Pietro MeloniPhD

22 ideas · D.1, D.2, D.3 · 2025 syllabus

IB Physics IA ideas: gravitational, electric and magnetic fields

Field investigations measure a magnetic field with a Hall probe or phone magnetometer, the force on a current, or charge and capacitance. Gravitational fields are hard to probe in a lab, so most ideas here are electric and magnetic.

By Pietro Meloni, PhD · Updated on

22 of 22 ideas

D.1 Gravitational fields: 2 ideas

Testing Kepler's third law with moons of Jupiter

Research question. Does the period squared against orbital radius cubed for the four Galilean moons follow a straight line, and what mass of Jupiter, in kg, results?

  • D.1 Gravitational fields
  • SL and HL
  • Easy data
  • database
  • Rarely listed

My take. Safe and easy, but only fine if you add depth. Measuring positions from real images or a simulator gives you your own data and lifts it above a table lookup.

Method, physics and where marks are lost+
Independent variable
Orbital radius cubed for 4 moons of Jupiter, or 6 to 8 bodies if more moons or another planet are added, from a public data table.
What you measure
Orbital period squared, taken from the table or measured from sequential telescope or simulator images; central mass calculated from the gradient as M = 4 pi2/(G x gradient).
Controlled variables
Same central body for all satellites. Radii measured from the planet's centre, not surface. Same data source and units throughout. Near circular orbits chosen so the radius is well defined.
Physics and graph
Newton's gravitation with circular motion gives T2 = (4 pi2/GM) r3. Plot T2 against r3; gradient 4 pi2/GM. A log-log plot tests whether the exponent is 3.
SL and HL
SL: linear graph and a mass of Jupiter compared with the accepted value. Top band: log-log fit for the exponent with its uncertainty, and a check for eccentricity effects. HL: discuss deviations from a point mass, such as planetary oblateness.
Where marks are lost
Research design: using only tabulated values with no reason or check on source. Data analysis: too few points, and no uncertainty. Conclusion: comparison with accepted mass without a percentage difference. Evaluation: only generic comments, no mention of eccentricity.
Data
Only a data table and spreadsheet needed; to be more original measure moon positions yourself from images, where pixel scale is the main uncertainty.

Weight against mass and local g from a force sensor

Research question. How does the weight of brass slotted masses from 0.050 kg to 0.500 kg vary with mass, and what value of gravitational field strength g results from the gradient?

  • D.1 Gravitational fields
  • SL and HL
  • Easy data
  • Rarely listed

My take. As it stands it is too easy and might score poorly on depth. Only choose it if you turn it into a comparison of two methods for g, or a measurement of g at different heights.

Method, physics and where marks are lost+
Independent variable
Mass hung from the sensor: 10 values from 0.050 to 0.500 kg, each measured 3 times, with the masses checked on an electronic balance.
What you measure
Force read from a digital force sensor (or a calibrated newton meter) in newtons; g is taken as the gradient of weight against mass.
Controlled variables
Location: same bench and height throughout. Sensor zero: reset with nothing hanging before each reading. Mass hanging still: readings taken once oscillation has stopped. Temperature and sensor orientation: kept vertical and unchanged.
Physics and graph
W = mg in a uniform field. Plot W against m; the gradient is g, expected near 9.8 N per kg. Compare with the accepted local value and with g from a free-fall or pendulum measurement.
SL and HL
SL students find g with an uncertainty and compare it with 9.81. Extra depth: use a different method for g and see whether the two values agree within uncertainty, and estimate how g changes with altitude between floors of the school using a very sensitive sensor.
Where marks are lost
Research design: the question is trivial and lacks an unknown. Conclusion: stating a gradient without comparing with an accepted value. Evaluation: not addressing sensor calibration and zero error.
Data
A force sensor, slotted masses and a balance; the main uncertainty is the calibration and zero offset of the sensor.

D.2 Electric and magnetic fields: 16 ideas

Turns on an electromagnet and its pulling force

Research question. How does the number of turns (20 to 120 in steps of 20) on an iron-cored electromagnet at constant current of 1.0 A affect the magnetic field measured 1.0 cm from its end, in mT?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Common: on 3 sites

My take. Overdone, listed on three sites, so choose it only if you handle the changing coil length and heating. Test the iron core against an air core for your own angle.

Method, physics and where marks are lost+
Independent variable
Number of turns N: 20, 40, 60, 80, 100, 120, rewound on the same core, 3 repeats each.
What you measure
Field strength from a Hall probe or phone magnetometer at a fixed distance, in mT. Optional second measure: mass of paper clips or steel washers lifted.
Controlled variables
Current held at 1.0 A by adjusting a power supply and checked with an ammeter each run. Same core and coil diameter. Same probe distance, fixed by a clamp. Short readings only, to stop the wire heating and the resistance drifting.
Physics and graph
For a solenoid B = μ₀nI, so B is proportional to N at fixed length. Plot B against N and compare the gradient with the prediction; with an iron core expect a larger effective permeability and eventual saturation.
SL and HL
SL students can plot B against N and test proportionality. Top band work compares the gradient with μ₀I/L, discusses the core's relative permeability and finds where the graph departs from linearity.
Where marks are lost
Research design: current falling as turns are added because resistance rises, and no control of it. Data analysis: ignoring that winding length also changes. Evaluation: overlooking Earth's field and background offset in the probe.
Data
Needs a Hall probe or a calibrated phone magnetometer; the main uncertainty is probe position and coil heating.

