Physics IA · D · Fields · topic 24 of 30
Solenoid: does doubling the turns double the field? · IB Physics IA idea: guide, sample graph and step-by-step plan
two variables want to move: don't let themAdd turns and the field grows, but only if you keep everything else fixed, and a longer coil quietly changes two things at once.
- ✓ you like building your own apparatus
- ✓ you can wind a neat coil
- ✓ you enjoy separating two variables that want to move together
The big idea
← that's your real challengeInside a long solenoid the field is B = μ₀NI/L, so for a fixed length and current, B should be proportional to the number of turns. The catch is in the words fixed length: winding more turns tends to make the coil longer, spread out or thicker. Your investigation is to build a coil where only N changes, then test how close a real, finite solenoid gets to the ideal formula and where it falls short.
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 →
Wind extra layers on the same former so the length stays fixed.
→ Keeps L constant but the average radius grows, so the finite-length correction changes; a good evaluation point.Keep N the same and stretch or squeeze the coil to change L.
→ The graph becomes field against 1/L, testing the other half of the formula.Compare your data with the infinite-solenoid formula and with the on-axis formula for a finite coil.
→ Two models on one graph; the finite one should do better, and you can quantify by how much.Fix N and I and move the probe from outside the coil to the middle.
→ Shows the end effects directly; the analysis is field against position.Treat the gradient of B against N as μ₀I/L and find μ₀ with an uncertainty.
→ A neat quantitative goal: compare your value with the accepted one.Repeat with an iron core in the coil.
→ Tests how the core changes the gradient; magnetisation effects make this a stretch.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 (how many turns, from how few to how many).
- Names the dependent variable and how you will measure it (what is "the field", and where is the probe?).
- Says how the coil length, current and probe position are kept the same when you change the turns.
- Says what you will compare with (the ideal formula, a finite-length model, or both).
- Is narrow enough for about 10 hours, including the time it takes to wind and rewind a coil.
The set-up
Keep the length L fixed on a former and count every turn as you wind. Draw your own version with your real measurements.
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 solenoids are used (relays, valves, MRI) and what you expect for the graph.
The solenoid formula, the assumption of a long coil, and what changes for a coil that is not long.
Independent, dependent and controlled variables, with how L, I and the probe position are kept constant.
How you wind the coil and count turns, fix the length, position the probe, and set and read the current.
Hot wire, sharp wire ends, power supply limits.
Turns, current and field readings with units, resolution and repeats.
Uncertainty in B, in N and in L, and one worked example of each type of calculation.
Field against turns with error bars, a fit, and what the gradient means in terms of μ₀, I and L.
Is B proportional to N? How does the gradient compare with theory, including the finite-length model?
Weaknesses ranked by effect on the gradient, linked to specific improvements.
Sources for the formulas and the sensor.
What your Physics IA graph could look like
check the intercept too ✓Made-up data, to show the form of a strong graph, not the answer. Yours will look different. example only
- Turns are counted, so there is no error bar on N. Say that, and say what is uncertain instead (the length the turns occupy).
- A straight line through the origin is the test of proportionality: check the intercept as well as the gradient.
- Drawing both models lets the reader see which fits. The gap between them is the effect of the coil's finite length.
- Start the vertical axis at zero so the proportionality is easy to judge.
- Another graph to consider: field per turn (B/N) against N. If the field is truly proportional, that is a flat line, and small trends become obvious.
How big is each effect?
Rank by the effect on B, not on the raw readings. Here the biggest item is not a measurement error at all, it is the model.
Your game plan
wind a test coil in the trial runTick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictFind the solenoid formula and its assumptions. Predict how B should depend on N for fixed L and I, and what a short, wide coil does to that.
- Trial run (1 lesson)Wind a coil, fix its length, and take readings at a few currents. Does the field change when the probe moves a little? Does the wire warm up?
- Plan how to change NDecide how you will vary the turns while keeping the length fixed. Test one method for the first two or three values before committing.
- Choose range and repeatsChoose at least 6 values of N and a single current. Decide how many repeats, and write down why.
- Collect the dataZero the sensor, switch the current on only for each reading, and record raw values with units. Note the coil length every time.
- ProcessWork out uncertainties in B, L and I, and decide how to show them on the graph.
- Graph and fitPlot B against N, add error bars, and find the gradient with its uncertainty from best and worst lines.
- Compare with theoryCompare the gradient with μ₀I/L and with a finite-length model. Which is closer, and by how much compared with your uncertainty?
- EvaluateRank the effects by their influence on the gradient 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
- Changing the length of the coil while changing the turns, so two variables move at once.
- Leaving the current on so the wire heats and the current drifts.
- A probe that is not on the axis, or not at the same place for every coil.
- Not counting turns carefully, or losing count while winding.
- Using the ideal formula for a short, fat coil without checking the assumption.
- Forgetting the Earth's field or a stray magnetic object near the sensor.
More Physics IA ideas like this
- Magnet in a copper tube: how slow is its fall?●●○
- Two magnets: how does the force fall with distance?●●○
- Coil: how does the central field depend on current?●●○
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.