OCR A-Level Physics

## Designing an investigation An experiment tests how one quantity depends on another, so the first job is to decide which quantity you will change and which you will measure. The **independent variable** is the one you change deliberately, and the **dependent variable** is the one you measure as a result. Every other quantity that could affect the result is a **control variable** and has to be kept constant, because otherwise a change in the dependent variable could be caused by something other than the one you meant to alter. A plan should say how each variable will be measured, with which instrument and to what resolution, and how each control variable will be held steady. In a test of how the resistance of a wire depends on its length, for example, the length is the independent variable and the resistance the dependent variable. The controls are the wire's material, its diameter and its temperature. Temperature is the awkward one, because the current heats the wire, so the plan should use a low current and switch it off between readings. The instrument must have a resolution that suits the size of the quantity. A metre rule with 1 mm divisions is fine for a length of 80 cm, where a reading is uncertain by about 1 part in 400, but it is poor for a wire of diameter 0.5 mm, where the same uncertainty is larger than the thing being measured. That is why a micrometer is used for the diameter. Choosing apparatus therefore means estimating the size of each quantity first, then…