Capacitance of parallel plates with different dielectric sheets

Research question. How does the capacitance of a parallel plate capacitor with plates of 15 cm × 15 cm change when different sheets of thickness 1 mm to 5 mm of acrylic are placed between them?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. A good choice, as it gives a real relationship and a natural linear graph, though it is a low-capacitance measurement so care is needed. The twist is comparing your εᵣ with tables and explaining any air gap effect.

Method, physics and where marks are lost+
Independent variable
Thickness of the dielectric between the plates, d from 1 mm to 5 mm using stacked acrylic sheets (5 values); a second run compares acrylic, glass, paper and cardboard sheets at the same thickness.
What you measure
Capacitance measured with a capacitance meter (pF) or by the discharge time constant τ = RC through a known resistor with a data logger; charge from Q = CV.
Controlled variables
Plate area: same two aluminium plates, measured with a rule. Plate separation: set by the sheet thickness measured with a micrometer, with plates pressed evenly. Voltage: a fixed 5.0 V for charge calculations. Humidity and stray capacitance: same wires and layout, zero reading with wires alone subtracted.
Physics and graph
C = ε₀εᵣA/d. Plot C (y) against 1/d (x): the gradient is ε₀εᵣA, giving εᵣ. Stray capacitance from the meter leads gives a positive intercept. Q = CV then follows.
SL and HL
SL: a linear plot of C against 1/d and εᵣ from the gradient, compared with data. Top band: subtract the stray capacitance, deal with the air gaps between plates and sheets, and compare materials by εᵣ with uncertainty.
Where marks are lost
Research design: the original mixes materials and capacitance and charge without a clear plan, and does not say how C is measured. Data analysis: small pF values with a large stray contribution. Conclusion: the εᵣ found is not compared with accepted values. Evaluation: air gaps between the plates and sheets lower the apparent εᵣ, and this is missed.
Data
Needs two metal plates, sheets of insulator and a capacitance meter or logger; the main uncertainty is the air gap and stray capacitance.

Counting excess electrons on a balloon from its repulsion of a second charge

Research question. Using Coulomb's law, how many excess electrons are on a rubbed balloon, found from the deflection of a suspended second balloon at separations of 4 to 12 cm?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Hard data
  • Rarely listed

My take. Fun and cheap but hard to make rigorous. Use small foil-coated spheres instead of balloons to make the point charge model defensible.

Method, physics and where marks are lost+
Independent variable
Separation r between two identically charged balloons, 4 to 12 cm in 1 cm steps (at least six values), each charged by the same rubbing routine and repeated three times.
What you measure
Deflection angle of a light balloon or foil ball hung on a thread, measured from a video frame with a ruler grid. Force from F = mg tan(θ), charge q from F = kq2/r2, and number of electrons N = q/e.
Controlled variables
Humidity monitored with a hygrometer and kept in a narrow range; identical mass and size of the two objects; same rubbing material and count of strokes; thread length constant.
Physics and graph
Coulomb's law F = kq1q2/r2 and force balance on a suspended charge. Plot F against 1/r2, gradient = kq2, so q = √(gradient/k). Charge leaks with time so timing matters.
SL and HL
SL students get q and N and comment that the answer is of order 1010 to 1011. Top band work models charge leakage over time and separates the effect of induced charge on the neutral parts of the setup.
Where marks are lost
Data analysis: assuming point charges though balloons are large. Evaluation: charge decay during measurements and humidity ignored. Conclusion: no comparison with a known charge scale.
Data
Balloons, thread, camera and ruler are enough; charge leaks quickly and balloons are not point charges, so the uncertainty is large.

Elementary charge from balanced or falling oil droplets

Research question. What value of the elementary charge e is found from at least 15 oil droplets by measuring their rise and fall times in a Millikan apparatus at 300 to 500 V?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Hard data
  • Rarely listed

My take. Only worth choosing with real apparatus, and then it is strong. If you do not have the kit, a database or simulation study is much weaker, so consider another topic.

