WBBSE Madhyamik Physical Science common mistakes
WBBSE Madhyamik Physical Science · Understand the traps and how to avoid them.
Substituting given values straight into a formula without converting to SI first - for example putting u = 72 km/h into v = u + at with a in m/s², or using a length in cm next to a value in SI - and then writing the final answer as a bare number with no unit.
In most Class 9–10 practice sums the data already arrives in SI units, so conversion feels optional. The formula gets memorised as a string of letters, and units are treated as decoration to be attached at the end rather than as part of the quantity.
What to do instead
Enforce a fixed three-line layout for every numerical: (i) a 'Given' list with each quantity converted and its unit written down, (ii) the formula, (iii) substitution with units shown and the answer with unit and sensible significant figures. Run short conversion drills (km/h→m/s, g→kg, cm³→m³, °C→K) before each numerical set, and mark a solution wrong if the 'Given' line is skipped, even when the number is right.
Using the full length of a rod, or the distance from the force to the far end, as the moment arm; and calculating the work done by a force applied at an angle as W = F × s instead of F s cosθ.
The standard diagram shows a force at the end of a horizontal rod, so the rod itself gets used as the lever arm. The definition (perpendicular distance from the pivot to the line of action) is recited but never practised on tilted forces, a fulcrum in the middle, or a spanner at 30° to the handle.
What to do instead
Make drawing compulsory: extend the line of action of the force, drop a perpendicular from the pivot, label that perpendicular d, and only then substitute. Mark clockwise and anticlockwise moments with arrows and apply the principle of moments. Drill cases with a metre stick balanced at the 40 cm mark with weights on both sides, and with a force at 30°, 45° and 60° to a spanner so that cosθ is seen to matter.
In calorimetry, applying Q = mcΔθ to a change of state, or leaving out the heat absorbed by the calorimeter or container; also treating specific heat and latent heat as interchangeable when picking a value from a table.
Both relations have the shape Q = m × (a constant) × (a temperature term), and most textbook numericals involve a single substance at a single stage. The multi-stage problem (ice at 0°C → water at 0°C → water at θ) is rarely worked through end to end, so the two formulas blur.
What to do instead
Force a stepwise energy-flow sketch before calculating: list every stage with its own Q, use mL for the plateau and mcΔθ for the sloped parts, then set total heat gained = total heat lost, including the vessel's water equivalent. Teach the units as identifiers - J kg⁻¹ K⁻¹ means specific heat, J kg⁻¹ means latent heat. Work one full ice–water–steam mixture problem in every revision cycle.
Using the one-way distance in echo problems (t = d/v instead of t = 2d/v), and stating ultrasonic as simply 'above audible' without the 20 kHz figure, or quoting the audible range without attaching Hz and the word 'human'.
Echo is drawn as source→wall with the return path only implied, so the factor of 2 disappears. The range is memorised as the chant '20 to 20,000' with no unit and no subject, which makes it useless in a one-mark fill-in-the-blank.
What to do instead
In every echo sum draw the ray, mark 2d on the diagram and write 'total distance travelled by sound = 2d' as a line of working. Keep one reference card: audible range 20 Hz–20 kHz for humans, infrasonic below 20 Hz, ultrasonic above 20 kHz, minimum echo distance about 17 m at room temperature. Solve at least one problem where the time is given and the distance is the unknown, not only the reverse.
Mixing the sign convention between mirror and lens formulas - taking a real object distance as positive, or reporting a virtual image as real because the sign of v was ignored when interpreting the result.
Students copy a formula sheet without practising the rule (all distances measured from the pole or optical centre, against the direction of incident light negative), and rarely check an answer against a ray diagram. The magnification sign, which actually decides erect/inverted, is usually not written down at all.
What to do instead
Always sketch the ray diagram first, then assign signs to u, v and f before substitution, and finish by reading the sign of v and of m to classify the image as real or virtual, erect or inverted. Use one convention for mirrors and lenses and write it on the formula card. Include convex mirror and concave lens cases, which are the ones usually skipped, and require a one-line interpretation after every answer.
