3.3 Particle Model & Pressure

Figure 1 shows a spray-can containing gas particles which are in constant motion.

Figure 1

Explain how the gas particles produce a pressure on the walls of the spray-can.

A student presses the button and some liquid leaves the can. 

Explain what happens to the gas pressure in the spray-can as the liquid leaves.

The student wants to investigate gas pressure further. They blow up two balloons to the same size.

They put one balloon into a freezer.

After a while, the student compares the two balloons and finds that the balloon that has been cooled is smaller. 

Explain why the cooled balloon is smaller using ideas about particles.

The student decides to investigate the link between temperature and the size of the balloon.

They write a plan for this experiment:

There are several faults in the student's plan.

Identify three of these faults and suggest an improvement to correct each one. 

Figure 1 is a graph showing how the volume changes with pressure for a gas at a constant temperature.

Figure 1

Sketch the graph for the same gas at a higher temperature.

Sketch the graph of pressure p against  where V is the volume of a gas. Label this X. Add a second graph, labelled Y showing the same gas at a higher temperature.

The piston in Figure 2 is pulled out of the cylinder from position P to position Q, without changing the temperature of the air enclosed. when moving from P to Q, the distance between the piston and end of the cylinder doubles. The pressure when the piston is at position P is 2.5 × 10⁵ Pa.

Calculate the pressure when the piston is moved to position Q.

A student carries out an investigation to measure how the pressure of a gas changes as its volume changes.

Her results are shown in Figure 2 below.

Figure 2

What conclusion can be drawn from the data in the graph?

Use data from the graph in your answer.

Give the reason for your answer.

Explain, in terms of particles, why the pressure in the gas increases as its volume decreases.

Using ideas about particles, explain how air inside a container exerts pressure.

A car tyre exerts a pressure of 203 kPa on the ground.

The contact area between the tyre and the ground is 0.021 m².

Calculate the force that the tyre exerts on the ground.

The air in the tyre heats up when the car is driven.

Explain how this affects the contact between the tyre and the ground.

You should assume that the volume of the air remains constant.

Figure 3 below shows the basic details of the steam engine used in a steam locomotive.

Figure 3

The following is a description of how the steam engine works:

• Hot air from the firebox passes through a pipe to the chimney
• Water is heated as the pipe passes through the boiler
• The water boils to form steam
• The steam is trapped by the closed inlet valve and continues to be heated.

Explain why the pressure of the steam increases as its temperature increases.

When the inlet valve is opened, the steam moves into the cylinder.

The pressure of the steam is 1.53 MPa.

The area of the piston is 0.0285 m².

Calculate the force exerted by the steam on the piston.

When the steam flows through the outlet valve, its pressure changes from 1.53 MPa to 101 kPa.

The volume of steam in the cylinder is 0.022 m³

Calculate the volume of the steam after it has passed through the outlet valve.

Figure 4 shows the particles of helium inside a balloon.

Figure 4

Describe the motion of particles inside the helium balloon

A small canister of pressurised helium is used to fill some balloons.

Using ideas about particles, explain why the pressure in the canister decreases as the helium is used to fill the balloons.

The cannister supplies 20 000 cm³ of helium at a pressure of 2400 kPa.

The volume of each balloon is 1200 cm³.

The pressure inside a balloon is atmospheric pressure, 100 kPa.

Calculate how many balloons can be filled using the helium from the canister.

You should assume the temperature of the helium remains constant.