Independent variable: It is the variable that the experimenter changes, to get different values from dependent variable.
Dependent variable: The variable that depends on independent variable.
Controlled variable: The variable that doesn’t actually affect anything.
How to identify these variables?
Let’s take a look on a past question paper from IGCSE Physics 2013 Jan Paper 1.
Q: A student investigates how the resistance of a wire depends on its length. The photograph shows the circuit that the student uses. Identify the independent and dependent variable.
One easy technique to find the answers is to check what variable the experimenter is trying to measure. That variable will be the dependent variable. The variable that it is depending on is the independent variable.
Ans: Dependent variable: Resistance (as it is the result the student is trying to obtain)
Independent variable: Length of the wire (because the resistance is affected by the length)
Let’s take a look on another question from 2013 May Paper 1.
Q: A student investigates the vertical forces acting on the ends of a horizontal ruler when it supports a load. The ruler hangs from two newtonmeters with a weight suspended from it as shown.
The student moves the weight along the ruler and records forces B and C by taking readings from the newtonmeters. Identify the independent and controlled variable.
Ans: Independent variable: Distance A
Explanation: The experimenter investigates the forces B and C. There those are dependent variable. And this variables are depending on the distance A. So distance A is the independent variable.
Controlled variable: force D
Explanation: The weight do not actually affects the experiment. (Force B and C might increase by increasing force D, but that doesn’t matter. Investigation is upon the ratio of readings from B to C when A is changed. Changing the mass will keep the ratio same.)
What makes a fair test?
If an experiment consist of only one independent variable and dependent variable, and all other variables are controlled, then this experiment is a fair test.
For example, a question from 2013 June 2PR. A student investigates friction between a block of wood and different types of surface. The student uses the equipment shown in photograph A to measure the force needed to move the block of wood.
Explain why he keeps the masses constant during the experiment.
Ans: To keep a fair test.
Explanation: The independent variable of this experiment is type of surface. So there cannot be more than one independent variable. The masses, if different, can affect the force needed to move the block (the dependent variable). So it should be controlled to keep a fair test.