A sealed syringe contains a fixed amount of air. When the plunger is pushed inward, reducing the volume of the air inside, the pressure of the air increases. Explain, using ideas about particle collisions, why reducing the volume of the air inside the syringe causes the pressure to increase.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student pushes the plunger of a sealed syringe inward, reducing the volume of the trapped air. The student notices that the syringe becomes harder to push as the plunger moves further in.
Model answer (4 marks)
Reducing the volume confines the air particles to a smaller space, so they are closer together.
The particles therefore collide with the walls of the syringe more frequently.
Each collision exerts a perpendicular force on the wall, so the force per unit area on the walls increases.
Because pressure is the force acting perpendicular to a surface per unit area, the increased frequency of collisions per unit area raises the pressure.
The particles therefore collide with the walls of the syringe more frequently.
Each collision exerts a perpendicular force on the wall, so the force per unit area on the walls increases.
Because pressure is the force acting perpendicular to a surface per unit area, the increased frequency of collisions per unit area raises the pressure.
Examiner tips
- Use the term ‘frequency of collisions’ to show understanding of kinetic theory.
- Mention that the force is perpendicular to the wall to link to the definition of pressure.
- Keep the explanation concise – 4 points only.
Common mistakes
- Confusing pressure with force – forgetting to mention ‘per unit area’.
- Saying the particles move faster instead of being closer together.
- Using vague wording like ‘more force’ without linking to collisions or area.
Mark scheme (4 marks)
- Reducing the volume means the particles are confined to a smaller space / the particles are closer together
- The particles collide with the walls of the syringe more frequently
- Each collision exerts a perpendicular force on the wall / the force per unit area on the walls increases
- More frequent collisions per unit area means greater pressure, because pressure is the force acting perpendicular to a surface per unit area
Key terms in this question
pressure · volume · particle collisions
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Pressure questions
- A helium weather balloon is released from the Earth's surface and rises high int…
- A sealed metal can contains a fixed amount of air. The can is placed in a freeze…
- A cyclist pumps air into a bicycle tyre that has a fixed volume. After pumping, …
- A submarine dives from the surface of the ocean to a much greater depth. Explain…