Controlling Variables

Listing your controlled variables is one thing; actually keeping them constant is another. This is the practical craft of experimental physics — the calibrating, insulating, and shielding that stops unwanted factors from quietly skewing your data. The goal throughout is to minimise systematic errors: the sneaky kind that shift every reading in the same direction. Master this and your results become genuinely valid and fair.

Calibrating your instruments

Calibration is the process of checking an instrument’s readings against a known, reliable standard and adjusting it if necessary. It’s the first line of defence against systematic error — an uncalibrated instrument can be wrong by the same amount every single time, and no number of repeats will catch it.

A digital thermometer or temperature sensor is checked at two fixed points: pure melting ice, which should read 0.0 °C, and boiling distilled water at standard pressure, which should read 100.0 °C. If it’s off at these known points, you know it needs adjusting.

A digital balance must always be zeroed — tared — before use, so you measure only the mass of the object and not the container or any drift in the electronics.

Maintaining environmental conditions

The lab itself can affect your results. Key conditions to watch are temperature, pressure, humidity, air currents (draughts), and light intensity. Temperature is often the most important, because it directly changes other quantities — density, gas pressure, and electrical resistance among them.

Draughts from windows or air conditioning can cool a substance and wreck a calorimetry experiment, so close windows or use a draught shield. And when a variable simply can’t be perfectly controlled — ambient room temperature might drift — the best practice is to monitor and record it, then discuss its impact in your evaluation.

Insulating against heat loss or gain

In any thermal experiment, the biggest source of error is unwanted heat exchange with the surroundings. Insulating the system is essential for accurate temperature data. Common techniques: use a polystyrene cup instead of a glass beaker (far better insulator), nest it inside a larger beaker for a trapped air layer, and add a lid with holes for the thermometer and stirrer to cut heat loss by evaporation and convection.

Reducing friction and resistance

Two more unwanted energy losses come up constantly in mechanics and circuits.

Reducing friction

Frictional forces reduce motion and drain energy as heat and sound. Reduce it by lubricating moving parts, adding bearings, or using low-friction apparatus like an air track — which floats a glider on a cushion of air for a near-frictionless surface, ideal for studying collisions.

Reducing electrical resistance

Every component and connecting wire has some resistance, causing unwanted heating. Reduce it by using shorter, thicker wires, keeping the current low, and ensuring connections are secure — loose or dirty contacts add extra resistance.

Accounting for background radiation

In any experiment with radioactive sources, background radiation must be accounted for — it’s always present, from cosmic rays, rocks, and the air. Using a Geiger–Muller (GM) tube, first record a background count rate with no source present, then subtract it from every subsequent measurement, so you’re left with only the radiation from the source itself.