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Gas Pressure During Compression
A student investigated how the pressure in a fixed mass of air varies with the volume of the air using a syringe and pressure gauge.
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.
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Marking points
- 1 States that compression brings the fixed number of air particles closer together.
- 2 Links the reduced volume at constant temperature to more frequent particle collisions with the walls.
- 3 Links the collisions to a larger force per unit area/greater pressure.
Full-mark answer
Pushing in the plunger decreases the volume, so the fixed number of air particles are closer together. At constant temperature they collide with the syringe walls more frequently. This produces a greater force per unit area on the walls, so the pressure increases.
Why this answer loses marks
The particles move faster when the air is compressed.
The temperature is constant, so increased particle speed is not the cause. Compression brings particles closer, increasing wall-collision frequency and force per unit area.