The morning paper of 23 January 2026, the fifth shift of Session 1. Below is the Physics section question by question, the chapter split, where the hundred marks sat, how tough it was, and the official answer key from NTA.
The shift
| Date | 23 January 2026 |
|---|---|
| Shift | Shift 1 — 9:00 AM to 12:00 PM IST |
| Paper | Paper 1 (B.E./B.Tech) |
| Whole paper | 75 questions, 300 marks, 180 minutes |
| Physics | Q26 to Q50 — Section A 20 MCQ (80 marks), Section B 5 numeric (20 marks) |
| Marking | +4 right, −1 wrong, nothing for a blank |
Physics, question by question
| Q | What it asked | Chapter |
|---|---|---|
| 26 | Pendulum length needed to double the number of oscillations in the same time | Oscillations |
| 27 | Bob on a rod hits an identical bob, which then just completes a vertical circle | Work, Energy and Power |
| 28 | Screw gauge with the circular scale zero above the line — corrected thickness | Units and Measurements |
| 29 | de Broglie wavelength of an oxygen molecule at 27 °C | Dual Nature of Radiation and Matter |
| 30 | Critical angle from the speeds of light in the two media | Ray Optics |
| 31 | Four claims about the series limits of the hydrogen spectrum | Atoms |
| 32 | Two spheres stick after a head-on collision — rise in temperature | Thermal Properties of Matter |
| 33 | Four measurements of a rod — relative error in the average | Units and Measurements |
| 34 | Copper wire falling through a horizontal field — emf after 200 m | Electromagnetic Induction |
| 35 | Charge at a cube corner and another at a face centre — flux through the cube | Electric Charges and Fields |
| 36 | Match four integral equations to the laws they state | Electromagnetic Waves |
| 37 | Series resistor for a Zener voltage regulator | Semiconductor Electronics |
| 38 | Ring of wire with a diameter wire across it — resistance between the ends | Current Electricity |
| 39 | Two blocks on two springs — energy stored in the second one | Oscillations |
| 40 | Square loop of four solid cylinders — moment of inertia | Rotational Motion |
| 41 | Two unequal balls on a light rod — angular velocity from the angular momentum | Rotational Motion |
| 42 | Four materials on a stress-strain plot — which has the largest Young’s modulus | Mechanical Properties of Solids |
| 43 | Assertion and reason on domains in a ferromagnetic material | Magnetism and Matter |
| 44 | Two thin prisms giving dispersion without deviation — the second angle | Ray Optics |
| 45 | Projectile at 60° — kinetic energy at two later points compared with the start | Motion in a Plane |
| 46 | numeric Average power of a wave from the amplitude of its electric field | Electromagnetic Waves |
| 47 | numeric Capacitor gap filled with dielectric slabs — the new capacitance | Electrostatic Potential and Capacitance |
| 48 | numeric Two double slit set-ups with equal fringe width — ratio of screen distances | Wave Optics |
| 49 | numeric Swinging pendulum wire in a field — largest emf along the wire | Electromagnetic Induction |
| 50 | numeric LCR circuit at its largest current — the capacitance | Alternating Current |
Chapter by chapter
| Chapter | Questions | Marks |
|---|---|---|
| Electromagnetic Induction | 2 | 8 |
| Electromagnetic Waves | 2 | 8 |
| Oscillations | 2 | 8 |
| Ray Optics | 2 | 8 |
| Rotational Motion | 2 | 8 |
| Units and Measurements | 2 | 8 |
| Alternating Current | 1 | 4 |
| Atoms | 1 | 4 |
| Current Electricity | 1 | 4 |
| Dual Nature of Radiation and Matter | 1 | 4 |
| Electric Charges and Fields | 1 | 4 |
| Electrostatic Potential and Capacitance | 1 | 4 |
| Magnetism and Matter | 1 | 4 |
| Mechanical Properties of Solids | 1 | 4 |
| Motion in a Plane | 1 | 4 |
| Semiconductor Electronics | 1 | 4 |
| Thermal Properties of Matter | 1 | 4 |
| Wave Optics | 1 | 4 |
| Work, Energy and Power | 1 | 4 |
All 19 shifts of JEE Main 2026, counted together chapter by chapter: JEE Main Physics important chapters.
Nineteen chapters, and six of them took two questions each. Magnetism and Matter turned up here, in a shift where it was tested by an assertion and reason question on ferromagnetic domains.
