Practical Physics and Experimental Skills
What this domain tests
Practical Physics and Experimental Skills tests measuring instruments, least count, zero error, error analysis, significant figures and standard experiments such as the metre bridge, potentiometer, pendulum, lens and diode.
Core rules
- Vernier least count = 1 main scale division − 1 vernier division; typical 0.1 mm.
- Screw gauge least count = pitch/number of circular divisions; typical 0.01 mm.
- Correct for zero error: subtract a positive zero error and add a negative one.
- Metre bridge: unknown resistance R = S × l/(100 − l), where l is the balancing length in cm.
- Ohm's law experiment: plot V against I; the slope gives resistance.
- Focal length by the u–v method: 1/f = 1/v − 1/u; plot 1/v against 1/u.
Worked example
A screw gauge has pitch 0.5 mm and 50 circular divisions. Least count = 0.5/50 = 0.01 mm. A reading shows main scale 3.0 mm and circular scale 24 divisions. Observed diameter = 3.0 + 24 × 0.01 = 3.24 mm. If the zero error is +0.02 mm, the corrected diameter is 3.24 − 0.02 = 3.22 mm.
Traps
- Forgetting to apply zero error with the correct sign.
- Reading the main scale at the wrong edge of the vernier.
- Recording more significant figures than the instrument allows.
- Taking one reading instead of repeating and averaging to reduce random error.