IB Physics: the mistakes examiners report most often

Free-body diagrams gain an extra centripetal force arrow or a centrifugal force, while real forces such as buoyancy, friction between stacked blocks or weight in a vertical loop are left out. Constant acceleration equations are used where acceleration varies, momentum conservation is used to show a collision is elastic, and a rolling or rotating body is treated as a point mass.

Why it happens. Students go straight to an equation they recognise instead of first deciding which forces act and which quantities are conserved. Centripetal force sounds like a separate force, and drag and buoyancy are often ignored in practice questions, so they are forgotten when they matter.

How to fix it. Start every mechanics question with a free-body diagram for each object, drawn from the point of application, showing only real forces with the body that exerts them. Ask whether acceleration is constant before using the suvat equations, and recalculate it for each stage of the motion. For collisions remember that momentum is always conserved and kinetic energy only in elastic ones, so elasticity is tested by comparing kinetic energies.

Wave speed is given when the speed of a particle in the medium is asked, a position axis is read as time, and angles are taken from wavefronts instead of rays. In interference questions the source phase difference or the half-wavelength phase change on reflection is ignored, single-slit questions are answered with the grating formula, and the number of grating maxima is rounded up or counted on one side only.

Why it happens. Students rarely picture what the medium is doing, so a displacement graph is read as a picture of motion. Interference, single-slit and grating patterns are taught with similar-looking formulas, and students pick one without checking which situation it describes.

How to fix it. Before using a wave graph, say aloud whether the horizontal axis is time or position. Separate wave speed (the pattern) from particle speed (the oscillation). For grating maxima take the integer part of n, count orders on both sides and add the central maximum. For standing waves, place nodes and antinodes from the boundary conditions given in the stem before calculating anything.

Students cannot read coordinates in the second frame, draw the moving axes in random directions, send light paths backwards in time instead of at 45 degrees, or pick a worldline faster than light. When a distance can be read straight off the diagram they apply length contraction instead, and they confuse when an event happens with when an observer sees it.

Why it happens. Relativity is learnt as a set of formulas for time dilation and length contraction, and the diagram is treated as an illustration rather than as the calculation. The difference between an event occurring and its light reaching someone is rarely made explicit.

How to fix it. Practise drawing the second frame's axes from the speed: both tilt towards the 45 degree line by the same angle. Read coordinates by drawing lines parallel to the other axis, never perpendicular. For every question on order of events, decide first whether it asks when events occur in a frame or when light from them arrives.

Binding energy is defined as the energy holding the nucleus together, positron masses are forgotten in a mass defect, and comparisons of binding energy per nucleon have no justification. In decay, background is not subtracted or not added back, half-life is defined with mass, the fraction remaining is confused with the fraction decayed, and fusion conditions are mixed up with fission.

Why it happens. Students learn the words of definitions loosely and do not check them against the physics. Decay calculations look routine, so the extra step for background or for the daughter product is easy to drop.

How to fix it. Define binding energy as the energy needed to separate a nucleus into its nucleons, and half-life using the number of undecayed nuclei. List every particle on both sides of a reaction, including incoming neutrons and emitted positrons. With measured count rates, subtract background first, apply the decay, and add background back if a reading is asked for.

Explanations of threshold frequency do not compare photon energy with the work function, kinetic energy is confused with work function, the work function is not seen as the intercept of a graph, and joules and electronvolts are mixed. Students count only some transitions between energy levels, use a ratio of energies instead of energy differences, and miss that Compton scattering increases wavelength while the speed is unchanged.

Why it happens. The photoelectric equation is easy to substitute into, so the physical argument about one photon and one electron is not learnt. Energy level questions are done by recall of a diagram rather than by counting.

How to fix it. Learn a short explanation: photon energy is proportional to frequency, one photon gives its energy to one electron, and emission needs that energy to exceed the work function. Keep every energy in eV or every energy in joules, never both. For emission lines list every pair of levels systematically, and work with differences between levels.

Derived units such as the newton or pascal are not reduced to kg, m and s, squared velocities are left unsquared in units, and areas in mm squared are converted with the linear factor. Powers of ten are lost in multiple-choice and in Paper 2 calculations, and when two quantities change together students assume the effects cancel or invert the final ratio.

