Lower Saxony Zentralabitur Physics common mistakes
Lower Saxony Zentralabitur Physics · Understand the traps and how to avoid them.
Forgetting to convert millimetres, centimetres, or grams to SI units before substituting into formulas such as F = qvB or E = ½mv², then reporting the answer in mixed units.
Exam tasks often state values such as a plate spacing of 2 cm, a mass of 50 g, or a wavelength of 500 nm, and candidates plug the numbers in directly because they focus on the formula rather than the units. The resulting error is a power of ten or more, and it often goes unnoticed because the final value looks plausible.
What to do instead
Before any calculation, write every given quantity in a separate line with its SI unit, converting explicitly (2 cm = 2·10⁻² m, 50 g = 5·10⁻² kg, 500 nm = 5·10⁻⁷ m). Carry units through the algebra and check that the result's unit matches the physical quantity asked for. When a result seems off by orders of magnitude, recheck the conversion step first.
Treating velocity, acceleration, and force as positive scalars, so a braking car or a downward-thrown ball produces sign errors in v = v₀ + at and in the energy balance.
Candidates learn the equations in one direction (accelerating from rest, upward positive) and apply them unchanged when the motion reverses. The distinction between 'speed' and 'velocity' is blurred, so the sign of a is set by intuition ('braking means negative') rather than by a consistent axis convention.
What to do instead
At the start of every mechanics problem, draw the axis, label the positive direction, and write down v₀, v, a, and s with signs relative to that axis. Substitute signed values into the formulas and interpret a negative result as 'opposite to the chosen axis', not as an error. For energy problems, verify that kinetic energy cannot become negative, which catches sign mistakes quickly.
Mixing up the direction of the induced current or magnetic force by applying the right-hand rule to the wrong quantity (for example, using the field direction as the thumb instead of the current).
There are several rules (Lorentz force, right-hand grip rule for a coil, Lenz's law) and candidates memorise them as gestures without linking each finger to a physical quantity. Under time pressure, they reproduce the gesture from memory but misassign thumb, index, and middle finger.
What to do instead
Write a one-line legend for each rule ('thumb = current, index = B field, middle = force') and practise it on three standard situations: a wire in a field, a charge moving in a field, and a coil in a changing flux. For induction, always determine the change in flux first, then apply Lenz's law to find the direction that opposes that change, and only then use the right-hand rule for the current direction.
Confusing the formula symbols in electrical circuits: using U = R/I instead of U = RI, or adding internal resistance in parallel with the external resistor.
The letters U, I, R and the relationships between them are learned quickly and then recalled under stress with the operations swapped. Internal resistance is a small conceptual step that is often skipped because the textbook circuit diagram shows only the external resistor.
What to do instead
Derive Ohm's law once from the definition R = U/I and rearrange it deliberately each time rather than recalling a memorised form. For every circuit, redraw it with the internal resistance as an explicit series resistor next to the ideal source, then compute the total resistance before finding the current. Check with a limiting case: if the external resistance is zero, the current should be U/r, not infinite.
Assuming the period of a pendulum or spring oscillator is independent of amplitude in every case, and using T = 2π√(l/g) or T = 2π√(m/D) outside the small-angle or Hooke-region range.
The standard formulas are presented as universal, and the small-angle approximation (sin θ ≈ θ) or the linear force law is rarely emphasised as a condition. Candidates then apply them to large swings or to springs stretched beyond their elastic limit and are surprised by the mismatch with measured data.
What to do instead
State the validity condition explicitly whenever you use an oscillator formula: small amplitude for the pendulum (typically θ < 10°), Hooke's law for the spring. If the task gives measured periods at increasing amplitude, plot T against amplitude and describe the deviation rather than forcing the formula. In error discussions, name the approximation as a systematic source of deviation.
Describing interference patterns qualitatively ('there are bright and dark fringes') without linking fringe spacing to wavelength, slit separation, or grating constant, and without using the correct condition for maxima versus minima.
The double-slit and grating formulas look similar, and the factor of ½ for minima in the double slit is easily dropped. Candidates also describe the pattern from memory instead of deriving it from the path difference, so they cannot adapt when the task changes the setup.
What to do instead
Always start from the path difference Δs and write the condition for constructive (Δs = kλ) and destructive (Δs = (k+½)λ for two slits) interference before using any formula. For a grating, use d·sin θ = kλ with d = 1/N and state which order k you are calculating. When asked to explain a change in the pattern, refer back to the path-difference condition rather than to a memorised sentence.
Using the photoelectric equation with the wrong quantity as the stopping voltage or treating the work function and the photon energy as interchangeable, so the threshold frequency is miscalculated.
The equation E_photon = W + E_kin is short and easy to memorise in the wrong order, and the stopping voltage is often confused with the photon energy because both are given in electronvolts. Candidates also forget that intensity affects the number of electrons, not their maximum kinetic energy.
What to do instead
Write the energy balance in words first: photon energy = work function + maximum kinetic energy of the electron, with E_kin,max = e·U_stop. Then substitute numerically, keeping eV and joules separate and converting only at the end. Test your understanding by predicting what happens to the stopping voltage when intensity doubles (nothing) versus when frequency increases (it rises).
Balancing a nuclear reaction equation by mass number only, or forgetting that beta-minus decay increases the atomic number by one while beta-plus decay decreases it.
The mass numbers balance in many simple examples, so candidates stop checking the atomic number. Beta decay is also confused with electron capture because all three change Z in different directions, and the neutrino or antineutrino is often omitted from the equation.
What to do instead
For every nuclear equation, write two separate balance checks side by side: sum of mass numbers on the left equals the right, and sum of atomic numbers on the left equals the right. Memorise the Z and A changes for alpha, beta-minus, beta-plus, and gamma as a small table, and always include the emitted lepton in beta decays. Verify with a concrete example such as carbon-14 decay.
Applying classical kinetic energy ½mv² or classical momentum mv to electrons or particles moving at a significant fraction of the speed of light, without checking whether relativistic formulas are needed.
Relativistic formulas are introduced late in the course and are often treated as a separate topic, so candidates default to the familiar classical expressions. They rarely estimate v/c first, so they do not notice when the classical result is off by a large factor.
What to do instead
Before any energy or momentum calculation involving electrons, protons, or high-speed particles, estimate v/c from the given energy or voltage. If v exceeds about 0.1c, use the relativistic expressions E = γmc² and p = γmv, and state the approximation you are making when you use the classical form. In the exam, a quick check of the classical result against the rest energy mc² flags the problem immediately.
Reporting a measured value without an uncertainty, or quoting a result to six significant figures from data given to two, and then drawing a firm conclusion from a difference smaller than the uncertainty.
Candidates treat error analysis as a formality to add at the end and are not taught to propagate uncertainties through a calculation. The habit of copying the calculator display reinforces excessive precision, and the comparison of a difference with its uncertainty is rarely practised.
What to do instead
Carry uncertainties through each operation (add absolute uncertainties, add relative uncertainties for multiplication and division, double the relative uncertainty for a square) and round the final uncertainty to one significant figure, matching the value to the same decimal place. When comparing two results, explicitly state whether the difference is larger than the combined uncertainty before claiming a discrepancy, and name at least one systematic and one random source of error in every evaluation.