NEB Class 12 Physics common mistakes
NEB Class 12 Physics · Understand the traps and how to avoid them.
Writing the projectile range as R = u²sinθ/g, or using the launch angle instead of 2θ in the range formula, so a 30° launch is reported with half its true range.
The range formula is memorised as a single string of symbols and the factor 2 in sin2θ is dropped under exam pressure; candidates also confuse the maximum-height formula (sin²θ) with the range formula (sin2θ).
What to do instead
Re-derive range once from R = uₓ·T = (u cosθ)(2u sinθ/g) and keep that derivation in your notes; before substituting, write the three formulas side by side (H = u²sin²θ/2g, T = 2u sinθ/g, R = u²sin2θ/g) and check that R = 4H at 45° as a sanity test.
Treating a body on an inclined plane as if the normal reaction were mg, then computing friction as μmg instead of μmg cosθ, and omitting the mg sinθ component from the force balance.
Free-body diagrams are drawn from memory of the horizontal-surface case; the geometry of resolving weight into components parallel and perpendicular to the incline is not practised enough, so the cosθ/sinθ assignment gets swapped.
What to do instead
Always draw the incline, mark θ at the base, and resolve weight along the plane (mg sinθ, down the slope) and perpendicular to it (mg cosθ, into the plane). Then write N = mg cosθ and f = μN before any numbers; test the limits θ = 0 (N = mg) and θ = 90° (N = 0) to confirm the assignment.
Applying conservation of linear momentum to a collision without checking whether it is elastic, and separately claiming kinetic energy is conserved in a perfectly inelastic collision.
Momentum and energy conservation are learned together, so candidates assume both always hold; the distinction that momentum is conserved in all collisions (no external impulse) while kinetic energy is conserved only in elastic ones is blurred.
What to do instead
For every collision problem write two headers: 'momentum: always' and 'KE: only if elastic'. For perfectly inelastic collisions use the single combined-mass equation m₁u₁ + m₂u₂ = (m₁+m₂)v and then compute the lost KE explicitly as a check.
Using Celsius temperatures directly in the ideal-gas law or in Charles's/Gay-Lussac's law, e.g. substituting 27 instead of 300 K.
Laboratory data and everyday phrasing are in Celsius, and the conversion is treated as a formality rather than a physical requirement of the absolute scale.
What to do instead
Make 'T must be in kelvin' the first line of every gas or thermodynamic calculation, and convert at the moment you copy data (T = t + 273). If a ratio of temperatures appears, verify it is greater than 1 for a heating process, which immediately exposes a Celsius slip.
Stating the efficiency of a Carnot engine as 1 − T_c/T_h with Celsius values, or claiming an engine can have 100% efficiency when the sink is at 0 °C.
The formula is remembered symbolically but the absolute-temperature requirement and the physical impossibility of a zero-temperature sink are not connected to it.
What to do instead
Write η = 1 − T_c/T_h only after converting both temperatures to kelvin, then check η < 1 always and η = 0 when T_c = T_h. Remember 0 °C = 273 K, so a 0 °C sink still leaves a finite rejection of heat.
In ray optics, placing the image on the same side as the object for a convex lens when the object is outside the focal length, or mis-signing the focal length in the mirror/lens formula.
The Cartesian sign convention (distances measured from the pole/optical centre, positive in the direction of incident light) is memorised as a list of rules without being applied to a drawn ray diagram.
What to do instead
Draw the principal-axis diagram first, mark the direction of incident light with an arrow, and assign signs from that arrow before touching the formula. For a convex lens f is positive and a real image has positive v; for a concave mirror f is negative. Use 1/v − 1/u = 1/f for lenses and 1/v + 1/u = 1/f for mirrors, and never mix the two.
Using the wrong angle in the diffraction grating or Young's double-slit equation, e.g. substituting the angle of the first minimum into d sinθ = nλ, or forgetting to convert slit separation from mm to m.
The formula is applied mechanically without identifying which physical feature (bright fringe, dark fringe, order n) the angle corresponds to, and unit prefixes are dropped when data are copied.
What to do instead
Label the diagram: for maxima use d sinθ = nλ with n = 0,1,2…; for minima in single-slit diffraction use a sinθ = nλ with n = 1,2… (not 0). Convert every length to metres before substitution and state the unit of the answer.
Adding resistors in parallel by simply summing them, or using the series formula for capacitors, and then reporting a total resistance smaller than the smallest branch.
The two combination rules are mirror images of each other and are recalled by pattern rather than by the underlying physics (same current in series, same voltage in parallel).
What to do instead
Anchor the rules physically: in series the same current flows, so R adds; in parallel the same voltage appears across each branch, so 1/R adds. After computing, check that a parallel equivalent is smaller than the smallest resistor and a series equivalent is larger than the largest.
Describing the photoelectric effect by saying brighter light gives faster photoelectrons, or that emission occurs at any frequency provided the intensity is high enough.
Wave intuition (intensity → energy) is deeply ingrained and the photon model is memorised as a formula (hf = φ + K_max) without its conceptual consequences.
What to do instead
State the two independent facts explicitly in every answer: intensity controls the number of photoelectrons (current), frequency controls their maximum kinetic energy; and no emission occurs below the threshold frequency f₀ = φ/h regardless of intensity. Practise one sentence each way until the distinction is automatic.
Balancing a nuclear equation by matching mass numbers only, or writing beta-minus decay without the antineutrino and without increasing Z by one.
Conservation of charge (atomic number) is treated as secondary to mass number, and the decay schemes are recalled as 'beta = electron emitted' without the full balanced equation.
What to do instead
For every nuclear equation write two balance columns, A (top) and Z (bottom), and check both. For beta-minus: ᴬᶻX → ᴬ_(Z+1)Y + e⁻ + ν̄; for beta-plus: ᴬᶻX → ᴬ_(Z−1)Y + e⁺ + ν; for alpha: A decreases by 4 and Z by 2. Verify the neutrino/antineutrino is present, since it carries away the missing energy and lepton number.