RBSE SS Physics common mistakes
RBSE SS Physics · Understand the traps and how to avoid them.
When combining resistors, the candidate adds parallel branches arithmetically (R = R1 + R2) or applies the series formula to a parallel network, then carries the wrong equivalent resistance into every later current and voltage step.
The series formula is learned first and rehearsed more, so under time pressure the candidate pattern-matches on 'resistors together' without checking whether the same current or the same voltage is shared. The reciprocal form also invites arithmetic slips that go unnoticed.
What to do instead
Before substituting numbers, state in one line which quantity is common to the components (same current = series, same potential difference = parallel). For two parallel resistors use the product-over-sum form R1R2/(R1+R2) to avoid reciprocals, and sanity-check that the parallel equivalent is smaller than the smallest branch and the series equivalent is larger than the largest.
The candidate computes the net force on a charge in a multi-charge configuration by adding the magnitudes of the individual Coulomb forces, ignoring that force is a vector and that like charges repel while unlike charges attract.
Textbook drill questions often use collinear charges, so direction never matters and the habit of scalar addition is never broken. When a right-angle or triangular arrangement appears, the candidate has no practised routine for resolving components.
What to do instead
Draw the free-body diagram first and label each force arrow with its direction away from or toward the source charge. Resolve into x and y components with explicit signs, then combine with Pythagoras and arctan. Always ask whether the answer direction is physically sensible for the signs of the charges involved.
The candidate writes the lens or mirror formula with the wrong sign for the focal length or object distance, or reports a magnification with the wrong sign, so a real inverted image is described as virtual and erect.
Two different sign conventions (Cartesian and real-is-positive) are in circulation, and the candidate switches between them mid-solution. The magnification sign is treated as a formality rather than as the piece of information that encodes image orientation.
What to do instead
Commit to one convention and write it at the top of the answer sheet: distances measured against the incident light are negative, and for a convex lens f is positive while for a concave lens f is negative. Interpret the sign of m explicitly at the end, stating whether the image is real or virtual, erect or inverted, and check against a quick ray sketch.
For a charged particle entering a magnetic field, the candidate sets magnetic force equal to qvB with no angle factor, or assumes the path is always a circle, even when the velocity is parallel or at an arbitrary angle to the field.
The perpendicular case dominates worked examples, and the sin theta factor is dropped as a triviality. The candidate also forgets that the magnetic force does no work, so speed stays constant and only direction changes.
What to do instead
Always write F = qvB sin(theta) and evaluate the angle from the stated geometry before substituting. State the consequence: velocity parallel to the field gives zero force and straight-line motion, perpendicular gives a circle of radius mv/(qB), and oblique gives a helix. Note that kinetic energy is unchanged because the force is perpendicular to the velocity.
The candidate applies Ohm's law V = IR directly to a cell in a closed circuit, using the cell's emf as the terminal voltage and ignoring the internal resistance drop.
Ohm's law is over-learned as a universal relation, and the internal resistance is treated as an optional detail rather than part of the circuit. The candidate also confuses emf with terminal potential difference in definitions.
What to do instead
Write the circuit equation as emf = I(R + r) and terminal voltage as V = emf - Ir. Check the limiting cases: with the external circuit open, terminal voltage equals emf; as external resistance falls toward zero, terminal voltage falls toward zero while current tends to emf/r. Use these limits to test any numerical answer.
For the photoelectric effect, the candidate claims that increasing the intensity of incident light increases the maximum kinetic energy of emitted electrons, or that emission occurs for any frequency provided the light is bright enough.
Wave intuition says brighter light means more energy delivered, and the wave picture is not actively discarded when the photon picture is introduced. The threshold frequency is remembered as a formula but not as a hard cutoff.
What to do instead
Anchor every answer to the Einstein equation K_max = hf - phi. Intensity controls only the number of photons and hence the saturation photocurrent; frequency alone controls K_max and determines whether emission happens at all. State the threshold condition f > phi/h explicitly before discussing intensity.
When asked for the direction of an induced current, the candidate recites Lenz's law as a phrase but cannot convert it into a direction, or applies the right-hand rule to the wrong vector and gets the field direction reversed.
Lenz's law is memorised as 'opposes the change' without a practised two-step procedure. The candidate also confuses the direction of the induced current with the direction of the induced field or of the motion.
What to do instead
Use a fixed three-step routine: identify the change in magnetic flux, decide the direction of the induced field that opposes that change, then use the right-hand grip rule to convert that field direction into a current direction. Verify with a second method such as the motional emf relation for a moving rod, and check the sign against energy conservation.
The candidate uses the wrong formula for the energy of an electron in a hydrogen level, mixes up the n values for a transition, or reports the energy of an emitted photon as negative.
The negative sign of bound-state energy is memorised without interpretation, and the difference between the two level energies is confused with the energy of one of the levels. The Rydberg formula and the Bohr energy expression are also applied interchangeably.
What to do instead
Write E_n = -13.6/n^2 eV and state that the negative sign means the electron is bound. For a transition from n_i to n_f with n_i > n_f, compute photon energy as E_i minus E_f, which is positive, then convert to wavelength with 1240 eV nm divided by the energy in eV. Check that larger jumps give shorter wavelengths.
In a p-n junction question, the candidate says the depletion region widens under forward bias, or treats a reverse-biased diode as a good conductor because it 'has a battery connected'.
The forward and reverse cases are described with similar vocabulary, and the candidate never links bias direction to the direction of the applied field relative to the built-in field. The exponential versus near-zero current behaviour is not connected to the barrier width.
What to do instead
Reason from the applied field: forward bias opposes the built-in field, so the barrier and depletion width shrink and majority carriers cross; reverse bias reinforces the barrier, so the depletion region widens and only a small leakage current flows until breakdown. Draw the band diagram for both cases and mark the barrier height before answering.
The candidate reports a measured quantity with more significant figures than the instrument justifies, or adds percentage errors of quantities that are multiplied instead of added, and drops units or prefixes in the final answer.
Calculator output is copied verbatim, and the rules for combining uncertainties in products, quotients and powers are not practised. Prefix conversions are done mentally and slip by factors of a thousand.
What to do instead
Convert every quantity to SI base units before substituting, carry one extra digit through the working, then round the final result to the precision of the least precise input. For products and quotients add fractional errors, for powers multiply the fractional error by the exponent, and for sums add absolute errors. Attach the unit and check its dimensions against the target quantity.