Where the 100 marks sat
| Part of Physics | Questions | Marks | Share |
|---|---|---|---|
| Electrodynamics | 9 | 36 | 36% |
| Optics and Waves | 5 | 20 | 20% |
| Mechanics | 4 | 16 | 16% |
| Modern Physics and Electronics | 3 | 12 | 12% |
| Units, Dimensions and Measurement | 2 | 8 | 8% |
| Properties of Matter and Heat | 2 | 8 | 8% |
36 marks of electrodynamics is the highest of any shift in 2026, and mechanics fell to 16, its lowest. A student strong in electricity and magnetism and weak in mechanics got the paper they would have asked for.
How tough it was
| Level | Questions | Share | Which ones |
|---|---|---|---|
| Straight recall or one step | 7 | 28% | 26, 30, 31, 36, 43, 44, 48 |
| Two or three steps | 12 | 48% | 28, 29, 33, 34, 35, 38, 39, 41, 45, 46, 47, 50 |
| Long or awkward | 6 | 24% | 27, 32, 37, 40, 42, 49 |
The square loop built out of four cylinders needed four separate moment-of-inertia terms, and the colliding spheres needed a collision, an energy loss and a calorimetry step one after the other. Those two were the clock-eaters. The four hydrogen-spectrum claims, on the other hand, could be checked on sight by anyone who knew that a series limit is the shortest wavelength, not the longest.
All 25 Physics questions, solved
Q1 · Oscillations · on the pattern of Q26
A simple pendulum with a string 30 cm long completes 20 oscillations in 10 s. The string length needed for it to complete 40 oscillations in the same 10 s is ______ cm. (The mass of the bob is unchanged.)
Answer 7.5 cm
Where it goes wrong Halving the length to 15 cm. The period goes as the square root of the length, so the length must change four-fold.
Q2 · Work, Energy and Power · on the pattern of Q27
A bob A of mass m is fixed to a light rigid rod 1 m long pivoted at P, held at 60° to the vertical and released. At the lowest point it strikes an identical bob B resting on a smooth horizontal surface, the collision being elastic. B then runs into a vertical circular track and just manages to complete the circle. The radius R of the track is ______ m. (g = 10 m/s2.)
Answer 0.2 m
Where it goes wrong Using v2 = 2gR, the condition for reaching the top with zero speed. On the inside of a loop the string or track cannot push inward, so the speed at the top must still be √(gR), which makes it 5gR at the bottom.
Q3 · Units and Measurement · on the pattern of Q28
In a screw gauge of least count 0.01 mm, the zero of the circular scale sits 3 divisions above the horizontal pitch line when the studs are brought together. A sheet is then measured: the pitch scale reads 1 mm and the circular scale reads 51. The correct thickness of the sheet is ______ mm.
Answer 1.54 mm
Where it goes wrong Subtracting 0.03 and answering 1.48 mm. Above the line means the instrument reads low, so the correction is added.
Q4 · Dual Nature of Matter · on the pattern of Q29
The de Broglie wavelength of an oxygen molecule at 27 °C is x × 10−12 m. The value of x is about ______. (h = 6.63 × 10−34 J·s, k = 1.38 × 10−23 J/K, mass of an oxygen molecule = 5.31 × 10−26 kg.)
Answer about 26
Where it goes wrong Using the average speed or the most probable speed instead of the r.m.s. speed. The √3 in √(3mkT) is what fixes the answer.
Q5 · Ray Optics · on the pattern of Q30
Light travels from medium A into medium B across a flat boundary and undergoes total internal reflection. The speeds of light in A and B are 2.4 × 108 m/s and 2.7 × 108 m/s. The critical angle is ______.
Answer sin−1(8/9)
Where it goes wrong Putting the ratio the other way up and getting sin C = 9/8, which is impossible. The sine of the critical angle is always the smaller index over the larger.
Q6 · Atoms · on the pattern of Q31
For the hydrogen spectrum (R is the Rydberg constant), which of these are correct? A. The longest wavelength of the Lyman series is 4/3R. B. The Balmer series lies in the visible region. C. The shortest wavelength of the Paschen series is 9/R. D. The shortest wavelength of the Lyman series is 4/5R.
Answer A, B and C only
Where it goes wrong Treating the series limit as the longest wavelength. The limit is the biggest energy jump, so it is the shortest wavelength.
