Experiment-Based Physics Questions in JEE Main

Experiment-based questions in JEE Main physics have increased in frequency since 2021, consistently appearing as 1–2 questions per session. These questions test the NCERT Class 11 and 12 physics experiments and practical skills: instrument reading, error calculation, graph interpretation, and understanding of experimental design. Unlike pure theory questions, experiment-based questions require familiarity with real laboratory setups — many students who score well on theory fail these questions simply because they have never engaged with the practical physics content. This guide covers all the key experiments in JEE Main-relevant detail.

Precision Instruments: Vernier Callipers and Screw Gauge

Vernier callipers: The main scale reads in mm; the vernier scale provides additional precision. Least count (LC) = 1 MSD − 1 VSD. Standard: 10 VSD = 9 MSD = 9 mm, so 1 VSD = 0.9 mm, LC = 0.1 mm. Reading: Main scale reading + (vernier scale coincidence × LC). Zero error: if the zero of the vernier scale is to the right of the main scale zero when jaws are closed, there is a positive zero error — subtract it from all readings. If to the left, negative zero error — add it. JEE Main question type: "Vernier reading shows MSR = 2.3 cm, VSC = 5. Find the reading (LC = 0.01 cm)." Answer: 2.3 + 5×0.01 = 2.35 cm. Screw gauge: LC = pitch / number of circular divisions = 0.5 mm / 50 = 0.01 mm. Reading: MSR + (circular scale reading × LC). Backlash error: always approach the measurement from the same direction to avoid backlash. For the broader measurement context, see our Units, Measurement, and Errors Guide.

Metre bridge experiment: measures unknown resistance using Wheatstone bridge principle. At balance: R/S = l/(100−l). Potential divider connection: the metre bridge wire acts as a potential divider. End errors (contact resistance at the ends) affect accuracy — the experiment is more accurate when balance length is near the midpoint (40–60 cm range). If balance shifts to one extreme, the precision decreases. JEE Main tests: given balance length l1 with R in left gap, find R after interchanging R and S (balance shifts to l2 = 100 − l1 for same R and S values).

Young's Modulus, Resonance Tube, and Sonometer

Young's Modulus experiment (Searle's apparatus): Y = (MgL)/(pi·r²·l), where M is the load, g is gravitational acceleration, L is original length, r is wire radius, l is elongation. The most sensitive measurement is l (elongation) — read from an optical lever or micrometer. Graph: stress vs. strain, slope = Y. JEE question: if the wire radius is measured with 2% error and elongation with 3% error, what is the percentage error in Y? Using error propagation: delta·Y/Y = delta·M/M + delta·g/g + delta·L/L + 2·(delta·r/r) + delta·l/l. The factor of 2 for r is because Y depends on r² — a key error propagation application. Practise experiment-based JEE Main questions on our mock platform which includes specifically curated experiment-themed question sets.

Resonance tube experiment: determines the velocity of sound in air. At resonance, the length of the air column is approximately lambda/4 for the first resonance and 3·lambda/4 for the second. First resonance at L1, second at L2: L2 − L1 = lambda/2, so lambda = 2(L2 − L1). End correction e = (L2 − 3·L1)/2. Velocity of sound: v = f·lambda = 2f(L2 − L1). This experiment is directly tested in JEE Main. Sonometer: frequency of vibration f = (1/2l)·sqrt(T/m) where l is vibrating length, T is tension, m is mass per unit length. Melde's experiment: transverse modes vs. longitudinal modes differ by a factor of 2 in string vibration frequency.

Electrical Experiments: Potentiometer and Galvanometer

Potentiometer experiment (internal resistance): cell E with internal resistance r, balance length with circuit open = l1, with external resistance R = l2. Then r = R(l1 − l2)/l2. Derivation: in open circuit, E = k·l1; in closed circuit, terminal voltage V = k·l2 = E·R/(R+r). So k·l2 = k·l1·R/(R+r), giving r/R = (l1/l2) − 1 = (l1−l2)/l2. This is a standard derivation that generates both theory and numerical questions. Potentiometer sensitivity: increase battery EMF or use a thinner, longer wire to increase potential gradient and sensitivity. Galvanometer conversion to ammeter: shunt resistance S = I_g·G/(I−I_g), where G is galvanometer resistance, I_g is full-scale deflection current, I is desired full-scale current. Conversion to voltmeter: series resistance R = V/I_g − G, where V is desired full-scale voltage. These formulas appear annually in JEE Main.

p-n junction diode experiment: V-I characteristics show the knee voltage (forward biased conduction begins) at approximately 0.6–0.7 V for silicon and 0.2–0.3 V for germanium. In reverse bias, tiny reverse saturation current flows until breakdown voltage. Zener diode operates in the breakdown region — used as a voltage regulator. JEE questions test: reading V-I graphs, identifying dynamic resistance = delta·V/delta·I at a point, and calculating the forward bias resistance.

Graph-Based Experimental Data Analysis

JEE Main increasingly presents experimental data as tables or graphs and asks students to extract physical quantities. Key graph types in physics experiments: (1) v-t graph in free fall — slope = g (should be approximately 9.8 m/s²; deviations indicate air resistance or measurement error). (2) log(activity) vs. time in radioactive decay — slope = −lambda (decay constant), and t_1/2 = ln(2)/lambda. (3) V_s vs. frequency in photoelectric experiment — slope = h/e, x-intercept = threshold frequency. (4) Stress vs. strain for a metal wire — Young's modulus from slope, elastic limit from deviation from linearity. For each graph type, know what slope and intercept represent physically — JEE Main asks both "what does the slope represent?" and numerical calculations from given slope values. Create an account to access experiment-based physics question sets. Our subscription includes graph interpretation exercises drawn from actual NCERT experiments. For the electromagnetic induction experiment (coil and galvanometer), our Magnetic Effects Guide provides the theoretical foundation needed to interpret experimental observations correctly.

Before the JEE Main exam, review the NCERT physics practical books for Classes 11 and 12. The listed experiments are the exact source of experiment-based questions in JEE Main. A 3-hour session reviewing all experiments (not doing them — just reading and understanding the setup, principle, and formula derivation) can secure 8 marks that many underprepared students drop.