BCS Written Physics common mistakes
BCS Written Physics · Understand the traps and how to avoid them.
Writing the final numerical answer without carrying units through the working, then mixing SI and CGS quantities (e.g., using g = 980 cm/s² alongside masses in kg) in the same calculation.
Candidates memorise formulas symbolically and substitute numbers at the end, so unit conversions are skipped; under time pressure they grab whichever constant value is remembered first, and mixed systems silently produce answers off by powers of ten.
What to do instead
Adopt a fixed routine: convert every given quantity to SI at the start of the solution, write the unit beside each substituted number, and perform a dimensional check on the final expression before computing. Train by redoing past problems with a deliberate conversion line at the top.
Stating a result to more significant figures than the data justify, or rounding intermediate values (e.g., writing 9.8 for g mid-calculation) and then reporting a final answer to four decimals.
Calculators display many digits and candidates equate more digits with more marks; they also confuse precision with accuracy and have not internalised the rules for propagation of uncertainties.
What to do instead
Carry one extra guard digit through intermediate steps, round only at the end, and match the answer's significant figures to the least precise input. Practise by computing an uncertainty range for each final answer and stating it explicitly.
Describing an experimental procedure without stating how the measurement was actually made precise - omitting the instrument, its least count, the number of readings, and the method of averaging or plotting to reduce random error.
Laboratory work is often learned as a recipe to be reproduced rather than as a measurement problem; candidates describe apparatus but not the error budget, so they cannot answer questions about improving accuracy.
What to do instead
For every standard experiment, prepare a short written card listing: quantity measured, instrument and least count, systematic-error source and its correction, random-error reduction method (repeat readings, graph slope, vernier), and expected percentage error. Rehearse explaining one card aloud in two minutes.
Applying conservation of mechanical energy to a system where friction or an inelastic collision occurs, or treating momentum as conserved in a direction where an external impulse acts.
Textbook problems are usually single-concept, so candidates pattern-match to the most recently studied principle instead of auditing which quantities are actually conserved under the stated conditions.
What to do instead
Before writing any equation, list the forces/impulses acting and mark each candidate conserved quantity as yes or no with a one-line reason. For collisions, explicitly classify elastic, inelastic or perfectly inelastic and state which of energy and momentum survives.
Drawing a free-body diagram that omits a force (commonly friction, normal reaction on an incline, tension in a connecting string, or the centripetal requirement) and then resolving components about an axis that is not aligned with the acceleration.
Candidates sketch the diagram hastily after choosing equations, and they resolve along horizontal/vertical by habit even when the acceleration is along an incline or radial direction.
What to do instead
Draw and label the free-body diagram first, isolate each body, then choose axes along and perpendicular to the acceleration. Check the count of forces against the number of interacting bodies before resolving.
Confusing heat with temperature and internal energy in the first law, and mis-signing work done by versus on the gas; similarly applying the ideal-gas equation to a saturated vapour.
Everyday language treats heat and temperature as synonyms, and sign conventions differ between textbooks, so candidates flip ΔU = Q − W and ΔU = Q + W inconsistently; phase-change problems tempt them to use pV = nRT where it fails.
What to do instead
Fix one sign convention at the start of every answer and state it explicitly. Before solving, classify the process (isothermal, adiabatic, isobaric, isochoric) and write which of Q, W, ΔU is zero. For vapours, check saturation before using the gas law.
Treating a lens or mirror problem without a consistent sign convention, or assuming an image is real when the calculated image distance is negative; also ignoring diffraction when told the slit is narrow.
Sign conventions for object/image/focal distances vary between treatments, and candidates mix them within one problem; geometric-optics habits carry over into situations where the wave nature of light dominates.
What to do instead
Declare the sign convention (Cartesian, measured from the pole/optical centre) before substituting, and interpret the sign of every result physically (real/virtual, erect/inverted). Decide first whether the regime is ray optics or wave optics by comparing slit width or aperture with the wavelength.
Mixing up rms and peak values of alternating current and voltage, and forgetting the √2 or 1/√2 factor, or applying DC power formulas directly to AC circuits with reactance.
The formulas look identical in form, and candidates memorise one version; they also do not distinguish average power in a reactive circuit from the simple I²R result.
What to do instead
Write the full set of relations (V_rms = V_peak/√2, power factor cos φ) at the top of the answer, state which value is being quoted, and check the power expression includes the phase angle whenever inductance or capacitance is present.
Stating a quantum or relativistic result (photoelectric threshold, de Broglie wavelength, time dilation, mass–energy conversion) as a bare formula without checking the physical regime, e.g., using classical kinetic energy for electrons of a few keV or ignoring rest energy.
Candidates learn formulas in isolation and lack the habit of comparing the relevant energy or speed with the rest energy or the speed of light, so they cannot tell when a correction is required.
What to do instead
Before computing, compare the particle energy with its rest energy (0.511 MeV for an electron) or v with c; if comparable, use the relativistic expression. For photons, always pair the equation with the conservation-of-energy statement for the emitted electron.
In electronics questions, treating an operational amplifier or transistor as an ideal device without stating the assumptions, and not distinguishing forward and reverse bias conditions when predicting circuit behaviour.
Circuit problems are answered by recalling a memorised configuration rather than analysing the device state; candidates do not check whether the assumed ideal conditions (infinite gain, zero input current, negligible base current) are valid for the given circuit.
What to do instead
Begin each electronics answer by listing the device model and its assumptions, then verify the bias condition (V_BE ≈ 0.7 V for a conducting silicon junction, virtual short only with negative feedback). Solve the ideal case first, then comment on how real-device behaviour shifts the result.