Q7 · Thermal Properties of Matter · on the pattern of Q32
Two spheres of the same material, of masses 15 kg and 25 kg, move towards each other at 10 m/s and 30 m/s, collide and stick together. If all the heat produced stays in the spheres, their temperature rises by ______ °C. (Specific heat of the material = 31 cal/kg·°C, 1 cal = 4.2 J.)
Answer about 1.44 °C
Where it goes wrong Forgetting to turn joules into calories, since the specific heat is given in calories. One of the two has to be converted before dividing.
Q8 · Units and Measurement · on the pattern of Q33
Four people measure the length of a rod and get 20.00 cm, 19.75 cm, 17.01 cm and 18.25 cm. The relative error in the average length is ______.
Answer about 0.06
Where it goes wrong Letting the signs of the deviations cancel. The mean absolute error uses the sizes of the deviations, never their signs.
Q9 · Electromagnetic Induction · on the pattern of Q34
A copper wire 20 m long is held horizontally and allowed to fall freely through a uniform horizontal magnetic field of 0.5 gauss at right angles to its length. The emf across the wire after it has fallen 200 m is ______ mV. (g = 10 m/s2.)
Answer 20√10 mV, about 63 mV
Where it goes wrong Leaving the field in gauss. The conversion to tesla is a factor of 10−4, and missing it throws the answer out by ten thousand.
Q10 · Electric Charges and Fields · on the pattern of Q35
A charge 2q sits at one corner of a cube and a charge q sits at the centre of one of its faces. The total electric flux through the cube is ______.
Answer 3q/4ε0
Where it goes wrong Counting both charges as fully enclosed and answering 3q/ε0. A charge on the surface of the closed surface only ever contributes its fair share.
Q11 · Electromagnetic Waves · on the pattern of Q36
Name the law stated by each equation: A. ∮E·dl = −d/dt ∮B·da, B. ∮B·dl = μ0(I + ε0 dφE/dt), C. ∮E·da = (1/ε0)∫ρdv, D. ∮B·dl = μ0I.
Answer A → Faraday’s law of induction, B → Ampere-Maxwell law, C → Gauss’s law of electrostatics, D → Ampere’s circuital law
Where it goes wrong Mixing up B and D, which look almost the same. The one with dφE/dt in it is the Maxwell-corrected version.
Q12 · Semiconductor Electronics · on the pattern of Q37
A Zener diode rated at 5 V is used as a voltage regulator, with a series resistor Rs fed from an unregulated supply of 15 V. The load needs 10 mA, and the Zener can safely carry four times the load current. The value of Rs is ______ Ω.
Answer 200 Ω
Where it goes wrong Sizing Rs from the load current alone. The series resistor carries the load current and the Zener current together, and the Zener takes up whatever the load does not.
Q13 · Current Electricity · on the pattern of Q38
A wire of uniform resistance λ ohm per metre is bent into a circle of radius r. A second piece of the same wire, of length 2r, is joined straight across between the two ends of a diameter AB, passing through the centre. The resistance between A and B is ______.
Answer 2πλr / (π + 4)
Where it goes wrong Treating the two semicircles as being in series because they form a ring. Between A and B they are two separate routes, so they are in parallel.
Q14 · Oscillations · on the pattern of Q39
Blocks of 100 g and 200 g hang at rest from the lower ends of two springs A and B. The energy stored in A is E. If the spring constants satisfy 4kA = 3kB, the energy stored in B is ______.
Answer 3E
Where it goes wrong Taking the two extensions as equal. They are not: each block hangs until its own spring balances its own weight, so the extension depends on both m and k.
Q15 · Rotational Motion · on the pattern of Q40
A square frame is built from four uniform solid cylinders, each of radius R and length L, with R much smaller than L. The total mass of the frame is M. Its moment of inertia about an axis in the plane of the square passing through the midpoints of two opposite sides is ______.
Answer (3/8)MR2 + (1/6)ML2
Where it goes wrong Using the same formula for all four cylinders. Two are crossed by the axis and two run parallel to it, and those are different cases.
Q16 · Rotational Motion · on the pattern of Q41
Two small balls of masses m and 2m are fixed to the two ends of a rigid rod of length d and negligible mass. The angular momentum of the system about an axis A through its centre of mass, at right angles to the rod, is L. The angular velocity about A is ______.
Answer 3L / 2md2
Where it goes wrong Putting the axis at the middle of the rod. The centre of mass of m and 2m is one third of the way along, not halfway.