Method, physics and where marks are lost+
Independent variable
Droplet identity (charge on each droplet), at least 15 droplets, each timed over several rise and fall traverses; optionally two plate voltages such as 300 V and 500 V.
What you measure
Fall time and rise time across a fixed number of graticule divisions, using a stopwatch or video; charge on each droplet calculated, then the smallest common divisor of all charges found.
Controlled variables
Plate spacing fixed; oil density and temperature recorded; same graticule distance for each timing; plate voltage measured with a voltmeter and held constant during a traverse.
Physics and graph
Terminal velocity from Stokes' law 6 pi eta r v = weight minus upthrust; then q = mg/E at balance or from the rise and fall speeds. Plot charge against droplet number, sorted, and look for steps; gradient of q against integer n gives e.
SL and HL
SL students calculate q and find approximate multiples of e. Top band work applies the Cunningham slip correction, uses a statistical test for the divisor, and evaluates the effect of droplet evaporation.
Where marks are lost
Data analysis: small droplets and Brownian motion cause scatter, and students force the result to 1.6 times 10-19 C. Evaluation: reaction time in stopwatch timing not treated.
Data
Needs a Millikan apparatus, which many schools lack; timing droplets by eye is hard and reaction time is the main uncertainty.

Equipotential mapping between different electrode shapes

Research question. How does the electric field strength, in V m⁻¹, vary with distance from the centre of a circular electrode in conducting paper, compared with a parallel plate arrangement, over 1 cm to 8 cm?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. Simple to run and easy to get right, but examiners want quantitative analysis, not just a picture. Add a linearised graph for the radial case and it becomes worthwhile.

Method, physics and where marks are lost+
Independent variable
Position along a radial line, 8 values at 1 cm spacing, for two electrode geometries (parallel plates and a central disc with an outer ring).
What you measure
Potential measured with a digital voltmeter and probe on conducting paper (uncertainty ±0.01 V). Calculated: E = −ΔV/Δx between neighbouring points.
Controlled variables
Supply voltage: fixed at 10 V and checked. Paper: same sheet or same batch, to keep resistivity constant. Electrode contact: silver conductive paint for a uniform edge. Probe pressure: light and consistent.
Physics and graph
E = −dV/dx. For a parallel plate E is constant; for a point-like or radial arrangement E ∝ 1/r. Plot V against ln r (gradient related to charge factor) or E against 1/r and check for linearity.
SL and HL
SL students can map potential and estimate E from slopes. A top band study tests the 1/r prediction with a linearised graph and quantifies the deviation near the edges. HL depth can come from relating the results to potential and field theory for radial fields.
Where marks are lost
Research design: too few points near the electrode where the field changes fastest. Data analysis: computing E from widely spaced points without considering the error this brings. Evaluation: ignoring paper non-uniformity and edge effects.
Data
Needs conducting paper, a low voltage DC supply and a digital voltmeter; the main uncertainty is inhomogeneous paper and probe contact.

Field inside a solenoid against current, to find μ-zero

Research question. How does the current I in a solenoid (0.5 to 3.0 A, six values) affect the flux density at its centre, and what value of the permeability of free space follows?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. Solid and safe but common in spirit. Make it yours by adding an iron core or by comparing solenoid length against diameter and testing when the long solenoid formula breaks down.

Method, physics and where marks are lost+
Independent variable
Solenoid current from 0.5 to 3.0 A in steps of 0.5 A, using a variable DC supply and ammeter, with three readings for each current and the direction reversed once.
What you measure
Flux density B at the centre from a calibrated Hall probe, in mT; mu0 calculated from the gradient of B against I with the known number of turns per metre.
Controlled variables
Number of turns and solenoid length measured once; probe kept on the axis and centre, held by a clamp; probe zeroed with current off each time; coil not left on long to avoid heating.
Physics and graph
For a long solenoid B = mu0 n I. Plot B against I: gradient = mu0 n, so mu0 = gradient/n. Length to diameter ratio should be at least 10 for the formula to hold, otherwise an end correction is needed.
SL and HL
SL students get mu0 with a percentage difference from the accepted value. Top band work adds an iron core to find relative permeability, or maps B along the axis and compares with theory.
Where marks are lost
Data analysis: the probe zero offset and Earth's field give a non-zero intercept and are ignored. Evaluation: coil heating raises resistance and lowers current. Note the RQ should say permeability of free space, not relative permeability.
Data
Solenoid, DC supply, ammeter and Hall probe; probe calibration and background field are the main uncertainty.

Field strength at different distances from a straight wire

Research question. How does the magnetic flux density B change with perpendicular distance r, from 1 cm to 8 cm in 1 cm steps, from a long straight wire carrying 5.0 A?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. Worthwhile and hands-on. Use a fit of B = k/(r + r₀) so that the unknown sensor offset becomes a result of your own, which makes it more than a routine test.

