roll smoothly on the road.
Gas cylinders: Gases like oxygen or cooking gas are stored in strong metal containers. They are kept under high pressure so a lot of gas can fit in a small space.
Industrial and medical manometers: Manometers are used in places like factories and hospitals to keep track of gas pressure in machines and equipment. This helps make sure everything works safely and correctly.
Bunsen burners: In science labs, gas pressure helps control how much gas comes out of the burner. More pressure means a bigger or stronger flame.
Pressure-Volume Relationship
Boyle’s Law: This law says that if we keep the temperature the same, then making the gas take up less space (volume) will make the pressure go up. In other words, if we squeeze the gas into a smaller space, the molecules bump into the walls more often, increasing pressure.
Molecular collision increase: When the container is smaller, gas molecules don’t have as much room to move. So they bump into the walls more often, and this makes the pressure go higher.
Factors Affecting Gas Pressure
Temperature effect: When you heat a gas, the molecules move faster. They hit the walls of the container more often and harder, which raises the pressure.
Molecule count effect: If you add more gas molecules into the same space, they will bump into the container walls more often. More molecules mean more collisions and higher pressure.
Volume influence: If you make the container smaller while keeping the temperature the same, the pressure goes up. That’s because the same number of molecules now have less space to move, so they hit the walls more often.
Units of Measurement
Pascal as SI unit: The Pascal (Pa) is the standard unit we use to measure pressure. One Pascal means one newton of force pressing on one square metre of area (1 Pa = 1 N/m²).
Other pressure units: Sometimes, we use other units to measure pressure, like:
- mm Hg (millimetres of mercury): Often used in medicine (e.g., for blood pressure).
- cm Hg (centimetres of mercury): Also used in measuring pressure.
- m H₂O (metres of water): Used in some engineering systems.
- Millibar (mbar): Common in weather forecasts to show air pressure.
Manometer Types and Uses
Open-ended manometer use: This type is open to the air on one side and the gas on the other. It compares the gas pressure to the air pressure. If the gas is stronger, it pushes the liquid down on its side and up on the air side.
Closed-ended manometer use: This type is sealed on one side, so no air pressure is involved. It measures only the gas pressure. The height the liquid rises shows how strong the gas pressure is.