Q17 · Mechanical Properties of Solids · on the pattern of Q42
Four wires A, B, C and D give stress-strain graphs that are straight lines through the origin, with A the steepest and D the shallowest. Which wire has the largest Young’s modulus?
Answer A
Where it goes wrong Not checking which quantity is on which axis. If the plot is strain against stress, the modulus is one over the slope, and then the shallowest line wins instead.
Q18 · Magnetism and Matter · on the pattern of Q43
Assertion: the individual atoms of a ferromagnetic material have magnetic dipole moments and interact so as to line themselves up spontaneously, forming domains. Reason: at a high enough temperature the domain structure breaks up and the magnetisation disappears, at what is called the Curie temperature. Choose the correct option.
Answer Both are true, but the reason is not the explanation of the assertion
Where it goes wrong Marking the reason as the explanation because both sentences are about domains. A correct statement on the same topic is not automatically the cause of the first one.
Q19 · Ray Optics · on the pattern of Q44
A thin prism of angle 5° made of material of refractive index 1.72 is combined with a second thin prism of refractive index 1.9, so that the pair gives dispersion without deviation. The angle of the second prism is ______.
Answer 4°
Where it goes wrong Using μ in place of (μ − 1). It is the excess over 1 that produces the bending.
Q20 · Motion in a Plane · on the pattern of Q45
An object is thrown with kinetic energy K at 60° to the horizontal. Ignoring air resistance, the ratio of its kinetic energy at the highest point of the flight to K is ______.
Answer 1 : 4
Where it goes wrong Answering zero, as though the object stopped at the top. Only the vertical motion stops; the horizontal motion carries on untouched.
Q21 · Electromagnetic Waves · numeric · on the pattern of Q46
A plane electromagnetic wave travelling through free space has electric field E = 377 sin(6.27 × 103t − 2.09 × 10−5x) N/C. The average power carried per unit area is ______ W/m2. (Take √(μ0/ε0) = 377 SI units.)
Answer 188.5 W/m2
Where it goes wrong Leaving out the factor of 2. The average of sin2 over a cycle is a half, and that is where the 2 comes from.
Q22 · Capacitance · numeric · on the pattern of Q47
A parallel plate capacitor of capacitance C has air between its plates. The gap is now filled with two slabs, each of half the gap thickness and of the full plate area, with dielectric constants 2 and 3. The new capacitance is ______.
Answer 12C/5, that is 2.4C
Where it goes wrong Treating the slabs as being in parallel and adding. They would be in parallel only if each covered part of the area over the whole gap; stacked across the gap they are in series.
Q23 · Wave Optics · numeric · on the pattern of Q48
Two separate Young’s double slit set-ups, using light of different wavelengths, give fringes of exactly the same width. The ratio of their slit separations is 2 : 1 and the ratio of their wavelengths is 1 : 2. The ratio of the screen distances D1/D2 is ______.
Answer 4
Where it goes wrong Inverting one of the two ratios. Write β out for each set-up and divide; do not try to see the answer in one step.
Q24 · Electromagnetic Induction · numeric · on the pattern of Q49
A simple pendulum has a bob of mass 10 g on a metallic wire 10 cm long, hanging in a uniform magnetic field of 2 T at right angles to the plane in which it swings. It is released from 60° to the vertical. The largest emf between the point of suspension and the bob is ______ mV. (g = 10 m/s2.)
Answer 100 mV
Where it goes wrong Using Blv with the bob’s speed, which gives 0.2 V. Different points of the wire move at different speeds, and the integral along it brings the factor of a half.
Q25 · Alternating Current · numeric · on the pattern of Q50
A series LCR circuit with L = 1 H is driven by a source whose frequency can be varied. The largest current measured is 50 mA, and it occurs for V = 5 sin(100t). The capacitance is ______ μF.
Answer 100 μF
Where it goes wrong Trying to use the impedance formula with a guessed R. The phrase “largest current” is the whole clue: it means resonance, and at resonance only L and C matter for the frequency.
The official answer key
NTA did not publish the Session 1 question papers. They were shown only inside each candidate’s own login during the answer key challenge window in February 2026, and that window is closed. The one official file for this shift is the final answer key, which covers all ten January shifts. The other January 2026 dates were 21, 22, 24 and 28 January, two shifts on each.
All shifts, date by date: JEE Main Previous Years Papers.
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