Method, physics and where marks are lost+
Independent variable
Perpendicular distance r from the wire, 1 to 8 cm (8 values), measured to the sensor's element with a ruler and repeated 3 times.
What you measure
Flux density measured with a Hall probe or a phone magnetometer, corrected by subtracting the reading with the current off. Field is reported in microtesla.
Controlled variables
Current: hold at 5.0 A with a power supply, checked with an ammeter. Wire orientation: keep it vertical and straight, away from steel. Probe orientation: keep the sensitive axis tangential to the field. Earth's field and the surroundings: take a zero reading each time and use the same location.
Physics and graph
B = μ₀I/(2πr). Plot B against 1/r; gradient is μ₀I/2π, so μ₀ = 2π·gradient/I, compared with 4π × 10⁻⁷ N A⁻². At 5 A and 5 cm, B is only about 20 µT, comparable to Earth's field.
SL and HL
SL students plot B against 1/r and compare μ₀ with the accepted value. Top band adds the uncertainty on r from the sensor's unknown internal position, treated as an offset in the intercept. HL can add the vector sum with the Earth's field.
Where marks are lost
Research design: current too small so the signal is buried in noise. Data analysis: forgetting to subtract the background. Evaluation: not considering the finite wire length and the sensor's position inside its casing, and wire heating.
Data
Needs a Hall probe or magnetometer and a supply giving 5 to 10 A; a loop of wire and heating may limit the current, and the uncertainty from probe position is dominant.

Field uniformity between two coils at varying spacing

Research question. How does the separation of two identical coils, varied from 0.5R to 1.5R in six steps, affect the percentage variation of the on-axis field over the central 4 cm?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. An excellent choice with a clear theory target at d = R. Do it as one focused study of coil separation and leave out the solenoid turn-density variant.

Method, physics and where marks are lost+
Independent variable
Coil separation d, 6 values from 0.5R to 1.5R where R is the coil radius (for example 5 cm to 15 cm for R = 10 cm), with the field mapped at 9 or more positions for each.
What you measure
On-axis field measured with a Hall probe along a ruler; uniformity is calculated as (Bmax − Bmin)/Bcentre × 100% across the central 4 cm.
Controlled variables
Coil current: hold constant with a DC supply and check with an ammeter. Coil alignment: keep the axes collinear with a common rod. Probe orientation: keep it along the axis. Background field: subtract the current-off reading.
Physics and graph
For a Helmholtz pair, B = (4/5)3/2 μ₀NI/R at the centre when d = R, and the second derivative of B along the axis is zero there. Plot uniformity (%) against d/R and find the minimum; compare with d = R.
SL and HL
SL students plot uniformity against separation and identify the best spacing. Top band compares the centre field to the theory value and considers the effect of coil thickness. HL can derive the on-axis field formula and show why the second derivative vanishes at d = R.
Where marks are lost
Research design: too few axial positions so that uniformity is not resolved. Data analysis: not using a percentage measure consistently. Evaluation: ignoring the finite thickness of the coils and the probe's position error.
Data
Needs two matching coils, a 1 to 2 A supply and a Hall probe; the position uncertainty of about 1 mm and probe noise limit the uniformity to about 1%.

Levitation height of a magnet above a fixed base magnet

Research question. How does the mass of a floating ring magnet (adding 5 g to 40 g of weights in steps of 5 g) affect its equilibrium height above a fixed magnet on a plastic rod?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Easy data
  • Rarely listed

My take. Simple and doable, and good for a power law graph. Change the IV to mass and keep magnet field for a secondary measurement, so the RQ is actually testable.

Method, physics and where marks are lost+
Independent variable
Added mass on the floating ring magnet from 5 g to 40 g in 5 g steps, eight values, three trials each.
What you measure
Gap between the magnets, measured from a photo taken with a ruler in frame, in mm; the field of the base magnet is measured separately with a Hall probe against distance.
Controlled variables
Same pair of magnets; alignment on a smooth vertical plastic rod; same time allowed for settling; no steel or magnetic objects nearby.
Physics and graph
At equilibrium repulsion equals weight, F = mg. For dipoles F falls roughly as 1/h4, so plot ln(m) against ln(h) with gradient about -4. The stronger the magnet, the higher it floats for a given load.
SL and HL
SL students give the power law and its exponent. Top band work compares different magnet grades and models the finite size of the magnets rather than point dipoles.
Where marks are lost
Research design: field strength cannot be varied directly, so vary mass instead and measure field separately. Data analysis: friction against the rod adds hysteresis. Evaluation: photo parallax.
Data
Ring magnets, a plastic rod, small masses and a camera; friction on the rod and parallax in the photo cause most of the scatter.

Magnetic flux density of a neodymium magnet from 0 to 80 degrees Celsius

Research question. How does the flux density measured 5 mm from the pole of a ferrite magnet change as its temperature is raised from 20 to 90 degrees Celsius in eight steps?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Hard data
  • Rarely listed

My take. Original and personal. Do it with a thin waterproof coat on the magnet and a probe kept away from the heat, or your data will show the probe rather than the magnet.

