Assam Higher Secondary Physics common mistakes
Assam Higher Secondary Physics · Understand the traps and how to avoid them.
When a numerical problem gives a value in cm, µC, or mA, the candidate substitutes the number straight into a formula such as F = q1q2/(4πε0r²) without first converting to SI (C, m, A), then reports a force or field off by factors of 10⁴ to 10¹².
School practice often works in 'convenient' units and candidates memorise answers rather than tracking units; the constant 9×10⁹ is quoted with C and m but the given data are not, so the mismatch is invisible unless units are written at every step.
What to do instead
Adopt a fixed habit: before any calculation, write a 'given' line converting every quantity to SI with its power of ten (µC → ×10⁻⁶, cm → ×10⁻², so r² gains ×10⁻⁴). Carry the powers of ten separately from the mantissa and check the final order of magnitude against intuition (two 1 µC charges 1 cm apart give ~90 N, not 9×10⁻⁵ N).
The candidate treats a capacitor circuit as a set of parallel resistors: for two capacitors in series they add C1 + C2, and for parallel they use the reciprocal rule, so equivalent capacitance and stored energy come out inverted.
Resistor formulas are drilled far more heavily in earlier chapters, and the reciprocal pattern feels 'natural' for series because resistors are usually the first network element encountered; the physical reason (same charge in series, same voltage in parallel) is never reconstructed.
What to do instead
Derive the rule once from first principles each time: in series the charge Q is common, so V = Q/C1 + Q/C2 gives 1/Ceq = 1/C1 + 1/C2; in parallel the voltage is common, so Q = (C1 + C2)V. Then state the mnemonic 'capacitors behave opposite to resistors' and re-check by computing energy ½CV² before and after combining.
In a potentiometer or metre-bridge experiment the candidate writes the balance condition as P/Q = l/(100 − l) but substitutes the lengths in the wrong order, or forgets that the null point must lie on the wire and reports a length greater than 100 cm as valid.
The formula is memorised as a pattern without attaching each length to a named resistance; candidates also skip drawing the circuit, so they lose track of which segment corresponds to which arm.
What to do instead
Before substituting, sketch the bridge with P and Q labelled on the top arms and l, (100 − l) labelled on the wire segments, then read the ratio directly off the sketch. Treat any computed l outside 0–100 cm as a signal that the known and unknown resistances were swapped, and correct by inverting the ratio rather than forcing the number.
The candidate applies Kirchhoff's loop rule but assigns the same sign to a battery's emf and to the potential drop across a resistor traversed in the direction of current, producing equations that give negative or obviously wrong currents and then 'fixing' the sign arbitrarily to match an expected answer.
Sign conventions are treated as arbitrary bookkeeping rather than physical statements about potential; without a marked current direction on the diagram, the candidate cannot tell whether a term should be +IR or −IR.
What to do instead
Fix one convention and write it at the top of the page: going round the loop, a resistor traversed with the assumed current contributes −IR, against it +IR; a battery traversed from − to + contributes +ε. Mark assumed current arrows on the circuit before writing equations, and accept a negative answer as meaning the true current is opposite, not as an error to be erased.
For a charged particle moving in a magnetic field the candidate uses F = qvB sinθ with θ taken as the angle in the plane of the page rather than the angle between v and B, and concludes that a particle moving parallel to B experiences maximum force.
Textbook diagrams usually show v perpendicular to B, so the sinθ factor is never exercised; candidates conflate the angle of the velocity vector with the angle between the two vectors.
What to do instead
Always write v and B as vectors and identify the angle between them explicitly; note that when v ∥ B, sinθ = 0 and the force vanishes, and the maximum occurs at 90°. Reinforce with the right-hand rule: if the cross product v × B is zero, there is no force regardless of speed.
The candidate computes the emf induced in a coil as ε = −dΦ/dt using only the change in flux density B, ignoring that Φ = BA cosθ and that a change in area or in orientation also induces emf; in rotating-coil problems they then use peak emf where rms is required (or vice versa).
Most drill problems vary B only, so the full flux expression is never needed; the distinction between peak and rms is learned as a formula pair without the context of what a meter or a power calculation actually uses.
What to do instead
Write Φ = NBA cosθ every time and ask which of N, B, A, θ is changing before differentiating. For AC, state explicitly whether the question asks for instantaneous, peak, or rms value; remember rms = peak/√2 for sinusoidal quantities and that average power uses rms. Check a rotating-coil result against ε₀ = NBAω at θ = 0.
In ray-optics problems the candidate applies the mirror formula 1/v + 1/u = 1/f and the lens formula 1/v − 1/u = 1/f with Cartesian signs chosen to make the arithmetic positive, so a real inverted image is reported as virtual or the magnification sign is lost.
The two formulas differ only in one sign, and the sign convention (distances measured from the pole, against incident light negative) is stated once and then dropped; candidates optimise for getting a number rather than a physically meaningful one.
What to do instead
Draw the principal axis, mark the pole, and place object, image, and focus with signed coordinates before substituting. Use the convention consistently: for a concave mirror f is negative, for a convex lens f is positive, object distance u is negative in both. Interpret the sign of v and of m = v/u at the end (negative m = inverted real image) and check against the ray diagram.
The candidate answers photoelectric-effect questions by increasing the intensity to raise the maximum kinetic energy of emitted electrons, or claims emission occurs at any frequency provided the light is intense enough.
Wave intuition (brighter light = more energetic) is strong and is not displaced unless the photon picture is used quantitatively; the threshold-frequency condition is memorised as a phrase rather than as hν = φ + Kmax.
What to do instead
Write Einstein's equation hν = φ + Kmax for every problem and read off the dependencies: Kmax depends on frequency and work function only, intensity changes the number of photons and hence the saturation current. Test any claim by asking whether it changes ν or only the photon count.
In nuclear decay and binding-energy problems the candidate balances only the mass number A and not the atomic number Z (or vice versa), and computes the Q value as a sum of masses without taking the mass difference, giving an energy release of the wrong sign.
Decay equations are often written quickly as 'A before equals A after' because alpha decay is the only case practised; Q value is treated as a formula to plug into rather than a mass defect.
What to do instead
Balance both A and Z explicitly on each side, using the periodic table for Z of the daughter. For Q, always compute Q = (mass of reactants − mass of products)c², keeping the sign, and convert with 1 u = 931.5 MeV. A positive Q means energy is released; a negative result signals a reversed mass subtraction.
For a half-wave or full-wave rectifier the candidate draws the output waveform with the same period as the input, or states that a full-wave rectifier doubles the output frequency without being able to justify it, and misidentifies which diode conducts in which half-cycle.
The circuit is memorised as a picture rather than traced through time; candidates do not step through one complete input cycle marking which diodes are forward-biased.
What to do instead
Trace one full input cycle on paper: mark the positive half-cycle, identify which diodes are forward-biased and hence which path the current takes through the load, then repeat for the negative half-cycle. Count how many output pulses occur per input cycle (one for half-wave, two for full-wave) and read the output frequency directly from that count.