Why it happens. Unit and power-of-ten work is seen as arithmetic, not physics, so it is not checked. Ratio questions are tackled by guessing from the options instead of writing the equation and substituting the changes.

How to fix it. Learn the base units of the common derived units (N, J, W, Pa, C, V) and practise reducing an expression step by step. Square or cube the conversion factor for areas and volumes. For a ratio question, write the relevant equation, put the changed values in, and divide, then check that the answer is physically sensible.

Potential is added as a vector, field strength is found as V/r instead of the gradient of the potential graph, and the field midway between opposite charges is taken as zero. In orbits and energy changes students forget to square the radius, leave kilometres unconverted, quote the data booklet instead of deriving, ignore potential at the surface, or use a point-charge formula in a uniform field.

Why it happens. Field, potential, force and potential energy have similar equations, and students do not keep track of which are vectors and which are scalars. Geometry is also rushed: distances from a diagram or from a diameter are not checked.

How to fix it. Learn the four quantities in a grid (field, potential, force, energy) with the scalar or vector nature of each. Sketch the geometry and mark each distance before substituting, and convert to metres at once. Find field strength as the gradient of a potential graph, and for energy changes use the potential at both positions.

Students restate the stem instead of explaining that greenhouse gases absorb more of the Earth's infrared radiation, invent an emissivity to reach a given result, stop after the first step of an energy balance, and use absorbed intensity as if it were incident intensity in albedo. Conduction is described without the mechanism of vibrations passed between neighbouring particles.

Why it happens. These topics are learnt as descriptive text, so students write general sentences rather than a chain of physics. The Stefan-Boltzmann law is remembered without the role of emissivity, area and the balance between incoming and outgoing power.

How to fix it. Write energy balance answers as an equation: power in equals power out, with each term named. Remember that incident intensity is absorbed plus reflected. For explanations, give the mechanism in steps: what absorbs, at which wavelength, what is re-emitted and in which direction.

Force directions are given at random, the field direction is given when the force is asked, and the right-hand rule is applied once where two steps are needed. In induction students use B instead of 2B when a field reverses, leave out the loop area, use the change in current for the change in field, and change only amplitude or only period when sketching a new emf graph.

Why it happens. Direction rules are practised in isolation and not combined, so a problem needing the field of a wire and then the force on a charge breaks down. Faraday's law is remembered as a formula without a clear idea of flux as field times area.

How to fix it. Apply direction rules in two explicit steps: first the field at the point, then the force on the charge or wire, and check whether the charge is negative. Write flux change as final minus initial flux with signs, and include area and number of turns. When the speed of a generator changes, change both the peak emf and the period on the sketch.

Students give the field instead of the change in field, write about Rutherford scattering when deviations from it are asked, do long calculations where a statement is wanted, and give generic explanations without the values when an explanation needs a supporting number. In show-that and derivation questions they work backwards from the given value or adjust a wrong result to match it, and later parts are left blank after an earlier part goes wrong.

Why it happens. Under time pressure students recognise a familiar topic and answer the question they expected. Command terms such as state, explain, deduce and show are not treated as instructions about what earns the marks.

How to fix it. Underline the quantity asked for and any qualifier such as change, difference or deviation. Match the length of the answer to the command term and the marks. In a show-that item never use the target value in the working and give one more significant figure than shown. Attempt every part, because later parts often do not depend on earlier answers.

Absolute uncertainties are added where fractional ones should be, a power is not applied to the fractional uncertainty, uncertainty is not converted with the units, and the uncertainty of repeated readings is taken from the instrument instead of the spread. Uncertainties are quoted to too many significant figures, and repeats are said to reduce systematic error.

Why it happens. The rules are learnt as a list without the reason behind each one, so they are mixed up under pressure. The new Paper 1B makes these data questions part of every session, and many students practise them far less than content questions.

How to fix it. Learn three rules and when each applies: add absolute uncertainties for sums and differences, add fractional uncertainties for products and quotients, multiply the fractional uncertainty by n for a power. Quote the uncertainty to one significant figure and round the value to the same decimal place. State clearly that repeats reduce random error only.