Adding resistances in parallel arithmetically (R = R₁ + R₂), drawing an ammeter in parallel with a bulb or a voltmeter in series, or ignoring the internal resistance when the question supplies it.
Series is taught first and is the intuitive case, so the reciprocal formula is memorised as a shape rather than understood as 'same voltage, currents add'. Circuit symbols are learned as pictures to be copied, not as rules about how an instrument must be connected.
What to do instead
Derive the parallel result from the physics (same potential difference across branches, currents adding) and build the habit of checking that the equivalent resistance is smaller than the smallest branch. Draw one fully labelled circuit per week with the ammeter in series and the voltmeter across the component, and mark misplaced instruments. Where internal resistance is given, use E = I(R + r) rather than V = IR alone.
Putting temperatures in °C into Charles' law or the combined gas equation; calculating amounts from a mass without balancing the equation first; and applying 22.4 L mol⁻¹ to a substance dissolved in water.
Everyday temperatures are quoted in °C and the gas relations look like simple ratio formulas, so the kelvin conversion is skipped. Mole ratios come from a balanced equation, but balancing is filed under 'equations' and not carried into numerical problems; 22.4 L mol⁻¹ is memorised without its 'gas at STP' condition.
What to do instead
Make T = t + 273 the first written line of every gas problem. For any reaction, balance the equation, then write the mole ratio underneath before touching numbers. Label 22.4 L mol⁻¹ explicitly as 'one mole of any gas at STP' and pair it with a concentration-based problem so the difference is visible. Set mixed exercises where the unknown is alternately volume, moles and mass.
Writing shell configurations such as 2,8,9 or 2,8,18 for the first twenty elements, and then reading the group from the number of shells and the period from the valence electrons, or vice versa.
The 2,8,8 rule is learned as a filling rule without the 3s/3p ordering, so the fourth shell is never properly distinguished. On a printed periodic table the group number and period number are both small figures near the symbol, which makes swapping them easy under time pressure.
What to do instead
Write configurations in both forms side by side, e.g. potassium as 2,8,8,1 and 1s²2s²2p⁶3s²3p⁶4s¹, then state explicitly: period = number of the highest occupied shell, group = number of valence electrons (for main-group elements). Drill twelve elements from atomic number 1–20 in both directions, and follow each with the ion formed and the expected bond type - ionic for metal plus non-metal, covalent for non-metal plus non-metal.
Naming electrolysis products wrongly - hydrogen at the anode and chlorine at the cathode for aqueous sodium chloride - or forgetting that the products depend on concentration and on whether the electrodes are inert or active.
The phrase 'cathode is reduction' is learned as a slogan without linking it to which ion gains or loses electrons. The aqueous case is taught after the molten case and the two merge, and the discharge series is recited without being applied to a real cell.
What to do instead
For every electrolyte, write the ions present, the two possible electrode reactions, and then choose using the discharge series plus concentration. Always state the product together with the electrode where it appears. Compare three cells side by side: molten NaCl, concentrated aqueous NaCl, and dilute sulphuric acid. Add copper electrodes in copper sulphate solution so the active-electrode difference is seen rather than memorised.
Naming an organic compound by counting carbons only and ignoring the functional group, or defining a homologous series as compounds with 'similar properties' without giving the same general formula and the −CH₂− increment of 14 u.
In condensed formulas the functional group sits at the end and is easily lost, and homologous series is memorised as a sentence for a two-mark question rather than as a structural pattern that generates names, formulas and molecular masses.
What to do instead
Name in a fixed order: longest chain, then functional-group suffix, then position number. Build a table of five series (alkane, alkene, alcohol, aldehyde, carboxylic acid) with general formula, first three members, functional group and the 14 u step between neighbours. Convert ten examples between condensed formula, full structural formula and IUPAC name, including one isomer-counting question.