Experimental Skills and Practical Physics
Experimental skills tests instrument least count, zero error, significant figures, error propagation, graph interpretation, and standard experiments such as Ohm law, metre bridge, potentiometer, lens focal length, prism deviation, and galvanometer figure of merit. The exam rewards precise reading and correct treatment of systematic versus random error.
Core rules and concepts
- Least count of a vernier = 1 main-scale division - 1 vernier division. For a screw gauge, LC = pitch/number of circular divisions.
- Zero error must be subtracted algebraically: if the instrument reads positive when closed, subtract that value from measurements.
- For addition/subtraction, absolute errors add. For multiplication/division, fractional errors add. For a power x^n, fractional error is n times the fractional error in x.
- A graph slope carries units of y per x; area under a graph carries product units. Linearise data before fitting when possible.
- In a metre bridge, balance condition R/S = l/(100 - l). In a potentiometer, emf ratio equals balancing length ratio.
- Avoid parallax, backlash, and heating of wires; repeat readings and use mean values.
Worked example
A screw gauge has pitch 0.50 mm and 50 circular divisions, so LC = 0.01 mm. Main scale reads 2.5 mm and circular scale reads 23 divisions: observed = 2.5 + 23(0.01) = 2.73 mm. If zero error is +0.02 mm, corrected reading = 2.73 - 0.02 = 2.71 mm.
Traps that cost marks
- Adding zero error instead of subtracting it with the correct sign.
- Using the wrong number of divisions for least count.
- Reporting more significant figures than the instrument allows.
- Forgetting to convert cm to m in a formula.
- Taking a graph slope without its units.