Physics IA · D · Fields · topic 25 of 30
Magnet in a copper tube: how slow is its fall? · IB Physics IA idea: guide, sample graph and step-by-step plan
looks like magic, needs real analysisDrop a magnet down a plastic tube and it's gone in a flash. Drop it down copper and it drifts. Your job is to put numbers on that drift.
- ✓ you like a result that looks like magic
- ✓ you can film and analyse video frame by frame
- ✓ you enjoy explaining *why* with a proper model
The big idea
← terminal speed is your numberAs the magnet falls, the changing flux drives eddy currents in the copper. By Lenz's law their field opposes the motion, so there is a drag force that grows with speed. The magnet speeds up, then reaches a terminal speed where drag equals weight. A graph of distance against time should curve at the start, then become a straight line. Change one thing (the magnet, the wall thickness, the mass, the metal) and find how the terminal speed responds.
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 →
Attach non-magnetic weights (brass or plastic) so the total mass changes while the magnet stays the same.
→ If the drag is proportional to speed, terminal speed should be proportional to the total weight: a clean quantitative test.Tubes of the same metal and bore but different wall thickness.
→ Tests how the amount of conductor affects the drag. Make sure the bore is the same, or you change the gap to the magnet too.Copper, aluminium and brass tubes of the same size.
→ Terminal speed depends on conductivity; link your results to resistivity values and discuss the order.Cool the tube in a freezer or warm it in a water bath (dry it carefully).
→ Resistivity depends on temperature, so does the drag. A small effect that needs good timing.Two, three and four identical magnets stacked.
→ Both the field and the mass change, so the analysis must handle two effects; a stretch.Film at the highest frame rate you have and track position against time.
→ Small timing uncertainty and a full distance-time graph, so you can fit the early acceleration and the terminal speed.Your Physics IA research question: run the check
write it FIRST, then check itWrite your own question first. Then make sure it ticks every box:
- Names one independent variable with its range (which masses, tubes, magnets or temperatures, and how many values).
- Names the dependent variable and how you will get it (terminal speed from a distance-time graph, not a hand-timed total).
- Says what stays fixed: the tube, the release point and the section of tube you measure.
- Says what you will compare with (free fall, a drag model, or a trend you predict from theory).
- Is narrow enough for about 10 hours, with at least 5 values and repeats.
The set-up
A phone on a stand, square to the tube, films the fall past marks a known distance apart. Draw your own version with real distances.
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, why eddy-current braking matters (trains, rides, induction cooktops) and what you expect.
Flux, Faraday's and Lenz's laws, eddy currents, the idea of terminal speed, and the free-fall comparison.
Independent, dependent and controlled variables, including the release height and the tube section used.
How you hold the tube vertical, release the magnet, film it, and read times from the video; how you stop the magnet touching the wall.
Strong magnets (pinching, swallowing, electronics), the magnet landing on a foot, sharp tube edges.
Frame times or positions from the video with resolution and frame rate stated.
How you got speeds and their uncertainties; one worked example for each type of calculation.
Distance against time for a run, then terminal speed against your independent variable with error bars.
How terminal speed depends on your variable, with numbers, and how it compares with free fall and with a drag model.
Weaknesses ranked by effect on the terminal speed, linked to specific improvements.
Sources for the theory and the material data.
What your Physics IA graph could look like
find where the line goes straightMade-up data, to show the form of a strong graph, not the answer. Yours will look different. example only
- A constant-speed section is a straight line: you read the terminal speed from its gradient, with best and worst lines.
- Free fall is drawn for comparison. The magnet is far from it almost immediately, which is the whole point.
- Error bars on both axes. The timing uncertainty is a frame, so its size depends on your frame rate.
- With strong drag the acceleration phase is very short (here it is over in a few millimetres), so use your video to show where the straight part begins rather than assuming it.
- Another graph to consider: terminal speed against your independent variable (mass, wall thickness), with each point from its own distance-time graph.
Which uncertainty matters most?
Rank by the effect on your result, here the terminal speed from a 300 mm section. It is often the timing method, not the equipment, that decides the quality.
Your game plan
film the trial run: it teaches you a lotTick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictRead about eddy currents and Lenz's law. Sketch the distance-time graph you expect and say where the straight part starts.
- Trial run (1 lesson)Drop the magnet down the tube and film it. Does it touch the wall? Where does it reach constant speed? Can you read frames clearly? Fix these now.
- Choose the independent variablePick one thing to change and one way to change it. Check by trial that the effect is big enough to measure.
- Choose a section and a methodDecide which section of the tube you measure, the frame rate, and the number of repeats. Write down your reasons.
- Collect the dataFilm each run with a scale in view. Note anything odd (tilt, a touch on the wall, a warm tube) as it happens.
- ProcessRead positions and times from the video, find the speed in the straight section and its uncertainty, and keep the method the same for every run.
- Graph and fitPlot terminal speed against your variable with error bars. Decide whether the trend is linear, a power law or something else, and why.
- Compare with theoryHow does your result compare with free fall and with the trend a drag model predicts? What does it tell you about how drag depends on speed?
- EvaluateRank the uncertainties and systematic errors by effect on your result and link each improvement to a specific weakness.
- Check before your teacher's draftRun the checklist above, then get your RQ or draft checked. Your teacher reads one draft only.
Physics you'll need
Traps to avoid in this Physics IA
- Timing the whole fall by hand: reaction time is bigger than the effects you are comparing.
- A magnet that touches or tilts against the wall, so friction is mixed with magnetic drag.
- Averaging over the first part of the fall, where the magnet is still accelerating.
- No plastic or non-metal tube as a control, so you cannot show the effect is magnetic.
- Changing two things at once (a thicker tube with a different bore, or a stronger magnet that is also heavier).
- Assuming linear drag without testing it.
More Physics IA ideas like this
- Two magnets: how does the force fall with distance?●●○
- Coil: how does the central field depend on current?●●○
- Solenoid: does doubling the turns double the field?●●○
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.