3.3 Particle Model & Pressure

• The molecules of a gas are in __________ _________ motion.

• The ______________ of the gas is related to the average __________ energy of the molecules.

• constant

• random

• temperature

• kinetic

What does changing the temperature of a gas, held at constant volume do?

Changes the pressure exerted by the gas

As a gas gets hotter, what happens to the speed of the particles?

They move faster.

What is gas pressure caused by?

Moving particles continuously colliding with the walls of their container — they exert a force

• When you heat a gas (constant __________):

  • the particles gain _________ energy — move __________

• They _________ with the container walls more frequently and with a greater _________ — results in a higher overall ______ __________

• volume

• kinetic

• faster

• collide

• force

• gas pressure

What can a gas be compressed or expanded by?

Pressure changes

• A gas can be ____________ or ___________ by pressure changes.

• The pressure produces a net force at _______ ________ (___________) to the wall of the gas container (or any surface).

• compressed

• expanded

• right angles

• perpendicular

When you increase the volume in which a gas is container (at constant temperature), what can it lead to?

Decrease in pressure

Explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure.

• The gas particles spread out → more room to move

• They have to travel further → hit walls of container less often

• Fewer collisions = less overall force exerted on the walls → pressure decreases

• A student investigated how the pressure in a fixed mass of air varies with the volume of the air.

• Figure 1 shows the equipment used.

• When the plunger was pushed slowly into the syringe, the pressure in the syringe increased.

• The temperature of the air remained constant.

Explain why the pressure increased. [3 marks]

• Air particles are closer together

• So frequency of collisions between air particles and syringe walls increased

• Larger total force on a smaller surface area

• A fire piston is a special type of syringe that can be used to start fires.

• Figure 2 shows a fire piston.

• The plunger is pushed quickly downwards and compresses the air.

• When the air is compressed quickly, the temperature of the air increases.

How does an increase in temperature affect the air particles inside the piston? [1 mark]

The mean kinetic energy of the particles increases.

• The image shows air being pumped into a car tyre.

Complete the sentence. [1 mark]

Air particles in the tyre move quickly in ___________________ directions.

random

• The image shows air being pumped into a car tyre.

• When the tyre is at the correct pressure, pumping more air into the tyre causes the pressure to increase further.

• The volume and temperature of the air in the tyre do not change.

Explain why the pressure increases as more air is pumped into the tyre. [2 marks]

• More air particles in the tyre

• Collisions with tyre walls are more frequent

• The image shows air being pumped into a car tyre.

• The air pressure in a car tyre changes if the temperature of the air in the tyre increases.

Explain why. [4 marks]

• As the temperature increases — particles have a greater mean kinetic energy

• Collisions with tyre walls are more frequent

• Each collision has a greater force

• Greater mean force per square metre causes greater pressure

What is the equation for a fixed mass of gas held at a constant temperature? State the units.

pressure × volume = constant

$$pV=cons\tan t$$

• pressure — pascals (Pa)

• volume — metres cubed (m³)

What is ‘work’?

The transfer of energy by a force.

What is the effect on a gas of doing work on a gas? (2)

• Increases internal energy

• Can cause an Increase in temperature

• Figure 3 shows a person using a bicycle pump to inflate a tyre.

• The internal energy of the air increases as the tyre is inflated.

Explain why. [2 marks]

• A temperature increase would increase the pressure in the tube.

• Because higher temperature means higher kinetic energy of particles.

What is the equation for pressure? State the units.

pressure = force ÷ area

$$P=\frac{F}{A}$$

• pressure — pascals (Pa) or newtons per metre squared (N/m²)

• force — newtons (N)

• area — metres squared (m²)