JEE Main (Engineering) · Physics (JEE & NEET)
Experimental Skills
JEE Physics laboratory methods, instrument least count, error handling and standard experiment formulas from mechanics, fluids, heat, optics and electricity.
Experimental skills is a formula-and-reading topic: instruments, graph slopes and standard lab setups are tested as directly as theory.
- JEE Main (Engineering)
- Medium level
- 5 concepts
- 5 practice questions
1Instrument reading skills
Every laboratory reading starts with the instrument, not the formula. Vernier calipers and screw gauges both read as a main value plus a coincidence multiplied by least count, then corrected for zero error. The reported value must respect the least count and significant figures.
Figure. Reading = main scale + (coinciding division × LC) − zero error. The bars walk the same arithmetic the undrawable vernier scale would have shown.
How it works
- Find least countUse vernier or screw-gauge definition before reading the scale.
- Add coincidenceMultiply the coinciding division by least count.
- Correct zero errorSubtract positive zero error and add negative zero error.
A screw gauge with zero error
A screw gauge has pitch 0.5\text{ mm} and 50 circular divisions, and reads +0.02\text{ mm} with the jaws closed. Holding a wire, the main scale reads 2.5\text{ mm} and the 27th circular division coincides. Find the diameter.
- Least count =0.5/500.01\text{ mm}
- Circular contribution =27\times0.010.27\text{ mm}
- Observed reading =2.5+0.272.77\text{ mm}
- Correct the positive zero error: 2.77-0.022.75\text{ mm}
Pro tip. Zero error carries its own sign into the subtraction, so a positive one always makes the reported diameter smaller than the raw reading.
A positive zero error of +0.02\text{ mm} means the true reading is
- Observed + 0.02 mm
- Observed - 0.02 mm
- Observed x 0.02 mm
Positive zero error means the instrument already reads too high at zero, so subtract it from the observed reading.
2Simple pendulum and graph reading
For small oscillations a simple pendulum obeys T=2\pi\sqrt{l/g}. The experiment is usually made straight-line by plotting T^2 against l, giving slope 4\pi^2/g. JEE asks this because many students read the slope as g instead of its reciprocal multiple.
Figure. Plot T² against length and the pendulum law becomes a straight line through the origin with slope 4π²/g. Read g from the slope — that is the experiment.
How it works
- Square the periodUse T^2 so the graph against l is a straight line.
- Read slopeCompare with T^2=(4\pi^2/g)l.
- Invert for gg=4\pi^2/\text{slope}.
Finding g from slope
T^2 vs l has slope 4.0\text{ s}^2\text{ m}^{-1}. Find g.
- Slope m=4\pi^2/gg=4\pi^2/m
- g=39.5/4.09.9\text{ m s}^{-2}
Pro tip. Graph slope questions are formula-comparison questions: identify y, x and m first.
3Mechanics and fluids experiments
Young modulus, capillary rise and Stokes viscosity look unrelated, but each is a balance law measured through dimensions you can read. Young modulus uses stress over strain from a stretched wire; capillary rise balances surface-tension pull against liquid weight; Stokes method balances viscous drag against effective weight at terminal speed.
Figure. Three balance laws, three readouts: extension for Young modulus, rise height for surface tension, terminal speed for viscosity. Each bar names the measured route, not a fake apparatus glyph.
How it works
- ElasticityY=FL/(A\Delta L) from a wire extension.
- Surface tensionCapillary rise has h\propto1/r for complete wetting.
- ViscosityAt terminal speed, net downward force equals 6\pi\eta rv.
| Experiment | Measured relation | Common trap |
|---|---|---|
| Young modulus | Y=FL/(A\Delta L) | Use wire radius in area \pi r^2 |
| Capillary rise | h=2T\cos\theta/(\rho g r) | h varies inversely with tube radius |
| Stokes viscosity | 6\pi\eta rv equals effective weight | Use terminal speed only |
In capillary rise, doubling tube radius approximately
- Doubles the rise
- Halves the rise
- Leaves the rise unchanged
h=2T\cos\theta/(\rho g r), so rise is inversely proportional to radius.
4Heat, sound and optics labs
Calorimetry is heat accounting: heat lost by the hot body equals heat gained by the cold body plus calorimeter. Resonance tube readings give air-column lengths separated by \lambda/2, so end correction cancels between consecutive resonances. Optics experiments use the same sign conventions as theory but ask you to identify u, v, angle of deviation or lateral shift from readings.
Figure. Heat lost equals heat gained; successive resonance lengths differ by λ/2; lens/mirror labs still close with 1/v + 1/u = 1/f. The bars index the three lab families.
How it works
- CalorimetryWrite heat lost = heat gained before substituting masses and specific heats.
- Resonance tubeUse l_2-l_1=\lambda/2 to avoid end-correction errors.
- OpticsMark object/image distances and angles with the chosen sign convention.
Mixing two masses of water
100\text{ g} of water at 80^\circ\text{C} is poured onto 200\text{ g} at 20^\circ\text{C} in a calorimeter of negligible heat capacity. Find the final temperature.
