Traps
How JEE Mains Physics costs you marks when you know the physics
A right answer earns 4 and a wrong one loses 1, on multiple-choice and numeric questions alike, on a clock shared with two other subjects. So the first traps below are about the paper, not the mathematics. The rest are the slips that produce the wrong options JEE reuses from shift to shift.
- trap shapes
- 24
- playbooks touched
- 27
- playbooks hit by the widest trap
- 13
- paper-wide, not chapter-specific
- 4
Paper traps — marks lost to the marking and the clock
Nothing to do with any one chapter: the guessing rule for each format, the numeric entry, and the shared clock.
The blank MCQ — expected marks left on the table
Paper-wide — not tied to any one chapter
How it happens
A right answer earns 4 and a wrong one costs 1. A blind pick among four options is worth 1/4 × 4 − 3/4 × 1 = +0.25 on average, and with one option ruled out it is worth about +0.67.
The check
Answer every MCQ before time runs out. Rule out what you can on sight — wrong dimensions, a value that grows when it should shrink, a limiting case that fails — and pick from what is left.
The guessed numeric answer — close to a sure −1
Paper-wide — not tied to any one chapter
How it happens
A numeric answer has no options to pick from, so a guess is almost never right. It still costs 1 mark when wrong, so its expected value is close to −1.
The check
Type a numeric answer only when you have worked it out. If you have not, leave it blank. The MCQ habit does not carry over.
The right physics, the wrong number typed
Paper-wide — not tied to any one chapter
How it happens
Numeric stems often ask for x in a stated form, such as x × 10⁻² or √x, or for the change rather than the new value. Typing the full quantity, the new value or an unrounded figure scores −1, the same as a wrong method.
The check
Before typing, reread the last line of the stem: what is x, in which unit, and is it the change or the final value. If your x is far from a whole number, recheck the last two steps for a slip.
The stuck Physics question on a shared clock
Paper-wide — not tied to any one chapter
How it happens
Physics, Chemistry and Maths share one three-hour clock, and every question pays the same 4 marks. Ten minutes on one long circuit or optics setup can cost several quick marks elsewhere.
The check
Work in two passes. Take what opens up quickly first and mark the rest. If a question has not opened up in about three minutes, mark it, move on, and come back after every subject has had its first pass.
Cornerstone traps — where the most marks go
The slips that cost marks in the chapters the paper leans on hardest. Each wrong answer they produce is printed among the options.
Vectors and phasors added as plain numbers
Affects: Electrostatics, Motion in a Plane, Work, Energy and Power, Moving Charges and Magnetism, Oscillations, Alternating Current, Wave Optics
How it happens
Fields, forces, momenta, SHM amplitudes, a.c. voltages and coherent wave amplitudes all combine as arrows. Adding their sizes gives the largest possible value, and that value is usually among the options.
The check
Draw the arrows and add by components, or use √(A² + B² + 2AB cos θ). In a series LCR circuit, V = √(V_R² + (V_L − V_C)²). Add sizes directly only when the arrows point the same way.
A diameter put in as a radius
Affects: Units and Measurements, Rotational Motion, Gravitation, Solids, Fluids, Current Electricity, Moving Charges and Magnetism, Waves
How it happens
Wires, bores, discs and planets are often given by diameter. Using d where r belongs makes an area four times too big and an r³ term eight times too big, and those wrong values sit among the options.
The check
Circle every diameter in the stem and halve it before it touches a formula. For areas use πd²/4 directly. Remember the end correction of a pipe is 0.3 times the diameter, not the radius.
A unit left unconverted
Affects: Units and Measurements, Laws of Motion, Rotational Motion, Fluids, Thermal Properties, Thermodynamics, Kinetic Theory, Current Electricity, Electromagnetic Induction, Dual Nature, Atoms, Nuclei, Semiconductor Electronics
How it happens
Stems mix km/h, rpm, grams, mm², gauss, poise, eV and degrees Celsius with SI constants. One skipped conversion moves the answer by a fixed factor, and the option built from that factor is waiting.
The check
Convert everything to SI before substituting: km/h to m/s before squaring, rpm to rad/s, mm² as 10⁻⁶ m², eV to joules unless you use hc = 1240 eV nm, and Celsius to kelvin in any ratio, gas law or Stefan's law.
Errors subtracted, or counted once when they count twice
Affects: Units and Measurements, Solids, Oscillations
How it happens
For Q = aᵐbⁿ/cᵖ the fractional errors add as mΔa/a + nΔb/b + pΔc/c, even for a quantity in the denominator. A distractor subtracts the denominator's error, or drops the power, so a diameter in Young's modulus or a period in g = 4π²L/T² is counted once.