Method, physics and where marks are lost+
Independent variable
Magnet temperature, eight values from 20 to 90 degrees Celsius, set in a water bath and checked by thermometer; three repeat readings per value, with a cooling run to check reversibility.
What you measure
Flux density B from a Hall probe or a phone magnetometer (calibrated) held at a fixed 5 mm distance by a 3D printed or wooden jig, in mT.
Controlled variables
Probe to magnet distance fixed by the jig; probe zeroed for background field before every reading; same orientation of the magnet; measurement made quickly after removal from bath while noting the cooling.
Physics and graph
Ferromagnetic order weakens as thermal energy rises towards the Curie temperature. Over a small range B is close to linear with T, so plot B against T with gradient in mT per degree. Ferrite decreases more slowly than neodymium and strongly hysteretic behaviour may show.
SL and HL
SL students report the gradient and compare with the supplier's temperature coefficient. Top band work checks reversibility, and separates reversible and irreversible loss.
Where marks are lost
Research design: the magnet cools between bath and probe and the temperature is wrong; probe sensitivity itself changes with temperature. Evaluation: heating too high causes permanent loss.
Data
Needs a Hall probe or calibrated magnetometer and a heated water bath; probe temperature drift and cooling during measurement are the main uncertainties.

Magnetic response of metals from force on a balance

Research question. How does the apparent change in mass of iron, aluminium and copper samples of equal volume depend on the current through an electromagnet, from 0.5 A to 4.0 A in steps of 0.5 A?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Hard data
  • Rarely listed

My take. Too ambitious as written, and I would narrow it to iron only, with hysteresis by running the current up and down. It is a real novelty and unusual, but keep the scope small.

Method, physics and where marks are lost+
Independent variable
Electromagnet current (0.5 A to 4.0 A, 8 values) at a fixed distance, with three materials as a second variable.
What you measure
Change in reading of a top pan balance (0.01 g) with the sample hung above the pole; the force is F = Δm g, and the field is measured with a Hall probe (mT).
Controlled variables
Sample volume and shape: cylinders of the same size, checked with calipers. Distance from the pole: fixed with a spacer. Temperature: coil allowed to cool between readings to avoid changing current. Position: sample held in place with a non-magnetic thread.
Physics and graph
Force on a sample in a non-uniform field is proportional to χVB(dB/dx)/μ₀, so F is proportional to B² for a constant gradient. Plot F (y) against B² (x): the gradient gives χ. Iron is ferromagnetic and not linear, and copper is diamagnetic, so it is repelled. A true hysteresis loop needs a B and H measurement on the way up and down.
SL and HL
SL: qualitative and quantitative comparison of the three materials and a plot of F against B². Top band: run the current up and then down for iron to show hysteresis, and estimate χ for aluminium. HL is not required, but the analysis is demanding.
Where marks are lost
Research design: the input idea asks for susceptibility and hysteresis together, which is too much for one IA. Data analysis: forces for copper and aluminium are tiny and comparable to balance noise, so the uncertainty is not treated. Conclusion: claiming a value of χ with no comparison to the tables. Evaluation: not commenting on the field gradient and remnant magnetism in the core.
Data
Needs an electromagnet, a sensitive balance and a Hall probe; the forces on copper and aluminium are near the noise level.

Mapping field uniformity between plates with a conducting paper probe

Research question. How does the plate separation d (1.0 to 6.0 cm, six values) affect the width of the region where the electric field is uniform to within 5 percent between two parallel plates at fixed voltage?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. A good fit if you define the uniformity threshold clearly. Add your own comparison with a numerical solver to make it yours.

Method, physics and where marks are lost+
Independent variable
Plate separation d from 1.0 to 6.0 cm in 1.0 cm steps, set with spacers, at least six values, three traverses each.
What you measure
Width of the central region where the potential gradient stays within 5 percent of the mid value, found by moving a voltmeter probe across conducting paper or a shallow water tray and taking potential at 0.5 cm intervals.
Controlled variables
Supply voltage fixed at about 10 V DC (or low AC with an AC probe in water); plate length and width fixed; same paper or same water depth and conductivity; probe moved along the same centre line.
Physics and graph
For ideal plates E = V/d. Edge fringing grows with d. Plot uniform width against d; ratio of uniform width to plate length against d/L is a good dimensionless graph. Equipotential lines are perpendicular to field lines.
SL and HL
SL students map potentials and find the trend. Top band work compares with a simulation (for example a free field solver) and explains fringing quantitatively.
Where marks are lost
Research design: 'area of uniform field' is not measurable until a 5 percent criterion is defined. Data analysis: probe positions with no uncertainty. Evaluation: paper resistivity not uniform.
Data
Conducting paper kit, power supply and a digital voltmeter; the main uncertainty is probe contact and paper inhomogeneity.

Motor speed against core material in a hand-wound electromagnet

Research question. How does the magnetic flux density at the end of a 200-turn electromagnet vary with core material (air, iron nail, steel bolt, copper rod, aluminium rod, ferrite) when a current of 0.50 A is used?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. I turned a vague motor efficiency idea into a clean field measurement. The saturation curve is the personal twist, and it makes the physics deeper.

