Practical Physics and Experimental Skills
What is examined
Practical Physics checks the instruments and experiments in the KSEAB practical list: vernier calipers, screw gauge, simple pendulum, metre bridge, potentiometer, Ohm's law, convex lens, prism and the figure of merit of a galvanometer. Questions ask for least count, zero error, formula, precautions and error analysis.
Core rules
- Vernier: LC = 1 main-scale division − 1 vernier-scale division; usual LC = 0.1 mm (0.01 cm). Reading = MSR + (VSD × LC) − zero error.
- Screw gauge: LC = pitch/number of circular divisions, usually 0.01 mm. Reading = main scale + circular scale × LC, corrected for zero error with sign.
- Simple pendulum: T = 2π√(L/g); plot L against T² to get a straight line of slope g/4π².
- Metre bridge: R/S = l/(100 − l); interchange the gaps and take the mean to cancel end corrections.
- Potentiometer: balance length is proportional to emf; at balance the galvanometer shows no deflection.
- Ohm's law: plot V against I; the slope is R.
- Convex lens: use the u–v graph; find f from the intercepts; power P = 100/f with f in cm.
- Prism: plot δ against i; the minimum gives δ_m and n = sin[(A + δ_m)/2]/sin(A/2).
- Galvanometer figure of merit k = I/θ, found from a known current and deflection.
Worked example
A screw gauge has pitch 1 mm and 100 circular divisions. Main scale reads 3 mm, circular scale 45, zero error −0.02 mm. LC = 1/100 = 0.01 mm. Observed = 3 + 45 × 0.01 = 3.45 mm. Corrected = 3.45 − (−0.02) = 3.47 mm.
Traps
Adding a negative zero error instead of subtracting it; reading the vernier in cm but quoting mm; using l instead of 100 − l in the metre bridge; and ignoring parallax when reading a pointer or scale.