Experimental Skills / Practical Physics
The practical paper checks whether you can handle instruments, record readings honestly, and process them into a graph or a computed constant. Typical experiments: vernier callipers and screw gauge measurement, metre bridge resistivity, potentiometer EMF comparison, Ohm's law, galvanometer figure of merit, focal length of a convex lens by the u–v method, prism angle and deviation, resonance tube for speed of sound, sonometer, and the I–V characteristics of a diode. Marks are lost far more often on least count and zero error than on physics.
Core rules
- Vernier least count = 1 main scale division − 1 vernier scale division; screw gauge LC = pitch/number of circular scale divisions.
- Record corrected reading as (main scale reading + LC × coinciding division) − zero error, keeping the sign of the zero error.
- Metre bridge: unknown resistance R = S·l/(100 − l); resistivity ρ = RA/L with A = πr².
- Potentiometer compares EMFs by balancing lengths: E₁/E₂ = l₁/l₂, because at balance no current flows.
- Lens formula 1/f = 1/v − 1/u; plotting 1/v against 1/u gives a straight line with intercepts 1/f. Prism: n = sin[(A + δm)/2]/sin(A/2).
- Resonance tube: v = 2f(l₂ − l₁); end correction e = (l₂ − 3l₁)/2.
- Always report the number of significant figures justified by the instrument, and repeat readings to average random error.
Worked example
A vernier has 1 main scale division = 1 mm and 10 vernier divisions coinciding with 9 main scale divisions. Main scale reads 12 mm and the 4th vernier division coincides. The instrument has zero error of −0.2 mm. LC = 1 mm − 0.9 mm = 0.1 mm. Observed reading = 12 + 4 × 0.1 = 12.4 mm. Corrected reading = 12.4 − (−0.2) = 12.6 mm. The sign matters: a negative zero error is subtracted as a negative, i.e. added.
Traps that cost marks
- Ignoring zero error or applying it with the wrong sign.
- Quoting a result to five decimal places when the instrument resolves only 0.01 mm.
- Plotting with an unlabelled axis or without choosing a scale that spreads the points.
- Mixing units (cm and m) inside one formula such as ρ = RA/L.