PSEB SS Physics common mistakes
PSEB SS Physics · Understand the traps and how to avoid them.
In electrostatics, adding electric potentials at a point as vectors or treating potential and field as interchangeable, e.g. using E = V/d for a non-uniform field.
Students memorize kq/r and E = V/d together and forget that potential is a scalar while field is a vector; the uniform-field formula is then applied everywhere.
What to do instead
Compute potential as an algebraic sum kq/r with charge signs. Compute field separately by vector addition of kq/r² components. Check special points: field can be zero where potential is not, and potential can be zero between opposite charges where field is not.
Using Ohm's law as V = IR with the battery emf in a closed circuit that has internal resistance, or assuming a bulb's resistance is the same at all operating voltages.
Ideal-battery problems dominate early practice, so students ignore r and treat 'rated 60 W, 220 V' resistance as constant.
What to do instead
Write the closed-circuit relation ε = I(R + r), terminal voltage V = ε − Ir, and only use R = V²/P at the rated voltage. For non-ohmic devices, read resistance from the V–I graph at the actual operating point.
Applying the right-hand rule incorrectly to magnetic force: treating the direction of B around a wire as the force direction, or forgetting to reverse the result for an electron.
The field-direction rule, the force rule, and the current direction are learned close together and get mixed; negative charge adds another sign flip.
What to do instead
Use F = q(v × B): right-hand rule for positive charge, reverse for negative. For a wire use F = IL × B. For a straight wire's field, use the right-hand grip rule separately and never use that circling field as the force direction.
Mixing peak and rms values in AC calculations, or using P = VI for a circuit containing a pure inductor or capacitor without including the power factor.
DC power formulas are reused automatically, and rms is treated as an extra label rather than the effective value for average power.
What to do instead
Convert every AC amplitude to rms before power calculations: V_rms = V_peak/√2, I_rms = I_peak/√2. Use P_avg = V_rms I_rms cos φ; for a pure inductor or capacitor, state that average power is zero and explain the phase difference.
Stating that electromagnetic waves need a material medium, that E and B are parallel, or that their speed in vacuum depends on frequency.
Mechanical-wave intuition and ray diagrams carry over, while the transverse mutually perpendicular field structure is often memorized only as a sentence.
What to do instead
Draw E and B perpendicular to each other and to the propagation direction. Use E₀/B₀ = c and c = 1/√(μ₀ε₀); in a medium use v = c/n. Emphasize that in vacuum all frequencies travel at the same speed.
Making sign-convention errors in mirror and lens formulas, such as substituting a positive object distance in the lens formula or giving a concave mirror a positive focal length without a consistent convention.
Signs are treated as arbitrary labels rather than as consequences of a chosen coordinate direction, and ray diagrams are skipped under time pressure.
What to do instead
Fix one Cartesian convention at the start: distances measured against incident light are negative. Write every known with its sign, sketch the ray diagram, then substitute. Check the sign of magnification to decide real/virtual and inverted/erect.
Using the wrong interference or diffraction condition, e.g. using d sin θ = nλ for dark fringes in Young's double-slit experiment or a sin θ = nλ for single-slit maxima.
The formulas look similar, and the 1/2 phase-shift origin of the dark-fringe condition is often memorized without meaning.
What to do instead
Keep the two sets separate: YDSE bright fringes d sin θ = nλ, dark fringes d sin θ = (2n − 1)λ/2; single-slit minima a sin θ = nλ, with the central maximum between the first minima. Derive fringe width β = λD/d from the path difference rather than recalling it alone.
Claiming that increasing the intensity of incident light increases the maximum kinetic energy of photoelectrons, or using hν = K_max without subtracting the work function.
Intensity feels like 'more energy', and the photoelectric equation is often remembered as a single term rather than as an energy balance.
What to do instead
Write K_max = hν − φ and stopping potential eV₀ = hν − φ. State explicitly that intensity changes saturation current, while frequency changes K_max and V₀; threshold frequency is ν₀ = φ/h.
Calculating binding energy from the total mass defect but forgetting to divide by mass number for binding energy per nucleon, or saying activity becomes zero after two half-lives.
Mass-defect arithmetic is rushed, and half-life is interpreted as a fixed lifetime rather than as a probability of decay.
What to do instead
Use BE = [Zm_p + (A − Z)m_n − M]c² and then divide by A for BE per nucleon. For decay use N = N₀(1/2)^(t/T) and note that activity is proportional to N, so it halves each half-life but never reaches exactly zero in the model.
In semiconductor problems, saying n-type material has more holes than electrons, or that forward bias widens the depletion layer.
The words 'n-type' and 'p-type' are remembered as labels rather than as majority-carrier descriptions, and bias effects on the junction are reversed.
What to do instead
State donor doping gives n-type with electrons as majority carriers and holes as minority; acceptor doping gives p-type the reverse. Forward bias lowers the barrier and narrows the depletion layer; reverse bias raises the barrier and widens it. Use this to justify diode conduction direction.