- Heat lost by the hot water = heat gained by the cold100(80-T)=200(T-20)
- 8000-100T=200T-4000300T=12000
- T40^\circ\text{C}
Pro tip. The specific heat cancels only because both sides are water. With a metal on one side, or a calorimeter whose own heat capacity is given, both belong in the balance.
Two successive resonance lengths are 17\text{ cm} and 51\text{ cm}. The wavelength is
- 34\text{ cm}
- 68\text{ cm}
- 17\text{ cm}
Successive lengths differ by \lambda/2, so 51-17=34\text{ cm}=\lambda/2 and \lambda=68\text{ cm}.
5Electrical experiments
Electrical laboratory questions test the circuit relation and the reading method together. Ohm law is a straight V-I graph; metre bridge and post office box are Wheatstone-bridge balance methods; half-deflection finds galvanometer resistance; p-n junction and Zener characteristics ask you to read the knee or breakdown region on an I-V curve.
Figure. Ohm's-law practicals are a V–I line through the origin; metre bridge and PO box are Wheatstone nulls (balance condition in the prose). Slope of this line is R.
How it works
- Ohm lawSlope of V vs I is resistance.
- Bridge balanceAt null deflection, ratio arms equal resistance ratios.
- Device curvesForward knee and Zener breakdown are read from I-V characteristics.
Metre bridge balance
Unknown R balances 5\ \Omega at 60\text{ cm} from the unknown end.
- R/S=l/(100-l)60/40=1.5
- R1.5\times5=7.5\ \Omega
Pro tip. The length beside the unknown resistance belongs in the numerator with the unknown.
In an Ohm law experiment, the slope of a V vs I graph gives
- Resistance
- Conductance
- Power
V=IR, so plotting V on the y-axis against I on the x-axis gives slope R.
Notes
- Instrument reading skills combine least count, zero error and significant figures. Vernier calipers and screw gauges test the same pattern: main reading plus coinciding division times least count, followed by zero correction.
- Mechanics experiments include simple pendulum T=2\pi\sqrt{l/g}, metre-scale moments \sum \tau=0, Young modulus from wire extension, capillary rise h=\dfrac{2T\cos\theta}{\rho g r} and viscosity by Stokes law 6\pi\eta rv.
- Heat and sound experiments include calorimetry by heat lost equals heat gained, resonance tube columns with end correction, and reading u-v optics data using mirror, lens, prism and slab arrangements.
- Electrical experiments include Ohm law, metre bridge, post office box, potentiometer, galvanometer resistance by half-deflection, and p-n junction/Zener characteristics.
- Experimental questions often ask for the slope or intercept of a graph, not the final formula. Identify which plotted quantity is on each axis before using a straight-line relation.
Formulas
- Simple pendulum: T=2\pi\sqrt{l/g}, so T^2=\dfrac{4\pi^2}{g}l
- Young modulus: Y=\dfrac{FL}{A\Delta L}=\dfrac{MgL}{\pi r^2\Delta L}
- Capillary rise: h=\dfrac{2T\cos\theta}{\rho g r}
- Stokes law terminal speed: 6\pi\eta rv=\dfrac{4}{3}\pi r^3(\rho_s-\rho_l)g
- Metre bridge balance: \dfrac{R}{S}=\dfrac{l}{100-l}
- Lens formula: \dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}; mirror formula: \dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f}
Exam traps & shortcuts
- When a graph is linear, compare it with y=mx+c before substituting values; slope and intercept usually carry the answer.
- In metre bridge problems, interchange of arms or balancing near the middle reduces end error; a balance length too close to 0 or 100 cm is suspect.
- For resonance tube, consecutive resonance lengths differ by \lambda/2, so the end correction cancels in the difference.
Reference tables
| Area | Experiment cues | Formula/readout |
|---|---|---|
| Measurements | Vernier, screw gauge | least count + zero correction |
| Mechanics | Pendulum, moments, Young modulus | T^2-l, torque balance, Y=FL/A\Delta L |
| Fluids | Capillary rise, Stokes law | h\propto1/r, terminal velocity |
| Sound/heat | Resonance tube, calorimetry | l_2-l_1=\lambda/2, heat lost = heat gained |
| Electricity | Metre bridge, Ohm law, Zener | balance length, V-I slope, breakdown voltage |
| Optics | Mirror/lens/prism/slab | u-v, deviation, lateral shift |
Recap
Read only this before an experimental-skills set.
- Reading
- Main reading plus coincidence times least count, then zero correction.
- Graphs
- Match the graph to y=mx+c; slope is often a reciprocal multiple of the wanted constant.
- Null methods
- At bridge balance or potentiometer balance, the detector current is zero and ratios become exact.
- Resonance
- Consecutive air-column resonances differ by \lambda/2, cancelling end correction.
- Devices
- p-n junction and Zener practicals ask for forward knee or reverse breakdown from I-V data.
Practise Experimental Skills
Reading is free and needs no account. Practice, mocks and progress live in the app.
- 5 exam-style questions on this topic, with explanations
- A 6-question practice set that ends the chapter
- Timed mocks scored with the real marking scheme
- Readiness tracked per topic, kept on your device