Physics IA · D · Fields · topic 23 of 30
Coil: how does the central field depend on current? · IB Physics IA idea: guide, sample graph and step-by-step plan
linear, but not boringB should be proportional to I. But your sensor also feels the Earth, and your coil is warming as you measure. Can you still get a straight line?
- ✓ you like a clean linear relationship
- ✓ you enjoy chasing hidden systematic errors
- ✓ you can use a data logger
The big idea
← why isn't the intercept zero?The field at the centre of a flat circular coil is μ₀NI/(2R), so a graph of B against I should be a straight line whose gradient depends only on the coil. Easy to predict, harder to measure: the Earth's field adds a constant, the coil heats up and its resistance changes, and the sensor must sit exactly at the centre. Your task is to find which of these matters most and to turn the gradient into a value you can test against theory.
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 →
Measure the coil's radius and turns carefully and compare the gradient with μ₀N/(2R).
→ Turns the experiment from "is it linear?" into a quantitative test with an uncertainty budget for the gradient.Repeat with coils of different radius or turns, same current range.
→ A second variable: how the gradient depends on R or N. Choose one as your main focus to stay within the word limit.Take readings for positive and negative current.
→ The Earth's field shows up as an offset between the two sets; averaging removes it. A strong data-processing point.Fix the current and move the sensor along the axis away from the centre.
→ The graph becomes field against position, compared with the on-axis formula; a different, richer analysis.Put an iron rod through the coil.
→ Tests how a core multiplies the field. Hysteresis and a non-linear graph appear, so it is a stretch.Link to electromagnets in cranes, loudspeakers or MRI.
→ Provides a purpose; the analysis is unchanged.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 currents, in amperes, and how many values).
- Names the dependent variable and the sensor you will use to measure it (what exactly is "the field"?).
- Says what stays fixed: the coil, the sensor position and orientation, the time the current is on.
- Says what you will compare with (the theoretical gradient, another coil, or both).
- Is narrow enough for about 10 hours, with at least 6 currents and repeats.
The set-up
The sensor on the axis, at the centre, facing along the axis. 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 coils produce fields (motors, speakers, MRI) and what the graph should look like.
The field of a circular coil, where the formula comes from and its assumptions; the Earth's field and how a Hall sensor works.
Independent, dependent and controlled variables with how each is set and kept constant.
How you position and orient the sensor, zero it, change and read the current, and limit heating.
Hot coil, power supply limits, short circuits.
Current and field readings with units, resolution, zero readings and repeats.
Correcting for the offset, uncertainties in B and I, and one worked example of each calculation.
Field against current with error bars, a fit with its uncertainty, and the physical meaning of the gradient and intercept.
Does the line support the theory? Your gradient compared with the theoretical value, with uncertainties.
Weaknesses ranked by effect on the gradient, linked to specific improvements.
Sources for the coil formula and the sensor.
What your Physics IA graph could look like
look at the intercept ✓Made-up data, to show the form of a strong graph, not the answer. Yours will look different. example only
- The intercept is physics: a non-zero value is a clue, here a constant field the sensor feels even at zero current.
- Error bars on both axes. For a straight line the gradient's uncertainty comes from best and worst lines.
- Both lines drawn so the reader can see what difference the offset makes.
- Readings taken as the current rises and then falls would show whether heating is changing the coil: a line that does not retrace itself is a warning.
- Another graph to consider: field against current for two coils on the same axes, to compare gradients.
Which uncertainty matters most?
Rank by the effect on your result, here the field at a current of 1.0 A. Notice how a hidden systematic error can beat a meter's resolution.
Your game plan
zero the sensor EVERY timeTick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictFind the formula for a circular coil and what it assumes. Predict the shape and gradient of your graph, and what you expect the Earth's field to do.
- Trial run (1 lesson)Set up the sensor on the axis, zero it, and take readings at five currents. Does the reading drift as the coil warms? Does turning the coil change the zero?
- Fix the geometryDecide how you will hold the sensor at the centre and keep it facing along the axis. Measure the coil radius and count the turns.
- Choose range and repeatsChoose at least 6 currents, how long the current stays on, and how many repeats. Write down your reasons.
- Collect the dataRecord raw readings with units. Take a zero reading before each run and switch off between readings.
- ProcessCorrect for any offset, work out uncertainties for B and I, and decide how to show both on the graph.
- Graph and fitPlot field against current, add error bars, and find the gradient with its uncertainty from best and worst lines.
- Compare with theoryCompare your gradient with the theoretical value. Is the difference bigger than your uncertainty? What does the intercept say?
- EvaluateRank the uncertainties and systematic errors by their effect on the gradient, and link every improvement to a weakness you found.
- 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
- Ignoring the Earth's field: a constant offset that bends your intercept and your conclusion.
- Leaving the current on between readings, so the coil heats and its resistance (and the current) drift.
- A sensor that is not at the centre or not along the axis, with no estimate of the effect.
- Using the wrong sensor range, or reading a value close to its resolution.
- Comparing with theory without measuring the coil radius and turns yourself.
- An evaluation that says "the sensor was inaccurate" without a number.
More Physics IA ideas like this
- Solenoid: does doubling the turns double the field?●●○
- Magnet in a copper tube: how slow is its fall?●●○
- Two magnets: how does the force fall with distance?●●○
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.