Practical Skills and Experimental Methods: Circuits, Mirrors and Data
This domain tests the skills assumed in the practical examination: correct circuit connections, finding the focal length of a mirror, tracing refraction through a glass slab, and recording and interpreting data.
Measurement discipline
Note the least count and zero error of every meter before use, and choose a range that keeps the pointer in the upper part of the scale. Read at eye level to avoid parallax, and take at least three readings, using the mean to reduce random error.
Common experiments
- Ohm's law: ammeter in series, voltmeter in parallel across the resistor. The V–I graph is a straight line through the origin and its slope gives R.
- Concave mirror: focus a distant object for a rough focal length, then use u–v pairs in 1/v + 1/u = 1/f.
- Glass slab: pin method. The emergent ray is parallel to the incident ray, and lateral displacement increases with slab thickness and angle of incidence.
- Electromagnetic induction: connect a coil to a galvanometer and move a magnet in and out; deflections in opposite directions show that the direction of change of flux matters.
Worked example
In a mirror experiment u = 30 cm and v = 15 cm, both measured in front of a concave mirror. Using 1/f = 1/v + 1/u with u = −30 cm and v = −15 cm: 1/f = −1/15 − 1/30 = −3/30, so f = −10 cm and the focal length is 10 cm. The image is real, inverted and diminished, consistent with the object lying beyond the centre of curvature.
Traps that cost marks
- Connecting the ammeter in parallel; its very low resistance short-circuits the circuit.
- Forgetting to convert centimetres to metres in SI calculations.
- Reporting a single reading without repeating the trial.
- Ignoring the sign convention in the mirror formula.