UPMSP Intermediate Physics - Code 151 common mistakes
UPMSP Intermediate Physics - Code 151 · Understand the traps and how to avoid them.
Using the scalar formula W = q(V_B − V_A) without tracking sign convention, so work done by the external agent versus work done by the field get swapped, and answers for moving a charge between two points come out with the wrong sign.
Textbooks define potential difference as work done by an external agent per unit charge, but many students memorize 'W = qV' and never fix which agent does the work; the sign is invisible in the plug-in step.
What to do instead
Before substituting, write one line: 'External work = q(V_final − V_initial); field work = −q(V_final − V_initial)'. Practice two cases - like charges moving apart and opposite charges moving apart - and check that the external work is positive when a charge is pushed toward a same-sign charge.
Applying the parallel-plate formula C = ε₀A/d to a conductor sphere or cylindrical capacitor, or forgetting the dielectric constant k when a slab fills the gap, producing capacitance values off by a factor of k.
The parallel-plate result is the most drilled formula, so it becomes the default; students also forget that inserting a dielectric of constant k multiplies C by k while keeping Q fixed if the battery is disconnected.
What to do instead
Keep a three-row table of C for parallel plate (ε₀A/d), sphere (4πε₀r), and cylinder (2πε₀L/ln(b/a)). For dielectric problems, first state whether the battery stays connected (V fixed) or is removed (Q fixed), then apply k accordingly.
Treating terminal voltage as always equal to emf, i.e. writing V = ε in a circuit with current flowing, and then being unable to explain why a cell's terminal voltage rises when it is being charged.
The ideal-cell habit is formed in early problems where internal resistance is negligible; the distinction between discharging (V = ε − Ir) and charging (V = ε + Ir) is memorized as one formula.
What to do instead
Always draw the cell with r in series and label the current direction. Write V_terminal = ε − Ir for discharge and V_terminal = ε + Ir for charging, and verify with a numerical example: ε = 2 V, r = 0.5 Ω, I = 2 A gives 1 V discharging and 3 V charging.
Using a single formula for resistors in parallel when the branches have unequal resistances, or adding internal resistance to the external parallel combination as if it were in parallel too.
Students pattern-match 'parallel means 1/R = 1/R₁ + 1/R₂' but then misplace r when reducing the circuit, and they often skip redrawing the circuit after each reduction step.
What to do instead
Reduce the circuit in numbered stages, redrawing after each stage and writing the equivalent resistance beside the new diagram. Check the result with limiting cases: equivalent parallel resistance must be smaller than the smallest branch, and total resistance must exceed r.
Reversing the direction of the magnetic force using the right-hand rule because the left hand is used, or forgetting that the force on a negative charge is opposite to v × B.
The right-hand rule is taught with several variants (palm rule, Fleming's rules) and students mix the left-hand motor rule with the right-hand generator rule; the sign of the charge is an easy afterthought.
What to do instead
Standardize on F = qv × B as the primary relation and use the right-hand rule only for v × B, then flip the result if q is negative. For wires use F = IL × B with L along conventional current. Do three quick checks: electron in a field into the page, proton moving east in a field north, and a current-carrying wire.
Writing the induced emf magnitude but ignoring Lenz's law direction, and confusing the flux change caused by changing B with that caused by changing area or orientation.
Numerical problems usually ask only for magnitude, so direction practice is skipped; students also forget that flux is Φ = BA cosθ and that rotating a coil changes θ, not B.
What to do instead
For every induction problem, state the three quantities that can change (B, A, θ) and which one is changing. Then apply Lenz's law explicitly: 'flux into the page is increasing, so induced current produces flux out of the page, hence anticlockwise'. Include one rotating-coil case where emf = NBAω sinωt.
Using the mirror formula 1/v + 1/u = 1/f with the wrong sign for u, or substituting object distance as positive in the lens formula 1/v − 1/u = 1/f, so the image position and magnification come out inverted.
The two formulas differ by a sign, and the Cartesian sign convention is stated once and then assumed; students memorize the formulas without anchoring them to a coordinate axis.
What to do instead
Draw the principal axis with the pole/optical centre at the origin and mark the incident-light direction as positive before substituting. Write the convention on the answer sheet: distances measured against incident light are negative. Verify with a convex lens of f = +10 cm and object at 15 cm, expecting v = +30 cm and m = −2.
Applying the photoelectric equation as hν = φ₀ + K_max but then using the stopping potential as if it were the kinetic energy in joules, mixing electron-volts and joules.
Stopping potential is quoted in volts while work function is often in eV, and students convert one but not the other; the relation K_max = eV₀ is treated as a definition rather than a unit bridge.
What to do instead
Keep every photoelectric calculation in eV until the final step: hν(eV) = φ₀(eV) + eV₀, so V₀ in volts is numerically the kinetic energy in eV. Convert to joules only if asked, using 1 eV = 1.6 × 10⁻¹⁹ J, and state the conversion explicitly.
Confusing the mass number and atomic number when writing nuclear equations, so beta-minus decay is written with the atomic number unchanged, or alpha decay is balanced only by mass number.
The symbols are visually similar and the decay particles are memorized as 'beta is an electron' without tracking the Z change; students also forget the antineutrino in beta-minus decay.
What to do instead
Balance every nuclear equation twice: once for A (superscripts) and once for Z (subscripts). Write beta-minus as ᴬzX → ᴬ(z₊₁)Y + ⁰₋₁e + ν̄, alpha as ᴬzX → ᴬ⁻⁴(z₋₂)Y + ⁴₂He, and check that both sums match before moving on.
Drawing a transistor circuit without identifying the forward-biased emitter-base junction and reverse-biased collector-base junction, or reading the output across the wrong terminals in a common-emitter amplifier.
Bias conditions are taught as a rule but not tied to the physical structure; students also mix common-base, common-emitter and common-collector configurations and their phase relationships.
What to do instead
For each configuration, sketch the input and output loops separately and label which junction is forward- and which is reverse-biased. For common-emitter, mark input between base and emitter, output between collector and emitter, and note the 180° phase inversion. Verify with a truth-table style check of bias polarities.