Experimental Skills & Laboratory Physics

Experimental Physics in NEET covers 18 experiments spanning measurement instruments, mechanics, waves, thermal physics, electricity, optics, and electronics. Mastering instrument readings, error analysis, and graph interpretation secures these marks reliably.

Measurement Instruments: The Vernier caliper measures external diameter, internal diameter, and depth. Its least count LC = 1 MSD - 1 VSD; for a standard caliper with 50 VSD = 49 MSD, LC = 1 mm - $\frac{49}{50}$ mm = 0.02 mm. Reading = MSR + (coinciding VSD x LC). Zero error is positive if the vernier zero is to the right of the main scale zero (subtract from reading) and negative if to the left (add to reading). The screw gauge measures thickness of thin sheets and wire diameter with LC = pitch / number of circular divisions = typically 0.5 mm / 50 = 0.01 mm. Reading = MSR + (CSR x LC). Backlash error is avoided by always rotating in one direction. Both have dimensional formula [L] and SI unit: m.

Mechanics Experiments: The simple pendulum gives T = $2\pi \; \sqrt{l/g}$ [T: [T], s]; plotting $T^{2}$ vs l yields a straight line through the origin with slope = $4\pi^{2}$/g.
Effective length = string length + bob radius.
The principle of moments (metre scale) uses $m_{1} \; l_{1}$ = $m_{2} \; l_{2}$ for balance.
Young's modulus is found by Searle's apparatus: Y = FL/(A $\Delta$-L) [Y: [M $L^{-1} \; T^{-2}$], Pa], where the stress-strain graph's slope in the linear region gives Y.
Surface tension by capillary rise: S = h r $\rho$ g / (2 cos $\theta$) [S: [M $T^{-2}$], N/m].
Viscosity by Stokes' law: at terminal velocity $v_{t}$ = $2r^{2}$($\rho$ - $\sigma$)g/(9 $\eta$) [$\eta$: [M $L^{-1} \; T^{-1}$], Pa s], weight = buoyancy + drag.

Waves and Thermal: The resonance tube experiment measures the speed of sound.
First resonance at $l_{1}$ = $\frac{\lambda}{4}$, second at $l_{2}$ = 3 $\frac{\lambda}{4}$.
Speed v = 2f($l_{2}$ - $l_{1}$) [v: [L $T^{-1}$], m/s] — this formula eliminates the end correction e = ($l_{2}$ - $3l_{1}$)/2.
Specific heat by the method of mixtures: $m_{1} \; c_{1}$($T_{1}$ - T) = $m_{2} \; c_{2}$(T - $T_{2}$) + $m_{cal} \; c_{cal}$(T - $T_{2}$) [c: [$L^{2} \; T^{-2} \; K^{-1}$], J/(kg K)].

Electrical Experiments: The metre bridge uses the Wheatstone bridge principle: R/S = l/(100 - l), and resistivity $\rho$ = RA/L [$\rho$: [M $L^{3} \; T^{-3} \; A^{-2}$], ohm m].
Interchanging R and S gives new balance at l' = 100 - l.
Ohm's law: V-I graph is a straight line with slope = R for ohmic conductors.
Galvanometer resistance by half deflection: G approximately equals S when S << R; figure of merit k = I/$\theta$ [k: A/div].

Optics Experiments: Concave mirror focal length by u-v method: 1/v + 1/u = 1/f with Cartesian signs (f negative for concave, u negative for real object, v negative for real image). Convex mirror uses an auxiliary convex lens (f positive, v positive).
Convex lens: 1/v - 1/u = 1/f (f positive); plotting 1/v vs 1/u gives intercepts = 1/f.
Prism: the i-$\delta$ graph is U-shaped with minimum deviation when i = e (symmetric passage); $\mu$ = sin((A + $\delta_{min}$)/2) / sin(A/2).
Glass slab: $\mu$ = real depth / apparent depth.

Electronics: The p-n junction diode shows exponential current rise in forward bias after the threshold voltage: ~0.7 V for Si, ~0.3 V for Ge (Si has wider band gap, hence higher threshold). In reverse bias, a very small saturation current flows until breakdown. The Zener diode shows sharp reverse breakdown at $V_{Z}$ with nearly constant voltage — used as a voltage regulator. Component identification: diode (arrow symbol), LED (emits light in forward bias, $V_{th}$ ~1.5-3 V), resistor (color code BBROYGBVGW = 0-9), capacitor (blocks DC, passes AC).

Solved Numerical 1: Vernier caliper: 1 MSD = 1 mm, 50 VSD = 49 MSD. LC = 1 MSD - 1 VSD = 1 mm - $\frac{49}{50}$ mm = 1 - 0.98 = 0.02 mm. MSR = 35 mm, 24th VSD coincides. Raw reading = 35 + (24 x 0.02) = 35 + 0.48 = 35.48 mm. Positive zero error = 0.06 mm. Correct reading = 35.48 - 0.06 = 35.42 mm. (Positive error is subtracted.)

Solved Numerical 2: Resonance tube: $l_{1}$ = 17.0 cm = 0.170 m, $l_{2}$ = 51.8 cm = 0.518 m, f = 480 Hz.
(a) Speed of sound: v = 2f($l_{2}$ - $l_{1}$) = 2 x 480 x (0.518 - 0.170) = 960 x 0.348 = 334.1 m/s.
(b) End correction: e = ($l_{2}$ - $3l_{1}$)/2 = (0.518 - 3 x 0.170)/2 = (0.518 - 0.510)/2 = 0.$\frac{008}{2}$ = 0.004 m = 0.4 cm.
Dimensional check for v: [Hz][m] = [$T^{-1}$][L] = [L $T^{-1}$].

Solved Numerical 3: Metre bridge: balance at l = 36.0 cm, R = 6.0 ohm (left gap). R/S = l/(100 - l) = 36/(100 - 36) = $\frac{36}{64}$. S = R x $\frac{64}{36}$ = 6.0 x $\frac{64}{36}$ = 6.0 x 1.778 = 10.67 ohm. On interchanging: new balance at l' = 100 - 36 = 64.0 cm from end A.

The key testable concept is instrument reading calculations (Vernier, screw gauge) and graph interpretation ($T^{2}$ vs l, V-I, i-$\delta$, I-V characteristics).