Experimental Skills and Practical Physics
What the exam tests
Practical physics questions test instrument least count, zero error, vernier callipers, screw gauge, simple pendulum, metre bridge, Ohm law, diode characteristics, lens and mirror focal length, prism and sonometer experiments. Expect reading interpretation and error analysis.
Core rules
- Vernier least count = 1 main scale division − 1 vernier scale division. Screw gauge least count = pitch/number of circular divisions.
- Zero error must be corrected with its sign: subtract a positive zero error and add a negative zero error to the observed reading.
- Simple pendulum: T = 2π√(L/g), so g = 4π^2L/T^2. Measure T for many oscillations and divide.
- Metre bridge balance: R/S = l/(100 − l), where l is the balancing length from the left end.
- Ohm law graph: V against I is a straight line through the origin; its slope gives resistance.
- Diode characteristic: forward current rises sharply after cut-in voltage; reverse current is small until breakdown.
- For a lens, plot u and v or use the u-v method; for a prism, plot deviation against incidence angle.
Worked example
A screw gauge has pitch 0.5 mm and 50 circular divisions, so least count = 0.5/50 = 0.01 mm. Main scale reads 2.5 mm and circular scale reads 23 divisions. Observed reading = 2.5 + 23×0.01 = 2.73 mm. If the zero error is +0.02 mm, the corrected reading is 2.73 − 0.02 = 2.71 mm. Positive zero error is subtracted.
Traps
Least count must be in the same unit as the reading. Zero error sign is a common mistake. Avoid parallax by keeping the eye perpendicular to the scale. Do not average readings taken with different least counts. In pendulum experiments, measure length to the centre of the bob. In metre bridge, check connections before balancing.