UEE Physics common mistakes
UEE Physics · Understand the traps and how to avoid them.
Treating a vector equation as a scalar one: adding magnitudes of forces or momenta that act along different directions, then reporting the scalar sum as the resultant.
Candidates memorize formulas like F = ma or p = mv in magnitude form and rarely draw a free-body diagram with axes, so direction information is silently dropped before the algebra begins.
What to do instead
Before substituting numbers, draw the free-body diagram, choose axes, and write each quantity as a component with an explicit sign. Resolve into components, sum components separately, then recombine with Pythagoras and report the direction angle. Check that the resultant lies inside the range spanned by the individual vectors.
Applying conservation of mechanical energy to a system where friction or an inelastic collision occurs, or applying momentum conservation to a system with a net external impulse.
Candidates pattern-match on the word 'collision' or 'slide' instead of checking the conditions that make each conservation law valid; the two laws are often taught together and get conflated.
What to do instead
State the condition explicitly before using each law: momentum needs zero net external force over the interval, mechanical energy needs no non-conservative work. For a block sliding to rest with friction, use the work-energy theorem with friction work; for a perfectly inelastic collision, conserve momentum and then compute the kinetic energy lost separately.
Using the ideal-gas equation with temperature in degrees Celsius, or mixing Celsius and kelvin within one calculation of a ratio such as P1V1/T1 = P2V2/T2.
Everyday familiarity with Celsius makes it the default, and candidates do not connect the absolute-temperature requirement to the derivation of the gas laws from kinetic theory.
What to do instead
Convert to kelvin at the very first line of working and keep the K unit visible. When using ratio forms, verify that every T is absolute; a quick sanity check is that halving the kelvin temperature at constant volume must halve the pressure, which Celsius would never predict.
Confusing the first law sign convention, writing ΔU = Q + W when the convention is ΔU = Q − W (work done by the gas), and then getting the wrong sign for expansion work.
Different textbooks and syllabi use opposite conventions for W, and candidates switch between them mid-problem without noticing.
What to do instead
Adopt one convention and write it at the top of every thermodynamics answer: ΔU = Q − W with W the work done by the gas. For an isothermal expansion, W > 0, Q > 0, ΔU = 0; for adiabatic compression, Q = 0 and ΔU > 0. Check the sign against the physical direction of piston motion.
Assuming the period of a simple pendulum is independent of amplitude for all angles, or applying T = 2π√(l/g) to a spring-mass system by analogy.
The small-angle approximation sin θ ≈ θ is used in the derivation but its validity condition is rarely stated, and the two oscillation formulas look structurally similar.
What to do instead
Attach the condition to the formula: the pendulum result holds only for small angular amplitude (roughly under 10°), and the period grows with amplitude beyond that. For a mass on a spring use T = 2π√(m/k), which is independent of amplitude and of g. Identify which restoring-force law applies before choosing a formula.
Treating the electric field inside a conductor in electrostatic equilibrium as nonzero, or assuming charge distributes uniformly over a non-spherical conductor.
Candidates apply Coulomb's law point-by-point to a conductor without invoking the electrostatic-equilibrium condition that the internal field must vanish.
What to do instead
State the equilibrium rules first: E = 0 inside the conductor, E perpendicular to the surface just outside, and all excess charge on the outer surface with higher density at sharper curvature. For a hollow conducting shell, then place the enclosed charge's field only outside the cavity radius.
Applying the thin-lens or mirror equation without a consistent sign convention, producing a virtual image where a real one is expected, or misplacing the focal point for a diverging element.
Sign conventions differ between the real-is-positive and Cartesian systems, and candidates memorize 1/f = 1/u + 1/v without attaching meanings to u, v, and f.
What to do instead
Declare the convention at the start: for the real-is-positive convention, f is positive for converging lenses and mirrors, u is positive for a real object, and v is positive for a real image. Sketch the three principal rays to confirm the sign and nature of the image before trusting the algebra.
Computing the photoelectric threshold and the stopping potential as if they were the same quantity, or using the incident frequency instead of the threshold frequency in the work-function relation.
The equation hf = Φ + K_max contains three energies, and candidates collapse them because the terms are not physically distinguished in their notes.
What to do instead
Write the energy balance as a sentence: photon energy = work function + maximum kinetic energy. Then stopping potential satisfies eV_s = K_max, while threshold frequency satisfies hf_0 = Φ. Compute each separately and check that increasing intensity at fixed frequency raises current but not V_s.
Quoting a measured value with more significant figures than the instrument resolution supports, or adding percentage uncertainties arithmetically when quantities are multiplied or divided.
Calculators return many digits and candidates copy them through, and the rules for combining uncertainties are often memorized as 'add them up' regardless of operation.
What to do instead
Record the resolution of each instrument, round the final answer to that resolution, and report absolute uncertainty with the same number of decimal places. For products and quotients add percentage uncertainties; for sums and differences add absolute uncertainties. Where a quantity is raised to a power, multiply its percentage uncertainty by the power.
Manipulating a formula algebraically and then substituting numbers without checking dimensional consistency, or dropping the constant of integration or the limits when evaluating a definite integral in a physics context.
Candidates treat algebra as symbol-pushing detached from physical meaning, so errors in rearrangement survive to the final numeric answer.
What to do instead
After rearranging, check that both sides carry the same base units, and substitute a limiting case (for example m → 0 or θ → 0) to see whether the expression behaves sensibly. When integrating, write the limits explicitly and confirm the result has the expected dimensions, such as velocity from an acceleration-time integral.