BSEH SS Physics common mistakes
BSEH SS Physics · Understand the traps and how to avoid them.
When combining two point charges of opposite sign, the candidate adds the magnitudes of the field vectors at a point between them instead of subtracting, or places the zero-field point between the charges even when the charges are unequal.
Textbook diagrams almost always show equal and opposite charges, so the null point sits at the midpoint. Candidates memorise 'null point between the charges' and never re-derive the superposition, so an unequal pair (e.g. +4q and −q) is handled with the wrong sign convention or the null point is put on the wrong side of the smaller charge.
What to do instead
Before substituting numbers, draw the field arrows from each charge at the test point and assign + and − by direction, not by charge sign. For a null point, always state the condition E1 = E2 in magnitude and set up the equation with the distance from the null point to each charge as an unknown; solve and then check the answer lies outside the segment joining the charges on the side of the smaller magnitude charge.
The candidate computes the equivalent resistance of a network by treating a bridge or a cube edge as a simple series–parallel combination, folding resistors that are actually at the same potential.
Series–parallel reduction is drilled on ladder networks, so any diagram gets folded left to right. Candidates do not check for symmetry or equipotential nodes, so a balanced Wheatstone bridge or a symmetric cube is collapsed incorrectly and the answer comes out as a plausible but wrong fraction.
What to do instead
Before any reduction, label every node and ask which nodes are connected by symmetry (identical paths to the source). Merge equipotential nodes first; only then apply series/parallel. For a balanced bridge, state explicitly that no current flows through the bridge arm and remove it. Verify with Kirchhoff's laws on one loop as a cross-check.
The candidate applies the right-hand rule to a current-carrying wire and reports the magnetic field direction at a point using the direction of current flow rather than the direction of the perpendicular from the wire to the point.
The rule is taught with a straight wire drawn horizontally and the field point above it, so the geometry is never varied. When the point is to the side or below, candidates rotate the current arrow instead of the radius vector, flipping the field direction by 90° or 180°.
What to do instead
Write the field direction as the cross product direction of (current element) × (vector from element to field point) and evaluate it with a fixed coordinate sketch. Practise the same problem with the field point placed in all four quadrants and check the answer changes sign correctly. For a loop, use the axial dipole convention and confirm the field on the axis points along the loop's magnetic moment.
In an AC circuit the candidate adds the rms voltages across R, L and C arithmetically (V = VR + VL + VC) to find the source voltage, or uses peak values in the impedance formula and rms values in the power formula interchangeably.
DC series circuits add voltages directly, and the phasor picture is often reduced to a memorised triangle without the candidate internalising that VL and VC are 180° out of phase. Mixing peak and rms slips through because both appear in the same problem statement.
What to do instead
Always draw the phasor diagram first: VR on the x-axis, VL up, VC down, source voltage as the vector sum. Compute V = √(VR² + (VL − VC)²). Then write the impedance Z = √(R² + (XL − XC)²) and insist on one convention: if the source is given as rms, all currents and voltages are rms; convert only at the end if a peak is asked for.
The candidate answers a question about the displacement current in a charging capacitor by saying the displacement current is zero in the gap because there is no conduction current, or claims the displacement current exists only inside the dielectric.
The candidate equates 'current' with moving charge and does not connect the changing electric flux to the displacement current. The Maxwell term is treated as a formula to recall for the exam rather than a physical statement about the gap.
What to do instead
State the rule: in the gap of a charging capacitor the conduction current is zero but the displacement current ε0 dΦE/dt equals the conduction current in the wires, so the circuit current is continuous. Work one numerical example: given dV/dt and plate area, compute both currents and show they match. Link this to why an AC ammeter reads the same current on either side of the capacitor.
In a two-lens or mirror–lens system the candidate applies the thin-lens formula to the second element using the object distance measured from the first element instead of from the second, and ignores the sign convention for virtual objects.
Single-element problems always measure u from the same lens, so the habit carries over. The intermediate image becomes a virtual object for the second lens and candidates do not recognise the sign flip, so the final image position and nature are wrong.
What to do instead
Draw a ray diagram and mark the intermediate image before touching any formula. For the second element, measure all distances from its own pole/optical centre, and if the intermediate image falls on the outgoing side, take u as positive (virtual object) under the Cartesian convention. State the sign convention at the top of the solution and check the final answer's nature (real/virtual, erect/inverted) against the diagram.
The candidate computes the de Broglie wavelength of an electron accelerated through a potential difference by using the non-relativistic formula λ = h/√(2meV) without checking the energy regime, or confuses the stopping potential with the photon energy in the photoelectric effect.
Both formulas are memorised as single-line expressions. Candidates do not track which quantity is the incident photon energy (hc/λ) and which is the maximum kinetic energy (eV0), and they rarely check whether the electron energy is a small fraction of its rest energy.
What to do instead
Write the photoelectric equation as hc/λ = φ + eV0 and label each term before substituting. For the de Broglie part, compute the kinetic energy in eV and compare with 511 keV; if it is a small fraction, the non-relativistic form is safe, otherwise use the relativistic momentum. Keep a one-line check: λ(Å) ≈ 12.27/√V for electrons, and verify the order of magnitude.
The candidate predicts the stability of a nucleus from the neutron-to-proton ratio alone without checking the mass–energy balance, or writes a beta-decay equation that does not conserve the lepton number and charge.
The N/Z band is the most visual fact in the chapter, so it becomes the only criterion. Candidates also treat beta decay as a simple neutron-to-proton switch and forget the emitted electron and antineutrino in the equation.
What to do instead
For stability questions, check both the N/Z ratio and the Q-value from mass excess; a nucleus can be energetically unstable even inside the band. For every decay equation, tally A, Z and lepton number on both sides before moving on: beta-minus emits e− and an antineutrino, beta-plus emits a positron and a neutrino, and electron capture emits a neutrino only.
The candidate analyses a transistor or diode circuit by assuming the base–emitter junction is always forward biased and the collector–base junction always reverse biased, without checking the actual bias from the resistor network, and applies the wrong current gain relation.
The standard CE amplifier picture is the only configuration practised, so the bias state is assumed rather than derived. The candidate also mixes up α and β, using IC = αIE in a CE circuit or IC = βIB in a CB circuit.
What to do instead
For every device circuit, first determine the bias of each junction from the supply polarity and resistor dividers, then select the operating region (cut-off, active, saturation). Write the relation that matches the configuration: CE uses IC = βIB and IE = (β+1)IB; CB uses IC = αIE with α = β/(1+β). Check the result against the supply voltage with a KVL loop on the collector side.
In the practical/laboratory questions the candidate reports a measured value without estimating the uncertainty or applying the instrument's least count, and quotes a final answer to more significant figures than the data support.
School practicals are often marked on the final number, so uncertainty analysis is treated as optional. Candidates also copy the calculator display without rounding, and do not convert units consistently before computing a derived quantity.
What to do instead
For every measurement question, state the least count of the instrument, take the mean of repeated readings, and compute the absolute and percentage error using the standard propagation rules (add absolute errors for sums, add fractional errors for products and powers). Round the final answer to the same number of significant figures as the least precise input, and always carry units through the calculation before writing the result.