Edexcel A-Level Physics 9PH0
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Thermodynamics
Boyle's law and investigating a fixed mass of gas
- 1 thing to know
- Papers 2 and 3
Overview
Recall questions on the statement of Boyle's law, why pressure must be changed slowly, why a column length can stand in for volume, and which graph confirms the law.
What you need to know
Answer each one before you reveal it.
Questions from this subtopic
Each one with the model answer underneath.
- 1
In molecular terms, why does the pressure of a gas rise when it is compressed at constant temperature?
Model answer
The molecules keep the same mean speed but have a shorter distance to travel between the walls, so they strike each unit area of wall more frequently and the mean force per unit area rises.
- 2
In molecular terms, why does the pressure of a gas rise when it is heated in a sealed rigid container?
Model answer
The molecules move faster, so each collision with the wall produces a larger change in momentum and collisions occur more often, increasing the force per unit area.
- 3
What does Boyle's law state?
Model answer
For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume.
- 4
How would you investigate the relationship between the pressure and volume of a gas at constant temperature?
Model answer
Trap a column of air above oil in a sealed vertical tube, use a foot pump to raise the pressure in stages, and record the pressure from the gauge together with the length of the trapped air column each time.
- 5
Why must the pressure be changed slowly and time allowed before each reading in this investigation?
Model answer
Compressing the gas warms it, so it must be left to return to room temperature if the temperature is to be genuinely constant.
- 6
Why can the length of the trapped air column be used instead of its volume?
Model answer
The tube has a uniform cross-sectional area, so volume is directly proportional to length and the constant of proportionality cancels.
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