Controlling Variables in Physics (DP IB Physics): Revision Note

Controlling variables in Physics

• This is the practical application of the 'Controlled Variables' you identified in your design

• The main goal of controlling variables is to minimise systematic errors - errors that consistently shift your results in one direction

• By keeping all other factors constant, you can:

  • be more confident that any effect you measure is due to the change in your independent variable

  • ensure that the results are valid

  • ensure that you are conducting a fair test

Methods of control

Appreciate when and how to calibrate measuring apparatus

• To ensure your data is accurate, the instruments you use must be checked and calibrated

• Calibration is the process of checking an instrument's readings against a known, reliable standard and adjusting it if necessary

• You should consider calibration for key instruments in your design:

  • Sensors:

    • A force sensor should be zeroed with no load on it

    • A motion sensor should be checked to ensure it accurately measures the distance to a known object

  • Digital thermometers and sensors

    • Their accuracy can be checked by placing them in boiling, distilled water (which should read 100.0°C at standard pressure) and a beaker of crushed ice (0.0°C)

      

      

  • Digital balances

    • Must always be zeroed (tared) before use to ensure you are measuring the mass of the object only
      

      

  • Ammeters and voltmeters

    • Should be checked to ensure they read zero when there is no current or potential difference

Appreciate when and how to maintain constant environmental conditions of systems

• The conditions in the laboratory can affect your results

• Key environmental conditions to control include temperature, pressure, humidity, air currents (draughts), and light intensity

• Maintaining temperature

  • For many experiments, temperature is one of the most important variables to control, as it directly affects other quantities, such as density, gas pressure, and electrical resistance

• Controlling air currents

  • Draughts from windows or air conditioning can cool a substance, causing significant errors in calorimetry experiments

  • This can be controlled by closing windows or using a draught shield around the apparatus

• Sometimes, a variable cannot be perfectly controlled (e.g., ambient room temperature might fluctuate slightly)

  • In these cases, the best practice is to monitor and record it

  • You can then discuss its potential impact in your evaluation

Appreciate when and how to insulate against heat loss or gain

• In any thermal physics experiment (e.g., measuring specific heat capacity), the biggest source of error is unwanted heat exchange with the surroundings

• Insulating the system is crucial for collecting accurate temperature data

• Common insulation techniques include:

  • using a polystyrene cup instead of a glass beaker, as polystyrene is a much better thermal insulator

  • placing the polystyrene cup inside a larger glass beaker to provide stability and an insulating layer of air

  • using a lid (e.g., made of cardboard or plastic) with holes for the thermometer and stirrer. This minimises heat loss by evaporation and convection

Appreciate when and how to reduce friction

• In mechanics experiments, frictional forces reduce motion and introduce energy losses to heat and sound

• Friction can be reduced by

  • lubricating moving parts, such as pulleys and wheels

  • adding bearings to prevent components from directly rubbing together

  • using low-friction apparatus, such as an air track

Appreciate when and how to reduce electrical resistance

• In circuit experiments, all components, including connecting wires, have some resistance

• This results in unwanted heat transfers to the wires, components and the surroundings

• Resistance can be reduced by:

  • using shorter, thicker wires

  • keeping the current as low as possible 

  • ensuring connections are secure, as poor connections can add extra resistance

Appreciate when and how to take background radiation into account

• In experiments involving radioactive sources, the background radiation must be taken into account

• To adjust for background radiation using a Geiger-Muller (GM) tube:

  • record a background count rate with no radioactive source present

  • subtract the background count rate from all subsequent measurements to ensure you are only measuring the radiation from the source itself