Practical Physics and Experimental Skills
Practical Physics and Experimental Skills tests instrument reading, least count, error treatment and standard experiments such as metre bridge, potentiometer, Ohm's law, lens focal length and prism deviation. You must describe the procedure and calculate from readings.
Core rules
- Least count of vernier callipers = 1 main scale division - 1 vernier scale division. For a screw gauge, LC = pitch/number of circular divisions.
- Zero error must be subtracted algebraically from every reading. Negative zero error is added with its sign.
- Metre bridge: unknown resistance R = S l/(100 - l), where l is the balancing length from the unknown side. Potentiometer compares emfs because it draws no current at balance.
- Ohm's law experiment plots V against I; the slope is resistance. Lens experiment uses 1/f = 1/v - 1/u or a u-v graph.
- Reduce random error by repeating readings and averaging; avoid parallax by keeping the eye perpendicular to the scale.
Worked example
A screw gauge has pitch 0.50 mm and 50 circular divisions. LC = 0.50/50 = 0.010 mm. A reading shows main scale 2.5 mm and circular scale 23 divisions, with zero error +0.02 mm. Observed reading = 2.5 + 23(0.010) = 2.73 mm. Corrected reading = 2.73 - 0.02 = 2.71 mm.
Traps
Adding zero error instead of subtracting it; using the wrong number of divisions; reading the main scale at the wrong edge; and reporting more decimal places than the least count allows.