Physics IA · B · The particulate nature of matter · topic 9 of 30
Trapped air: does pressure times volume stay constant? · IB Physics IA idea: guide, sample graph and step-by-step plan
looks simple. the dead volume makes it GOODBoyle's law is easy to quote. It is harder to measure. The air in the tube is the hidden villain.
- ✓ you like testing a gas law
- ✓ you can handle a sensor and software
- ✓ you like spotting hidden systematic errors
The big idea
← the intercept is your RQ hook!For a fixed amount of gas at constant temperature, pressure times volume is constant, so 1/P is proportional to V. But the gas does not only live in the syringe: the tubing and sensor hold some too, a dead volume V₀. Plotting 1/P against the syringe reading should give a straight line that does not pass through the origin. The intercept tells you V₀. Finding and correcting for V₀ is what turns a textbook demo into a strong IA.
Make it yours: pick one twist
Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.
Swipe for more twists →
Repeat the series with the syringe and tubing in warm and cool water baths.
→ The gradient of the 1/P-V line changes with T; your analysis tests whether PV tracks the temperature.Measure the volume of the tube with water or start at a second known volume.
→ An independent route to V₀ that you can compare with your intercept; the evaluation compares methods.Take readings with the plunger moving in and then out.
→ Hysteresis, seal friction and leaks show up; the graph has two sets of points to compare.Compress quickly, and then slowly with waits in between.
→ Tests the constant-temperature assumption; fast compression heats the gas, so the graph will bend.Use the gradient of P against 1/V and the temperature to estimate n.
→ Turns the constant into a quantity you can compare with the expected amount of air; stretch.Scuba diving, a bicycle pump or a bellows.
→ Gives the introduction a purpose; the physics and graph stay the same.Your Physics IA research question: run the check
Write your own question first. Then make sure it ticks every box:
- Names one independent variable and its range (volume set on the syringe, how many values).
- Names one dependent variable and how it is measured (absolute or gauge pressure, and which sensor).
- Says what stays fixed (temperature, amount of gas, speed of compression, leak check).
- Says what you compare with (the relationship PV = constant, and how you treat the dead volume).
- Is narrow enough for about 10 hours, with at least 7 volumes and repeats at each.
The set-up
One way to arrange it. Draw yours with the real syringe size, tubing and sensor, and mark the dead volume.
How your Physics IA could be laid out
The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.
Your question; where gas compression matters (pumps, diving, engines); what you expect and why.
Pressure, volume and temperature for a fixed amount of gas; Boyle's law; why a dead volume changes the graph; what a log or reciprocal graph shows.
Independent, dependent and controlled variables, each with how you set, measure or hold it, and why it matters.
How the syringe is sealed and checked for leaks; how the volume is set and read; how long you wait before reading; sensor calibration; repeats.
Short and specific: pressure in the syringe, tubing popping off.
One table: volume, each pressure reading, units and uncertainties in the headers.
Reciprocal of pressure and its uncertainty, how you use the intercept to find V₀; one worked example.
The graph you choose, with error bars and fit; what gradient and intercept mean physically.
Answer with numbers: whether PV is constant once V₀ is allowed for, and how well the data supports it.
Weaknesses ranked by their effect on your result, each linked to a realistic improvement.
Every source for theory and method.
What your Physics IA graph could look like
intercept = information ✓Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only
- Plotting 1/P against V makes Boyle's law a straight line; the intercept is now data.
- Error bars on every point: the volume reading on the syringe is the bigger uncertainty.
- Both lines are shown so the reader sees what the dead volume does.
- Say whether the pressure is absolute or gauge: it changes the gradient and the intercept.
- Another graph to consider: P against 1/(V + V₀), once you have an estimate of V₀, which should be a straight line through the origin.
Which uncertainty matters most?
Rank by the effect on your result, here the constant P(V + V₀) from one point. An unnoticed systematic error beats every instrument.
Your game plan
leak test before anything!Tick steps off as you go. We'll remember on this device.
0 of 11 done
- Read and predictFind Boyle's law and the idea of a dead volume. Predict what a 1/P against V graph should look like with and without one.
- Trial run (1 lesson)Connect everything and hold one reading for a minute. Is the pressure steady? If it falls, you have a leak. Does the pressure sensor read absolute or gauge?
- Fix the leaksTest every joint. A slow leak spoils the whole data set, so do not move on until the reading is stable.
- Decide range, repeats and waiting timeChoose at least 7 volumes, how many repeats, and how long to wait after each move before reading, so the gas returns to room temperature.
- Collect the dataRecord raw readings in the table with units and resolution. Note whether you went up or down the volume range.
- ProcessConvert pressure to 1/P and carry the uncertainty through. Decide how you use the intercept to estimate V₀.
- Graph and fitPlot 1/P against V with error bars. Fit best and worst lines and read off the gradient and intercept.
- Compare with theoryIs the line straight? What does the intercept say about V₀, and does it match what you can measure directly?
- EvaluateRank the weaknesses by effect on your result. Make each improvement specific to a weakness you found.
- Write, then cutWrite in the order of the layout and stay within 3,000 words. Tables, graphs and references do not count.
- Check before your teacher's draftRun through the checklist below, then get your RQ or draft checked. Your teacher reads one draft only.
Physics you'll need
Traps to avoid in this Physics IA
- Forgetting the dead volume of tubing and sensor, so PV looks like it is not constant.
- A small leak that you never tested for.
- Compressing quickly so the gas heats, then calling it constant-temperature.
- Mixing up absolute and gauge pressure.
- Reading the syringe scale at the wrong part of the seal.
- Starting each run at a different amount of gas, for example by re-connecting the tube.
- Fitting through the origin without asking whether the data goes there.
- A sensor that cannot read the highest pressure you reach: check its range before you compress to a small volume.
More Physics IA ideas like this
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- Capacitor discharge: what sets the time constant?●●○
This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.