Method, physics and where marks are lost+
Independent variable
Core material: 6 types of the same 100 mm length and diameter. A second run varies current from 0.2 to 1.0 A in 5 steps for the best core.
What you measure
Magnetic flux density B measured with a Hall probe (or phone magnetometer) at a fixed 10 mm from the core end, 3 readings each. Calculate the relative permeability from B against the air core value.
Controlled variables
Number of turns and coil geometry, one coil used for all cores. Current, held by a variable power supply and checked on an ammeter. Probe position, fixed by a clamp and ruler. Coil temperature, kept low by short switching on periods.
Physics and graph
Solenoid field B = μ0 μr nI. Plot B against I for the iron core. The gradient is μ0 μr n, so μr can be found. Compare across materials.
SL and HL
SL students compare the cores and one current series. To reach the top: explain saturation and remanence, correct for Earth's field, and show B against I bending over.
Where marks are lost
Research design: the original motor version has too many uncontrolled variables, so I moved to the electromagnet. Data analysis: not subtracting background field. Evaluation: heating changing coil resistance and current.
Data
Needs a Hall probe or phone magnetometer and a power supply; the main uncertainty is probe position, as B falls quickly with distance.

Photodiode current against distance from a lamp

Research question. How does the distance from a small LED lamp to a photodiode, varied from 10 cm to 60 cm in steps of 10 cm, affect the short circuit photocurrent, and does it follow an inverse square law?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Easy data
  • Rarely listed

My take. Safe and easy but it can look basic. Add a twist by fitting the offset in x, or by comparing an LED with a filament lamp to see how the pattern changes.

Method, physics and where marks are lost+
Independent variable
Distance x from lamp to photodiode along a metre rule, 6 to 8 values from 10 to 60 cm, each repeated 3 times.
What you measure
Photocurrent measured with a multimeter on the microamp range, or the voltage across a fixed resistor from a data logger.
Controlled variables
Lamp power: stabilised supply with a fixed current and warm up time. Ambient light: dark room or a black tube, with a background reading subtracted. Alignment: lamp and diode on the same axis, diode facing the lamp. Temperature of the diode: short readings so it stays constant.
Physics and graph
For a point source, intensity I = P/(4πx²), and the photocurrent is proportional to intensity within the linear range. Plot Iphoto against 1/x². Straight line through the origin means the law holds. A log-log plot gives the exponent. Curved data at short range shows the source is not a point.
SL and HL
SL students confirm the trend with a linear graph. Top band work subtracts background, tests that current is linear in intensity with filters, and includes an effective origin correction for the lamp filament position. HL students can connect to photon flux and the photoelectric effect.
Where marks are lost
Research design: stray light not controlled or subtracted. Data analysis: distance measured to the wrong reference point. Conclusion: forcing an inverse square fit at short distances where it fails. Evaluation: not checking saturation of the photodiode.
Data
Only a lamp, a photodiode, a multimeter and a ruler are needed, and the main uncertainty is stray light and the offset of the true source position.

Testing the inverse square law with charged spheres

Research question. How does the separation between two charged metal-coated polystyrene spheres, varied from 2.0 cm to 10.0 cm in steps of 2.0 cm, affect the electrostatic force between them, found from the deflection angle of a hanging sphere?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Hard data
  • Rarely listed

My take. Physically classic but hard to get clean data in a normal school room. Worth it only if you can control humidity and measure charge, otherwise choose something more forgiving. A personal twist is to test how fast the charge leaks and correct for it.

Method, physics and where marks are lost+
Independent variable
Centre to centre separation r of two charged spheres, 5 to 6 values from 2.0 to 10.0 cm, each repeated 3 to 5 times.
What you measure
Deflection angle of a suspended sphere, read from a photo against a protractor or by trigonometry on a ruler. Force is calculated as F = mg tanθ.
Controlled variables
Charge on each sphere: recharge from the same supply for a fixed time and check with a field meter or note the voltage. Sphere mass and thread length: same pair throughout. Humidity: run in one session, note it, use a dry room. Height of the spheres: aligned with a levelled stand.
Physics and graph
Coulomb's law F = kq1q2/r². Plot F against 1/r², a straight line through the origin if the law holds. Gradient is kq1q2, and if the charge is roughly known the value of k can be compared with the accepted one. Charge leakage means the exponent is worth fitting with a log-log plot too.
SL and HL
SL students verify the trend and the linear plot. Top band work fits the exponent with uncertainty, corrects for the fact that the force is not horizontal at large angles, and models charge leakage over time. HL students can link to field strength and potential.
Where marks are lost
Research design: charge not kept constant, so the variable is uncontrolled. Data analysis: uncertainty in r ignored, and no treatment of the angle error. Conclusion: claiming exactly r⁻² without a fitted exponent and its error. Evaluation: not discussing charge leakage and induced charges in nearby objects.
Data
Needs a high voltage supply or a charged rod, and humidity ruins repeatability, so the main uncertainty is charge decay between readings.

Where a bar magnet starts to behave like a dipole

Research question. How does the on-axis field B of a cylindrical neodymium magnet fall with distance x from 2 cm to 20 cm, and at what distance does the exponent reach 3?

