GCE A/L Physics common mistakes
GCE A/L Physics · Understand the traps and how to avoid them.
Ignoring the zero error of a vernier caliper or micrometer screw gauge when recording measurements, leading to systematic offset in all readings.
Candidates focus on taking the reading correctly but forget to check and apply the zero correction, often because they assume the instrument is perfectly adjusted.
What to do instead
Always check for zero error before use: close the jaws and record the reading. If it is not zero, subtract the zero error from each reading (or add if negative). Practice with instruments that have deliberate zero errors.
Using the wrong sign for acceleration due to gravity or choosing an inconsistent positive direction in kinematics problems, leading to incorrect equations and answers.
Candidates often take g as positive regardless of direction, or switch sign conventions mid-problem, especially when motion involves upward and downward phases.
What to do instead
At the start of each problem, define a positive direction and stick to it. Write down all vector quantities with signs according to that convention. For vertical motion, if up is positive, then a = –g. Check consistency in each equation.
Assuming that stress is directly proportional to strain for all extensions, even beyond the limit of proportionality, and using Hooke's law to calculate extensions for large forces.
Candidates memorise Hooke's law as a universal rule and do not check whether the material is still behaving elastically, often because the problem does not explicitly state the limit.
What to do instead
Always check if the problem states or implies the elastic limit. If the force is large or the extension is nonlinear, Hooke's law may not apply. Use energy methods or graphical areas if appropriate, and state assumptions clearly.
Confusing path difference with phase difference and using the wrong condition for constructive or destructive interference in two-source interference problems.
Candidates mix up the formulas: path difference = nλ for constructive, (n+½)λ for destructive, and phase difference = 2π/λ × path difference. They often apply the condition for constructive interference where destructive is required.
What to do instead
Write down the relationship: phase difference = (2π/λ) × path difference. For constructive interference, path difference = nλ; for destructive, path difference = (n+½)λ. Practice converting between the two and check the context (e.g., reflection may add a phase shift of π).
Treating the internal resistance of a cell as negligible in all circuits, leading to incorrect terminal potential difference and current calculations.
Candidates often use the simplified formula V = IR for the whole circuit, ignoring that the e.m.f. is shared between the external circuit and the internal resistance.
What to do instead
Always include internal resistance r in the circuit equation: e.m.f. E = I(R + r). Terminal p.d. V = E – Ir. If the problem does not mention internal resistance, state that you are assuming it is negligible, but check if that is reasonable.
Using the formula for gravitational field strength or potential without considering the direction and sign, especially when dealing with fields inside a planet or at points between two masses.
Candidates treat g and V as scalars and ignore vector nature or the negative sign of gravitational potential. They also forget that field inside a uniform sphere is proportional to distance from the centre.
What to do instead
Remember: gravitational field strength is a vector, always attractive. Gravitational potential is always negative (zero at infinity). Inside a uniform sphere, g = (GM/R^3)r. For potential inside, use the formula V = –GM(3R^2 – r^2)/(2R^3). Practice problems with hollow and solid spheres.
Applying the ideal gas equation pV = nRT without converting temperature to kelvin, or using the wrong value of R (8.31 J mol⁻¹ K⁻¹) when pressure is in different units.
Candidates often use Celsius in the equation or forget to convert cm³ to m³ or atm to Pa, leading to order-of-magnitude errors.
What to do instead
Always convert temperature to kelvin (T/K = θ/°C + 273.15). Use SI units: pressure in Pa, volume in m³. R = 8.31 J mol⁻¹ K⁻¹. Check unit consistency before calculating. For other units, convert first.
Forgetting to account for the mass defect when calculating the energy released in a nuclear reaction, or using atomic mass units incorrectly.
Candidates often subtract the mass of products from reactants but forget to convert the mass difference to energy using E = mc², or they use the mass of the nucleus instead of the atom, ignoring electron masses.
What to do instead
Calculate mass defect Δm = mass of reactants – mass of products. Convert to energy using 1 u = 931.5 MeV or E = Δm c². When using atomic masses, electron masses cancel if the number of electrons is the same on both sides; otherwise account for them. Practice with beta decay.
Assuming that the photoelectric effect can occur with any frequency of light, regardless of the work function, and not understanding the threshold frequency.
Candidates often apply Einstein's equation hf = φ + K_max without checking if hf > φ, leading to negative kinetic energy for low frequencies.
What to do instead
Always check that the photon energy hf is greater than the work function φ. If not, no emission occurs. The threshold frequency f₀ = φ/h. Remember that intensity affects the number of photons, not the maximum kinetic energy.
In simple harmonic motion, using the wrong expression for angular frequency (ω) or confusing it with frequency (f), leading to incorrect period calculations.
Candidates often use ω = 2πf but forget to convert, or use ω = f instead of 2πf. They also mix up the formulas for different systems (e.g., pendulum vs spring).
What to do instead
Memorise the standard formulas: for a mass-spring system, ω = √(k/m); for a simple pendulum, ω = √(g/L). Always use ω = 2πf and T = 2π/ω. Check units: ω is in rad s⁻¹, f in Hz. Practice deriving T from ω.