Dots are joined instead of drawing a best-fit line, gradients are taken from table points rather than from the line, maximum gradient lines are drawn parallel, and a straight line not through the origin is called proportional. Students suggest repeats or unavailable equipment as an improvement, give a controlled variable without saying how it would affect the result, and write conclusions that do not use the uncertainty or address the aim.

Why it happens. Graph work feels like practical routine rather than exam content, so it is done quickly. The experimental questions in Paper 1B are set in unfamiliar contexts, and students fall back on generic answers.

How to fix it. Draw best-fit lines and tangents with a ruler, and take gradient points far apart on the line itself. Know the test for proportionality: a straight line through the origin. When asked for an improvement or a controlled variable, refer to the equipment and the quantity named in the stem, and end conclusions by comparing with the aim using the uncertainty range.

Temperatures are left in degrees Celsius, molecular speed is taken as proportional to T instead of its square root, and internal energy is assumed constant during a phase change because the temperature is. In cycles the general gas law is applied to an adiabatic step, the signs of Q and W are lost, and efficiency is given as 100% or the meaning of theoretical efficiency is missed.

Why it happens. Gas questions look like substitution, so the model behind them (kinetic energy of molecules, potential energy during a change of state, energy transfers in each step) is skipped. The sign convention of the first law is learnt as a formula, not as a statement about energy in and out.

How to fix it. Convert every temperature to kelvin as the first line of working. Write the scaling explicitly when a quantity changes, for example speed proportional to the square root of T. For each step of a cycle make a small table of Q, W and change in internal energy with signs, and check which equation applies (isothermal, adiabatic or general gas law) before using it.

When a resistor is added or the load changes, students assume the current stays constant and forget the lost volts across the internal resistance. Resistance is taken as the gradient of an I-V curve instead of V/I at the point, parallel connections in an unusual drawing are not spotted, and peak values are read wrongly from an alternating current graph.

Why it happens. Most circuit practice uses ideal cells, so internal resistance is a separate formula rather than part of every circuit. Students also reason from the position of a resistor relative to the cell, as if current were used up along the wire.

How to fix it. Whenever the circuit changes, follow the chain: total resistance, current, lost volts, terminal p.d. Redraw unfamiliar networks so parallel branches are side by side. On any graph, check what the axis shows before reading a value, and find resistance from V/I at the point.

Students assume velocity has the same sign as displacement, invert energy ratios, use wrong fractions of the amplitude, work in degrees instead of radians, and for a vertical spring leave out gravitational energy (or add it for a horizontal one). On resonance curves the peak is put exactly at the natural frequency even with damping.

Why it happens. SHM is memorised as a list of equations, and the link between acceleration, displacement and energy is not used as reasoning. The energy expressions are compared by guessing rather than by writing the ratio algebraically.

How to fix it. Justify SHM by showing the restoring force or acceleration is proportional to displacement and opposite to it. Compare energies by writing kinetic and total energy in terms of amplitude and displacement and dividing. Set the calculator to radians before any SHM calculation, and list every energy store (kinetic, elastic, gravitational) for the actual orientation of the system.

Students use the moving-source formula for a moving observer, take the wrong sign, mix the sound and light forms, divide by the wrong frequency, or give an answer for a star in m/s when km/s is asked. Explanations describe the change in pitch without linking it to the spacing of wavefronts, and a shift is sometimes put down to measurement error.

Why it happens. Several versions of the equation are learnt together and the choice is made by memory, not from the situation. The physical picture of wavefronts bunching up or spreading out is skipped.

How to fix it. Sketch the source, the observer and the direction of motion, then decide whether frequency must rise or fall before choosing the sign. Use the sound form for sound and the approximate light form only for light. In any explanation describe compressed or stretched wavefronts and the resulting change in wavelength and frequency.

Each report lists, paper by paper, the areas candidates found difficult. Every item was assigned to one of the areas above, with its session and paper recorded. An area counts once per report, however many papers mention it; each time zone has its own report.

The count says how often examiners raise something, not how many marks it costs. The explanations and the advice are mine, from teaching the course, and no wording from the reports or from exam questions is reproduced. Session by session summaries are in the examiner reports. This is independent work and is not endorsed by the International Baccalaureate. Session codes: M is May, N is November. Corrections: pietro@pietromeloni.com.