  • D.2 Electric and magnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. A good option for a student who enjoys graphs. The twist is treating the exponent as a function of distance rather than assuming −3 from the start.

Method, physics and where marks are lost+
Independent variable
Axial distance x from the magnet centre, 2 to 20 cm in 2 cm steps (10 values), 3 repeats.
What you measure
Field strength measured with a Hall probe or a phone magnetometer along a ruler, with the zero-magnet reading subtracted. The exponent is found from the gradient of ln B against ln x for different ranges of x.
Controlled variables
Magnet orientation: keep the axis aligned along the ruler. Sensor orientation: keep the sensitive axis on the magnet axis. Nearby iron and electronics: clear the bench and use a wooden ruler. Background field: measure it before each run and subtract.
Physics and graph
For a dipole, B = μ₀m/(2πx³) on the axis, so B ∝ x⁻³ at large x. Plot ln B against ln x; gradient tends to −3 as x becomes larger than the magnet size. Compare gradients from near and far subsets.
SL and HL
SL students fit a power law and quote the exponent with its uncertainty. To reach the top band, separate the near-field and far-field fits and explain the change with the magnet length. HL can estimate the magnetic moment m and use a finite-magnet model.
Where marks are lost
Research design: readings too close where the sensor saturates. Data analysis: measuring from the wrong reference point on the magnet. Evaluation: not correcting for background, and taking centre versus end distance.
Data
Needs a strong magnet and a Hall probe or magnetometer; the position of the sensing element inside the phone is unknown and adds to the uncertainty of x.

D.3 Motion in electromagnetic fields: 4 ideas

Electron mass from a deflection tube using electric and magnetic fields

Research question. What value of the electron mass is obtained from a deflection tube when the accelerating voltage is varied from 2000 to 5000 V (six values) and the magnetic deflection radius is measured?

  • D.3 Motion in electromagnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. Good and classic if your school owns the tube. Frame it as e/m first, then mass, and be open that e is taken from the literature.

Method, physics and where marks are lost+
Independent variable
Accelerating voltage V from 2000 to 5000 V in 500 V steps, with Helmholtz coil current adjusted or held fixed, three readings each.
What you measure
Radius r of the electron beam path read from the tube scale, coil current measured with an ammeter to get B, then e/m calculated. The electron mass follows only by using the accepted value of e.
Controlled variables
Helmholtz coil current for each set kept constant with a stable supply; tube orientation aligned so that the field is perpendicular to the beam; Earth's field cancelled or its direction noted; warm up time kept the same.
Physics and graph
eV = 1/2 mv2 and evB = mv2/r give e/m = 2V/(B2 r2). Plot r2 against V at fixed B: gradient = 2m/(eB2). B from a Helmholtz coil: B = (4/5)1.5 mu0 N I / R. Then m = e/(e/m).
SL and HL
SL students get e/m and m from one linear graph. Top band work quantifies B uncertainty, corrects for Earth's field and discusses parallax on the beam radius.
Where marks are lost
Research design: mass cannot be measured directly, so the RQ should be e/m and then mass with a quoted e. Evaluation: parallax on radius and uniformity of B missed.
Data
Needs a school fine beam tube with Helmholtz coils and an EHT supply; parallax on the beam radius is the main error.

Magnetic force on a wire versus current

Research question. How does the current in a 5.0 cm straight wire, varied from 1.0 A to 5.0 A in steps of 0.5 A, change the force it feels between two magnadur magnets, measured in mN?

  • D.3 Motion in electromagnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. A solid, classic set-up that works in a school lab. Make it yours by comparing the balance-derived B with a Hall probe, or by testing how the field falls off near the magnet edges.

Method, physics and where marks are lost+
Independent variable
Current through the wire, 1.0 to 5.0 A in 9 steps (at least 5 values), each repeated 3 times, with the wire held perpendicular to the field.
What you measure
Force on the wire, found from the change in reading on a top-pan balance (0.001 g resolution) that supports the magnet yoke; F = Δm g. Calculated: F/L for each current.
Controlled variables
Active length of wire: fixed by using the same pole width, checked with a ruler. Angle to the field: set by clamping the wire and checking with a protractor. Field strength: same magnet pair, same position, checked with a Hall probe. Heating: current switched on only for a few seconds per reading.
Physics and graph
F = BIL sin θ. Plot F (y) against I (x): a straight line through the origin with gradient BL, so B = gradient / L. Compare B with a Hall probe reading.
SL and HL
SL students can get a clean linear graph and a value of B with uncertainty. Top band work checks the Hall probe value, discusses the fringe field beyond the poles, and tests whether the effective length is really the pole width. HL depth can come from repeating with the angle varied and fitting sin θ.
Where marks are lost
Research design: not controlling the effective length or the balance drift. Data analysis: ignoring the zero offset and uncertainty in the small mass changes. Evaluation: not commenting on fringe fields and wire heating.
Data
Needs a top-pan balance, a variable DC supply, an ammeter and magnadur magnets; the main uncertainty is the small force (a few mN) and the non-uniform field at the pole edges.

