Physics 30 Diploma common mistakes
Physics 30 Diploma · Understand the traps and how to avoid them.
When using a conservation-of-momentum equation, the candidate assigns a velocity a negative sign based on where the object is on the page (left/right of the diagram) rather than on an explicitly declared positive direction, so signs flip between the before and after lines and the final velocity comes out with the wrong sign or magnitude.
Diagrams invite 'left is negative' intuition, and candidates switch conventions mid-solution without noticing, especially when a collision reverses a direction. The algebra still 'works', so no red flag appears until the answer is compared to the key.
What to do instead
Write 'let right be positive' at the top of every momentum item, then convert each given velocity to a signed value in a small table (object, mass, v_before, v_after) before touching the equation. Carry those signed numbers, never the picture, into the equation, and check the sign of the result against physical sense (a rebound must be opposite to the incoming direction).
The candidate computes an electric force or field magnitude correctly but then answers with the direction taken from the charge's location (e.g., 'toward the left plate') instead of from the field direction and the sign of the charge, so a negative charge is given a force pointing the same way as the field.
Candidates memorize 'field points away from positive' and apply it to forces on any charge, forgetting that a negative charge feels force opposite the field. The numeric work consumes attention, leaving direction as an afterthought.
What to do instead
Adopt a fixed two-step routine: first draw the field direction at the charge's location, then apply 'force on positive = along field, force on negative = against field' as an explicit written line. For Coulomb-force pairs, state 'repulsive' or 'attractive' in words before converting that to a direction, and include the direction in the final answer sentence.
In magnetic-force items, the candidate substitutes the angle between the velocity and the page or the angle of the velocity from the horizontal into F = qvB sin(theta), instead of the angle between the velocity vector and the magnetic field vector, producing a wrong or zero force for a charge moving parallel to the field.
Textbook diagrams usually draw the field into or out of the page, so the angle with the field is 90 degrees and candidates stop thinking about it; when a problem tilts the field or the velocity, the habit carries over incorrectly.
What to do instead
Before substituting, explicitly write the angle between v and B and evaluate sin(theta); note that motion parallel to B gives zero force and perpendicular gives the maximum. Where the field is into or out of the page, say so and use 90 degrees deliberately, then use a right-hand rule (with a flip for negative charge) to get direction.
The candidate uses the photon energy relation E = hf or E = hc/lambda to find the energy of an incident photon, then sets that equal to the maximum kinetic energy of the emitted electron, omitting the work function.
The photoelectric equation is learned as a formula to rearrange, and candidates conflate 'energy of the photon' with 'energy given to the electron'. The threshold concept is abstract, so the work function term feels optional.
What to do instead
Always write the full statement hf = W + E_kmax, then identify which two of the three quantities the question supplies before substituting. When asked for threshold frequency or wavelength, set E_kmax = 0 and solve hf_0 = W. Sanity-check that E_kmax is smaller than the photon energy, and never larger than hf.
When finding the energy released in a nuclear decay or the mass defect, the candidate subtracts the mass of the parent nucleus from the sum of the products (reversing the order), or uses atomic masses without accounting for the emitted electron in beta decay, and reports a negative or wrong energy.
The phrase 'mass defect' suggests parent minus products, and beta-minus decay is taught with a reminder about the emitted electron that is easily dropped under time pressure.
What to do instead
Compute the mass difference as (total mass of products) minus (mass of parent), so a positive number means energy is released, then multiply by the appropriate energy equivalent. For beta-minus decay, use the neutral-atom masses directly (the emitted electron is already accounted for in the atomic mass bookkeeping) and state which masses you are using before calculating.
The candidate treats an induced current as producing a magnetic field that reinforces the change that caused it, or ignores the direction of the change (increasing vs decreasing flux) when applying Lenz's law, so the induced current direction is reversed.
The minus sign in Faraday's law is the hardest part conceptually; candidates can compute the magnitude correctly and then guess the direction, and 'oppose the change' is easily misread as 'oppose the field'.
What to do instead
Use a fixed four-step routine: identify the original field direction, decide whether flux is increasing or decreasing, choose the induced field that opposes that change (not the original field), then use a hand rule to convert the induced field into a current direction. Write each step as a short sentence so the reasoning is visible and checkable.
In a refraction problem, the candidate measures or reads the angle from the boundary (the surface) rather than from the normal, and substitutes that value into Snell's law.
Diagrams label the angle between the ray and the surface because it is easier to draw, and candidates copy the number they see without converting. The habit persists because the resulting index is plausible.
What to do instead
Before any substitution, redraw the normal at the point of incidence as a dashed line and label the angle between the ray and the normal. If the given angle is measured from the surface, write 'theta = 90 - given' explicitly. Also check that the ray bends toward the normal when entering a denser medium, which catches a swapped angle or index.
The candidate answers a projectile or circular-motion item with a scalar speed where a velocity vector is required, or reports only the horizontal component when the question asks for the velocity at a point.
Energy and momentum items reward scalar thinking, and candidates carry that habit into kinematics; also, components are computed separately and the resultant is never recombined.
What to do instead
Read the question for the words 'velocity', 'speed', 'magnitude', and 'direction', and underline them. When a vector is requested, compute both components, combine them with Pythagoras, and give the angle relative to a stated axis. For circular motion, state that the velocity is tangent to the path and the acceleration points to the centre before answering.
The candidate substitutes values into a formula without converting to SI base units, mixing grams with kilograms, centimetres with metres, microcoulombs with coulombs, or nanometres with metres, and reports an answer off by powers of ten.
Diploma items deliberately use mixed units to test conversion, and candidates under time pressure pattern-match numbers into formulas rather than tracking units.
What to do instead
Write a conversion line for every given quantity before the main equation (e.g., 25 g = 0.025 kg, 4.0 microC = 4.0 x 10^-6 C), and carry units through the calculation so the final unit is forced to come out correctly. If the answer's order of magnitude looks odd, recheck the exponents rather than the arithmetic.
On multi-step items the candidate stops at an intermediate result (a field, a force, an energy) and reports it as the answer, or rounds intermediate values so aggressively that the final answer misses the accepted range.
Each stage produces a number that looks like an answer, and candidates who are short on time skip the final step. Premature rounding is a habit carried from mental arithmetic.
What to do instead
Restate the actual target quantity in a final line ('the question asks for the speed, so I must convert energy to speed') and only then answer. Keep at least one extra significant figure in intermediate values and round only at the end, to the precision the given data supports.