Control Variables & Fair Tests (Edexcel International A Level Physics)

• In an experiment, a variable is any factor that could change or be changed
• There are different types of variables within an experiment
  • The independent variable: the only variable that should be changed throughout an experiment
  • The controlled variables: any other variables that may affect the results of the experiment that need to be controlled or monitored
  • The dependent variable: the variable that is measured to determine the outcome of an experiment (the results)

• It is essential that any variable that may affect the outcome of an experiment is controlled in order for the results to be valid and to have a fair test
  • A fair test is one in which only the independent variable has been allowed to affect the dependent variable
• Preliminary research and preliminary studies can be used to identify variables within an experiment and to determine ways of controlling these variables effectively
  • The science surrounding the issue / problem being investigated is likely to contain information about different factors or variables that may exist
• A common control variable in circuits (unless using a thermistor) is temperature
  • The temperature of a wire or electrical component increases as current goes through for a long period of time (think of a laptop charger)
  • Therefore, to keep temperature (the control variable) constant, it is common to disconnect components from the power supply to cool down between readings

Identifying Control Variables: An Example - Science Surrounding Ideal Gases

• There are several experiments that can be carried out to investigate the properties of ideal gases
• Factors that can be changed include:
  • Temperature
  • Pressure
  • Volume
  • Number of moles of gas

• The key point with ideal gas experiments is to ensure that only one of these variables is changed during a particular experiment
  • This is known as the independent variable

• All other variables must be controlled (they must stay the same)
  • These are known as the control variables

• When investigating Charles’ law:
  • The temperature varies (independent variable)
  • The volume is measured (dependent variable)
  • The pressure and number of moles must be kept the same (control variables)

• When investigating Boyle’s law
  • The pressure varies (independent variable)
  • The volume is measured (dependent variable)
  • The temperature and number of moles must be kept the same (control variables)

• If these control variables are not kept constant, they could affect the results of the experiment
  • This would make the results unreliable

• The control variables are:
  • Using the same beaker (this implies the same size and material)
  • Volume of water
  • Position in the room
  • Surrounding temperature