The check
Write the formula as a product of powers, then add every term with its power as a multiplier. Signs never cancel. For a sum or difference, add the absolute errors instead.
A sign dropped in the mirror or lens formula
Affects: Ray Optics
How it happens
The mirror formula is 1/v + 1/u = 1/f and the lens formula 1/v − 1/u = 1/f, with m = −v/u for a mirror and v/u for a lens. One dropped sign turns a virtual image real, an erect image inverted, or a concave mirror convex.
The check
Fix the convention before any number goes in: distances from the pole or centre, positive along the incident light. A real object has u negative; a virtual object has u positive. Check the result's nature against a quick ray sketch.
The angle measured from the wrong line
Affects: Electrostatics, Electromagnetic Induction, Moving Charges and Magnetism, Magnetism and Matter, Ray Optics, Motion in a Plane, Laws of Motion
How it happens
Flux, torque on a coil and refraction all use the angle with the normal, while stems often give the angle with the plane or surface. Using that angle swaps sine and cosine, and the swapped value is an option.
The check
Before writing cos θ or sin θ, name the line θ is measured from: the normal, the axis, the vertical or the magnetic meridian. If the stem gives the angle with the plane, use its complement.
The missing ½ — rolling bodies and stored energy
Affects: Rotational Motion, Work, Energy and Power, Electrostatics, Electromagnetic Induction, Electromagnetic Waves, Solids
How it happens
A rolling body has ½mv² + ½Iω², so it reaches the bottom of a slope slower than a sliding one and accelerates at less than g sin θ. Stored energy carries a ½ too: ½CV², ½LI², ½kx², and ½ × load × extension in a wire.
The check
For any rolling body write KE = ½mv²(1 + k²/R²) first. For stored energy, remember the battery or load does twice the work that gets stored; the other half goes as heat or into the source.
The moment of inertia about the wrong axis
Affects: Rotational Motion, Oscillations
How it happens
The parallel-axis theorem starts from an axis through the centre of mass, and the perpendicular-axis theorem holds only for flat bodies. A physical pendulum needs I about its pivot, and a disc about a diameter has MR²/4, not MR²/2.
The check
Name the axis first, then the body's standard I about its centre of mass, then shift it with Md². For a removed piece, subtract its I about the same axis, using the piece's own mass.
Kinetic energy conserved where only momentum is
Affects: Work, Energy and Power, Rotational Motion, Laws of Motion, Electrostatics
How it happens
When bodies stick, a mass lands on a moving belt or a child walks on a turntable, momentum or angular momentum is conserved but kinetic energy is not. In the same way, joining two charged capacitors or spheres keeps the charge and loses energy.
The check
Ask which quantity has no outside push or torque, and conserve only that. Work out the energy afterwards from the new state; the difference is the loss, never zero.
Not asking what stays fixed
Affects: Electrostatics, Current Electricity, Thermodynamics, Kinetic Theory
How it happens
A capacitor still on the battery keeps V; one taken off keeps Q, so a dielectric raises the energy in one case and lowers it in the other. A bulb at fixed voltage follows V²/R, one carrying a fixed current follows I²R, and a gas heated in a sealed vessel uses Cv, not Cp.
The check
Before using any formula, write down what the setup holds constant: V or Q, voltage or current, pressure or volume. Pick the formula that keeps that quantity in it.
The outside formula used inside
Affects: Electrostatics, Gravitation, Moving Charges and Magnetism
How it happens
Outside a sphere or wire the field falls with distance, but inside a solid one it grows linearly with r, and inside a hollow shell it is zero while the potential is not. Gravity below the surface falls as g(1 − d/R), not as an inverse square.
The check
Mark where the point sits: inside the body, on it or outside. Inside, use only the charge, mass or current enclosed. Inside a shell, the potential equals its surface value.
Core traps — slips in the middle chapters
A sign, a unit, a condition skipped: slips in method rather than hard questions.
Internal resistance, meter resistance or diode drop left out
Affects: Current Electricity, Moving Charges and Magnetism, Semiconductor Electronics
How it happens
A real cell's terminal voltage is E − Ir on discharge and E + Ir on charge, and maximum power comes at R = r. A galvanometer has its own resistance, and a conducting diode adds its forward resistance and its cut-in drop.
The check
Put r, the meter's resistance or the diode's drop into the loop before anything else. For a voltmeter range, subtract the galvanometer's own resistance from the total.