Neodymium magnet stack size and simple motor rotation rate

Research question. How does the number of stacked neodymium magnets (1 to 6, each 10 mm diameter) under a battery-and-wire homopolar or coil motor affect its rotation rate, in revolutions per second?

  • D.3 Motion in electromagnetic fields
  • SL and HL
  • Needs care
  • Rarely listed

My take. Worth doing if you measure B directly rather than just counting magnets. Adding a fixed load and comparing torque is a good personal twist.

Method, physics and where marks are lost+
Independent variable
Number of identical stacked magnets, 1 to 6, giving six values; each value repeated 5 times. Field strength at the coil is measured separately with a Hall probe.
What you measure
Rotation rate from slow-motion phone video (240 fps) counting frames per revolution, in rev/s; Hall probe reading of B in mT at the coil position.
Controlled variables
Same battery, checked with a voltmeter before each run; same coil or wire shape and mass; same contact resistance, cleaned and rewound each run; same distance from magnet to coil, set with a spacer.
Physics and graph
Force on a current-carrying conductor F = BIL, so torque and speed rise with B until friction and back emf balance. Plot rotation rate against B measured by the Hall probe. Check whether the line is straight and what the intercept says about friction.
SL and HL
SL: measure rate against B and describe the trend with uncertainties. Top band: model back emf and friction to explain why rate levels off. HL can link to induced emf in D.4.
Where marks are lost
Research design: field strength is assumed from magnet count instead of measured. Data analysis: rate is timed by eye with large uncertainty. Evaluation: battery voltage drift and contact friction ignored.
Data
Needs a Hall probe, magnets and a phone camera; the main uncertainty is inconsistent electrical contact and battery drain between runs.

Number of coil turns and speed of a simple DC motor

Research question. How does the number of turns on the coil of a simple DC motor (10 to 60 turns, 6 values) affect its no load rotation rate at a fixed supply voltage of 3.0 V?

  • D.3 Motion in electromagnetic fields
  • SL and HL
  • Hard data
  • Rarely listed

My take. Only worth it if you can build a repeatable motor, and expect surprises, since more turns may not mean faster. Twist: measure current as well and show how back emf explains the trend.

Method, physics and where marks are lost+
Independent variable
Number of turns of enamelled copper wire on the rotor coil, 10, 20, 30, 40, 50, 60 turns, rewound each time on the same former. Three repeats per value.
What you measure
Rotation rate measured from video in slow motion at 240 fps by counting frames per revolution, or with a phone strobe app, in revolutions per second. Angular velocity ω = 2π f. Current from an ammeter recorded alongside.
Controlled variables
Supply voltage: fixed at 3.0 V from a bench supply and checked with a voltmeter under load. Magnet strength and gap: same magnets, same position. Coil size and wire gauge: same former and same wire. Friction at the bearings: same supports, same lubrication, coil balanced by eye.
Physics and graph
Torque on a coil τ = NBIA sin θ, and back emf ε = NBAω sin θ. With more turns the resistance also rises, so the current falls. At steady speed the supply voltage balances back emf and resistive drop, so ω is roughly V/(NBA) if resistance is small, meaning speed may fall with N, not rise. Plot ω against 1/N and see if the result is linear.
SL and HL
SL: measure and describe the trend, and explain using force on a current in a field. Top band or HL: use back emf and Faraday's law to predict the shape, and link the torque and resistance to the fitted curve, from D.4 induction (HL).
Where marks are lost
Research design: unbalanced hand made coils give erratic speeds and this is not controlled. Data analysis: speed not converted to a proper unit and no uncertainty. Conclusion: a prediction of faster speed with more turns not tested against the physics. Evaluation: unreliable starts, wire heating and contact friction at the commutator.
Data
Needs a simple motor kit, magnets, enamelled wire and slow motion video; the main uncertainty is friction and contact resistance at the brushes.

Ideas were collected from public lists and published IA titles, merged when they are the same investigation, and rewritten from scratch. “On N sites” counts how many public lists carry the same investigation. How this list was made.

Frequently asked questions

What are good IB Physics IA ideas on gravitational, electric and magnetic fields?

+
Good starting points with easy data that few sites list are testing kepler's third law with moons of jupiter, weight against mass and local g from a force sensor and levitation height of a magnet above a fixed base magnet. Each one gives a straight-line graph from school equipment, which is what the Data analysis and Conclusion criteria need.

Which gravitational, electric and magnetic fields IA ideas are overdone?

+
turns on an electromagnet and its pulling force appear on three or more public lists. They still work, but they need a twist that shows your own thinking.

Can I do a gravitational, electric and magnetic fields IA at SL?

+
Yes. All 22 ideas on this page use physics from the SL syllabus. HL students can take the same ideas further, and each idea says how.

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