Gas Pressure

Section 1 of 8Particles in a gas

Constant random motion

• Constant random motion Gas molecules travel at high speed in all directions and change direction only when they collide.
• They collide with each other and with the walls of their container.

Temperature and energy

• Temperature and energy Temperature tells us the average kinetic energy of the molecules.
• A hotter gas has faster molecules; a cooler gas has slower ones.

Section 2 of 8How pressure arises

1

Each molecule hits the wall and rebounds, which changes its momentum.

2

The wall therefore pushes on the molecule, and the molecule pushes back on the wall with an equal force.

3

The enormous number of impacts each second gives a steady total force on the wall.

4

Pressure = force ÷ area (p = F ÷ A), measured in pascals, Pa.

Section 3 of 8Heating a gas at constant volume

1Temperature rises

• Temperature rises Molecules gain kinetic energy and their average speed increases.

2Wall impacts change

• Wall impacts change They strike the walls more often and with more momentum each time.

3Force per area grows

• Force per area grows The total force on each square metre of wall goes up.

4Pressure increases

• Pressure increases Cooling the gas reverses every step, so the pressure falls.

Section 4 of 8Linking the variables

Condition	Change	Result for pressure
Volume fixed	Temperature increases	Pressure increases
Volume fixed	Temperature decreases	Pressure decreases
Temperature fixed	Volume decreases	Collisions per second rise, so pressure increases

Section 5 of 8Pressure and volume at constant temperature

Fixed mass of gas, constant temperature

Volume decreased

• Volume decreased The same molecules are squeezed into a smaller space, so they reach each part of the wall more often and the pressure rises.

Volume increased

• Volume increased Molecules have further to travel between collisions with the walls, so impacts are less frequent and the pressure falls.

Speed unchanged

• Speed unchanged Because the temperature is constant, the average speed of the molecules does not change;
  • only the collision frequency does.

Everyday links

• Everyday links Pumping up a bicycle tyre squashes more air into the same volume, so the pressure inside the tyre goes up.

Section 6 of 8Worked example: pressure from force and area

1

Question: a gas pushes on a piston of area 0.020 m² with a force of 600 N. Find the pressure.

2

Write the equation: p = F ÷ A.

3

Substitute the values: p = 600 ÷ 0.020.

4

Calculate: p = 30 000 Pa, which is 30 kPa.

5

Check the units: newtons divided by square metres gives N/m², and 1 N/m² is 1 pascal.

Section 7 of 8Key terms

Term	Meaning
Pressure	Force acting per unit area of a surface, in pascals (Pa).
Kinetic energy	The energy a particle has because it is moving.
Random motion	Movement in unpredictable directions that keeps changing after collisions.
Constant volume	The container cannot expand or shrink, so the space available stays the same.
Collision frequency	The number of times per second that molecules hit a given area of the container wall.
Fixed mass of gas	A sample in which no gas enters or leaves, so the number of molecules stays the same.

Section 8 of 8Exam technique

1

Saying heated molecules expand. The molecules stay the same size; they move faster.

2

Describing pressure as molecules hitting each other. Pressure comes from collisions with the container walls.

3

Mentioning only more frequent collisions. Faster molecules also hit harder, giving a larger force per collision.

4

Forgetting to convert the area into square metres before using p = F ÷ A.