EJU Science common mistakes
EJU Science · Understand the traps and how to avoid them.
In mechanics, substituting grams and centimetres directly into F = ma, kinetic energy, or momentum equations without converting to kg and m, then reporting an answer that is off by powers of ten.
EJU stems often give values such as 20 g, 5.0 cm, or 2.0 mA, and candidates pattern-match the numbers into a memorised formula before checking units.
What to do instead
Write an explicit conversion line (20 g = 2.0 × 10^-2 kg; 5.0 cm = 5.0 × 10^-2 m) before substitution, and finish by checking that the result has the expected SI unit.
Applying Kirchhoff's loop rule with a chosen traversal direction but assigning the same sign to a battery's emf and to an opposing voltage drop, especially when two batteries oppose each other.
The rule is remembered as 'sum of voltages = 0' without a fixed sign convention for rises and drops.
What to do instead
Draw current arrows, mark the loop direction, and use a two-row sign table: traversal from − to + through a battery is positive; traversal with the current through a resistor is negative.
Placing a virtual image on the same side of a convex lens as the object, or mixing the sign conventions for a convex lens and a concave mirror.
Candidates memorise 1/f = 1/a + 1/b as a formula without defining a coordinate axis or the positive direction.
What to do instead
Draw a ray diagram first, define positive as the direction from object to lens, and only then substitute signs; check that a virtual image has b < 0 for a lens.
Reading a graph's slope or area without checking the axis units and scale, for example treating a current axis marked in mA as amperes or a distance axis marked in cm as metres.
Time pressure leads candidates to read numbers off the grid and substitute them immediately.
What to do instead
Before calculating, annotate each axis with its SI unit and multiplier (mA → 10^-3 A; cm → 10^-2 m), then compute slope or area with those annotated units.
Changing the numerical value of the equilibrium constant when a reactant is added, a product is removed, or the total pressure is changed.
Le Chatelier's principle is over-applied: candidates know the position of equilibrium shifts and incorrectly transfer that shift to K.
What to do instead
State explicitly that K depends only on temperature; use the reaction quotient Q to decide the direction of shift, and reserve changes in K for temperature changes.
Using 22.4 L/mol for a gas at 25 °C or at a pressure other than 1.013 × 10^5 Pa because 22.4 is remembered as a universal molar volume.
The standard-state value is memorised without its conditions, and EJU problems sometimes specify different temperature or pressure.
What to do instead
Read the stated conditions first; if they are not 0 °C and 1.013 × 10^5 Pa, use PV = nRT rather than 22.4 L/mol.
Confusing organic reagents: using Tollens' reagent to oxidise a secondary alcohol, or using acidified KMnO4 where Fehling's solution is required to distinguish an aldehyde from a ketone.
Reagent lists are memorised as a block without being linked to the functional-group change and the observable result.
What to do instead
For each reagent, write the substrate → product transformation and the visible change; for example, Tollens' reagent gives a silver mirror with an aldehyde but not with a ketone.
Counting chromosomes after meiosis without distinguishing homologous chromosomes from sister chromatids, for example stating that a human secondary spermatocyte contains 46 chromosomes.
The symbols n and 2n are conflated with DNA content, and chromosome number is not separated from chromatid number.
What to do instead
Draw a four-column table for each stage: chromosome number, chromatid number, DNA content, and ploidy; fill it in before answering.
Reversing the direction of a negative-feedback response, for example saying that ADH decreases water reabsorption or that insulin raises blood glucose.
Hormone names are memorised without the stimulus–sensor–effector loop that defines their action.
What to do instead
Write the loop explicitly: stimulus → sensor → hormone → effector → response that reduces the original stimulus; check that the final response opposes the initial change.
In genetics, treating a phenotype ratio as a genotype probability, or ignoring that the probability of an affected child depends on whether a parent's genotype is known or only inferred.
Punnett squares are used mechanically without assigning genotypes to each individual or conditioning on the information given.
What to do instead
Assign genotypes explicitly to every individual, mark unknown carriers with a question mark, and compute conditional probabilities only after stating what is known about the parents.