IGCSE Physics (0625)

Specific techniques (ray-tracing, balancing, timing)

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

Heating a liquid at one spot creates uneven temperature (hotter near the heater), so stirring ensures the thermometer measures the uniform temperature of the whole liquid, not just a local region.
Common mistake: Students often forget heat spreads slowly without mixing.
To minimize error in ray-tracing, pins must be placed directly on the path of the ray and spaced far enough apart (at least 5 cm) so that small placement errors do not greatly affect the drawn line's direction.
To show the reflected ray, you must draw a single straight line through the points marking the aligned pins (P3 and P4), extending it back to the mirror, because the reflected ray follows the path of light as seen from the observer’s eye.
Common mistake: Students often incorrectly connect all pin positions or draw multiple lines.
Recognise and accurately draw the thermistor symbol (a resistor rectangle with a bent diagonal line) and connect it in series, not parallel, so current flows through it as part of the circuit.
A voltmeter must be connected in parallel across the component being measured, not in series, so it measures the potential difference between the two ends without affecting the circuit's current.
To measure a liquid's temperature accurately, keep the whole thermometer bulb under the surface and away from the container's sides and base, so it reads the liquid and not the metal or air.

Worked example

Explain why the oil is stirred just before each thermometer reading is taken.

Try it first, then show the working

to make the temperature uniform / even throughout the liquid, / hotter oil near the heater and colder oil elsewhere

Answer: Stirring mixes the oil so that its temperature is the same all through the calorimeter, with no hotter regions near the heater, so the thermometer records the true temperature of the whole liquid rather than of one spot.

Try one yourself

Describe where on the line GD you would put the crosses for P1 and P2, and state the separation you would choose for a ray-trace experiment of this…Answer it and get it marked → Describe in writing exactly what you would draw, and where, to show the reflected ray.Answer it and get it marked → Complete the circuit by describing the symbol you would draw for the thermistor between M and N and how it is connected.Answer it and get it marked →

30 exam-style questions on Specific techniques (ray-tracing, balancing, timing), each marked to the scheme.

Specific techniques (ray-tracing, balancing, timing)

Key ideas and common mistakes

Worked example

Try one yourself

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