AP® Physics 2: Algebra-Based · Laboratory Investigations
Lab: Boyle's Law and the Gas Laws
Boyle's Law says that for a fixed amount of gas at constant temperature, pressure times volume stays the same. A sealed syringe and pressure sensor let you prove this by squeezing the plunger and recording pairs of readings.
Push the plunger on a capped syringe and it fights back harder the smaller the volume gets. The trapped air molecules are hitting the walls just as often per collision, but as you shrink the space they hit the walls more frequently, so pressure climbs. Boyle's Law packages this as PV = constant, for a fixed amount of gas held at constant temperature.
The lab setup is simple: a syringe with a known starting volume, sealed at one end, connected to a pressure sensor. You set the plunger to a series of volumes and record the pressure at each one. Plot pressure against volume and you get a curve, a hyperbola, which is hard to check by eye. Plot pressure against 1/volume instead and the same data falls on a straight line through the origin, because P = constant x (1/V). A straight line is something you can actually test: does it pass through zero, is the slope equal to nRT, are the points tight?
This lab is a template for the whole unit: pick variables that turn a curve into a line, then judge the fit by how straight the line is rather than by eyeballing a curve. The same move shows up anywhere one quantity varies inversely or as a power of another. It also sets up the ideal gas law PV = nRT, since Boyle's Law is just that law with n and T held fixed, and it hands you a ready-made way to spot bad data: a real physical problem, like a leak, shows up as a bent line, not as noise around a good one.
Key terms
- Boyle's Law
- For a fixed amount of gas at constant temperature, pressure times volume is constant.
- Ideal gas law
- PV = nRT, relating pressure, volume, moles, and temperature of a gas.
- Systematic effect
- A consistent bias that pushes measurements the same direction throughout a run, unlike random scatter.
- Linearization
- Choosing variables, like plotting P against 1/V, so that data expected to follow a curve instead falls on a straight line.
Watch it explained
- How it worksHow does a syringe, a pressure sensor, and a few volume readings prove pressure times volume is constant?
- Worked exampleGraph pressure against volume and then against one over volume for a sealed syringe of air.
- Common mistakeWhy doesn't a small air leak show up as scatter instead of as a bent line?
- Going furtherWhat changes in the data when you compress the syringe fast enough to heat the trapped gas?
Check yourself
End of topic.