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

Revision Note

Katie M


Katie M



Control Variables & Fair Tests

  • 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

Worked example

A student carries out an experiment to measure how the rate of cooling of a beaker depends on its initial temperature.

List the control variables in this experiment.

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

Exam Tip

When answering practical questions, try and use scientific terms where possible. For example, avoid saying 'amount' of water and instead say 'volume', as this is technically the variable that you can measure in a beaker.

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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