A ratio turned upside down
Affects: Ray Optics, Wave Optics, Fluids, Alternating Current, Atoms, Dual Nature, Motion in a Straight Line
How it happens
Telescope magnification is fₒ/fₑ, a transformer's currents go as the inverse of its turns, Rydberg's formula gives 1/λ, and a floating body sinks by the fraction ρ_body/ρ_liquid. The flipped ratio is usually one of the options.
The check
Read the asked ratio aloud in the stem's order, A to B, and write it as A/B before working. Then check one limiting case: a longer focal length eyepiece must lower the magnification.
Work with the wrong sign
Affects: Thermodynamics, Work, Energy and Power, Electrostatics
How it happens
Work by the gas and work on the gas differ in sign, as do work by a field and work by an agent. A compression, a leg running to the left on a P–V graph, or an area below the axis gives negative work.
The check
State whose work you are finding and in which direction the system moves. In thermodynamics write ΔU = Q − W with W done by the gas, and check that a compression gives W below zero.
A direction reversed by a charge or a cross product
Affects: Moving Charges and Magnetism, Electromagnetic Induction, Electromagnetic Waves, Current Electricity, Dual Nature, Electrostatics
How it happens
An electron feels a force opposite to E and drifts against the field. F = qv × B and the travel direction E × B depend on the order of the product, and Lenz's law opposes the change in flux, not the flux.
The check
Put the sign of the charge in at the start. Apply the right-hand rule in the order the formula prints. For Lenz, ask first whether the flux is rising or falling.
Series and parallel swapped
Affects: Current Electricity, Electrostatics, Oscillations, Thermal Properties, Moving Charges and Magnetism, Solids
How it happens
Slabs stacked across a capacitor gap are in series; slabs side by side are in parallel. A block between two springs on opposite walls sees them in parallel, slabs in a heat path add thermal resistances, not conductivities, and a shunt sits in parallel.
The check
Ask what the parts share. The same charge, current, heat flow or tension means series. The same voltage, stretch or temperature difference means parallel.
A result used outside the condition it was derived for
Affects: Ray Optics, Wave Optics, Gravitation, Oscillations, Atoms, Laws of Motion, Motion in a Plane, Current Electricity, Moving Charges and Magnetism
How it happens
δ = (μ − 1)A is for thin prisms, y = nλD/d for a far screen and small angles, g(1 − 2h/R) for small heights, and T = 2π√(l/g) for small swings. Bohr's formulas hold for one-electron atoms, and maximum range at 45° only on level ground.
The check
When a stem gives a large angle, a height comparable to R, a near screen or a slope, the shortcut does not apply. Go back to the full form: sin θ, GM/r², or the full projectile equations.
Long-tail traps — units, ratios and conditions
A power of r, an rms value read as a peak, a quantity assumed fixed: slips in the smaller chapters.
The wrong power in a scaling law
Affects: Atoms, Nuclei, Gravitation, Fluids, Dual Nature, Kinetic Theory
How it happens
Bohr radius goes as n²/Z and energy as Z²/n², nuclear radius as A^(1/3), orbital period as r^(3/2), terminal velocity as r², matter wavelength as 1/√K, and rms speed as √T. A distractor uses the first power, or squares where it should take a root.
The check
Write each answer as a proportionality with its power before any number goes in, then take the ratio. Check one direction: a higher orbit must have a smaller speed and a longer period.
Peak for rms, ω for f
Affects: Alternating Current, Electromagnetic Waves, Electromagnetic Induction, Oscillations, Waves
How it happens
Meters read rms, I_rms = I₀/√2, and average power is ½V₀I₀ cos φ. The coefficient of t in sin ωt is ω, not f, so f = ω/2π; resonance is ω₀ = 1/√(LC), and f₀ carries an extra 1/2π.
The check
Label every a.c. value as peak or rms and every frequency as ω or f the moment you read it. Convert once, at the start, and keep the labels on through the working.
Zero velocity read as zero acceleration
Affects: Motion in a Straight Line, Motion in a Plane, Oscillations, Laws of Motion
How it happens
At a turning point, the top of a throw and the extreme of an SHM the velocity is zero but the acceleration is not; at an SHM extreme it is the largest. A lift moving at steady velocity changes no reading, because only acceleration does.
The check
Find acceleration from the force or the slope of v against t, never from the velocity's value. For distance, add the areas on both sides of a turning point as positive numbers.
Two habits cover most of this page
Before you solve
Check the options first
Rule out anything with the wrong units, the wrong sign or an impossible size. It often leaves one option, and when it does not, it makes the end-of-paper guess worth more.
On the paper
Two passes, then fill the MCQs
Answer the quick ones first and mark the rest. At the end, every multiple-choice question gets an answer; a numeric answer you have not worked out stays blank.