The Gas Pressure Lab

The Gas Pressure Lab

A gas seems to be nothing at all, but it pushes. It inflates a balloon, drives a piston, and can burst a sealed can. That push is pressure. To understand it, we have to zoom in to the particles.

Billions of tiny hits

The particles in a gas move fast and at random in all directions. Constantly, they collide with the walls of their container, and each collision pushes on the wall with a tiny force. Add up billions of those collisions every second across the whole wall and you get gas pressure: the force per unit area on the walls. Two things follow that people get wrong. Because the motion is random, the pressure acts in every direction, not just downwards. And it has nothing to do with the weight of the gas: what matters is how hard and how often the particles strike.

Mark the claims

Five statements about gas pressure. Tap the TWO that are WRONG.

• Gas pressure is caused by particles colliding with the walls of the container.
• The pressure a gas exerts comes from the weight of the gas pressing down on the container.
• Each collision exerts only a tiny force, and the pressure is billions of them added together.
• Gas particles stick to the walls, and it is that sticking that produces the pressure.
• Gas pressure acts in every direction, not only downwards.

Two ways to raise the pressure

Temperature measures the average kinetic energy of the particles: hotter means faster on average. Pressure depends on how hard and how often the particles strike the walls, so there are two separate ways to raise it. A third works too, if you are allowed to add gas: pumping in more particles means more collisions every second, even with nothing moving any faster.

What raises the pressure?

Select ALL THREE changes that would increase the pressure of a gas.

• Raising the temperature while keeping the volume constant
• Squeezing the same gas into a smaller volume
• Pumping more gas particles into the same container
• Cooling the gas down
• Letting the gas expand into a bigger volume
• Removing some of the gas particles

Heating a sealed can

A sealed can of gas is thrown onto a fire. Trace what happens to the gas inside, step by step.

• The can heats up. What happens to the kinetic energy of the gas particles inside?
• The particles are now moving faster. How does that change their collisions with the can walls?
• The can is rigid, so the volume stays fixed. With harder, more frequent collisions, what happens to the pressure?

The squeeze, step by step

A syringe of air is sealed and the plunger is pushed slowly in, so the gas stays at the same temperature. Put the chain of reasoning into order.

• The plunger is pushed in, so the gas occupies a smaller volume
• The same number of particles is now in a smaller space
• Each particle travels a shorter distance before it reaches a wall
• So the walls are struck more often every second
• The force per unit area on the walls rises: the pressure has increased

Work on a gas

You can raise a gas's energy without heating it at all: by doing work on it. When you compress a gas quickly, the moving surface pushes on the particles and transfers energy to them, increasing the gas's internal energy and raising its temperature. Internal energy is the total energy stored in the particles of a system. There are two ways to increase it: heating the gas, or doing work on it. The bike pump is the second one, and it is why the barrel gets warm as you push air into the tyre even though no flame is anywhere near it.

Two routes, one result

Two identical sealed samples of gas end up at the same higher temperature. One was warmed over a flame. The other was compressed quickly, with no heat entering it at all. Which statement is true?

• Both gained internal energy: one by heating, the other by work done on it
• Only the heated sample gained internal energy
• The compressed sample got hotter without gaining any energy
• Compressing a gas always cools it, so the second sample cannot have got hotter

Explain the hot tyre

A bike tyre is pumped up on a cold morning, and the bike is then left in strong sunshine all day. By the afternoon the pressure in the tyre has risen. Explain why, in terms of the particles.

• Say what the sunshine does to the temperature, and what temperature measures
• Say what that does to the speed of the particles
• Give BOTH changes to the collisions with the tyre wall, not just one
• Say what stays roughly the same about the tyre, and why that matters

Lab summary

Gas pressure is caused by particles _____ with the container walls. Temperature measures the average _____ energy of the particles, so heating a gas at constant volume makes them hit the walls harder and raises the _____. Squeezing the same gas into a smaller _____ raises the pressure too, because each particle reaches a wall sooner.