IBPhysicsDPHLRevision NotesInquiry ProcessInquiry 1: Exploring and Designing in PhysicsDesigning in Physics

Designing in Physics (DP IB Physics): Revision Note

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

DP Physics: HLDP Physics: SL

Designing in Physics

  • This is where you turn your well-explored research question into a practical, step-by-step plan or methodology

  • Your goal is to design a valid procedure that allows you to collect sufficient, high-quality data to answer your research question properly

  • The report of the design must be explained clearly, considering variables, measurements, and all safety aspects

Designing your methodology

Identify and justify the choice of variables

  • At the start of your design, you must clearly list and explain your choice of variables

    • Independent variable (IV): The single variable you deliberately change to see its effect

    • Dependent variable (DV): The variable you measure to see how it is affected by the change in the IV

    • Controlled variables (CVs): All other factors that could plausibly affect the outcome. You must explain how you will keep these constant to ensure a fair test

Justify the range and quantity of measurements

  • It is not enough to state your measurements; you must justify them

  • Range of the independent variable:

    • You should plan to collect data across a suitable range

      • A minimum of five different values for the independent variable is recommended to establish a clear trend

  • Justify your choice

    • For a pendulum experiment, you might state: "A range of lengths from 0.20 m to 1.00 m was chosen because lengths shorter than this result in a period that is too rapid to time accurately, while lengths longer than 1.00 m are impractical in a standard laboratory."

  • Quantity of measurements:

    • You must repeat the experiment for each value of the independent variable to ensure the results are reliable

      • A minimum of three trials is recommended

    • Repeating trials allows you to calculate the mean average, which:

      • reduces the effect of random error

      • helps you to identify and discard any anomalous results

Design and explain a valid methodology

  • This is the detailed, step-by-step procedure of your experiment

  • It must be a logical sequence of instructions that is clear enough for another physicist to follow and replicate your experiment exactly

  • Include precise details of the apparatus used.

    • For example, "Measure the length using a 1.0 m ruler with 1 mm divisions", not "use a ruler"

  • Creativity in design can be shown in how you solve a measurement problem

    • For example, using video analysis software to plot the position of a falling object is more creative and reliable than using a stopwatch and the naked eye

  • When writing your report, a good way to structure your methodology is:

    1. Materials and apparatus:

      • A list of all equipment used and, if appropriate, a clear labelled diagram of the experimental set-up

    2. Safety, ethical, and environmental:

      • A brief risk assessment identifying key hazards and outlining specific precautions

    3. Procedure:

      • The numbered, step-by-step instructions

Develop investigations using different approaches

  • While hands-on laboratory work is most common, your investigation could be based on other sources of data:

    • Databases

      • Using an established scientific database (e.g. from NASA or CERN) to find data for a physical property (like star luminosity) and then processing and analysing it to find a trend

    • Simulations

      • Using a simulation (e.g. from PhET) to collect data for a process that is too difficult or dangerous to conduct in a school lab, such as investigating particle collisions or gravitational fields

Pilot methodologies

  • A pilot study is a small-scale trial run of your experiment

  • This is extremely useful to check that your planned methodology works

  • For example, a quick pilot test of your circuit for the resistance experiment will ensure the ammeter and voltmeter are connected correctly and that the chosen power supply provides a suitable range of current and voltage

Worked Example

Designing an oscillation investigation

Research question:

  • "What is the relationship between the length of a simple pendulum and its period of oscillation?"

Identifying and justifying variables:

  • Independent variable:

    • The length L of the pendulum, measured in metres (m), from the point of suspension to the centre of mass of the bob

  • Dependent variable:

    • The period T of oscillation, measured in seconds (s), as the time taken for one complete swing

  • Control variables:

    • The mass of the pendulum bob must remain constant

    • The amplitude of the swing must be kept small (less than 10°) and constant to satisfy the small-angle approximation

    • Air resistance should be minimised by conducting the experiment away from draughts

Justifying the methodology:

  • To measure the period accurately, the time for 20 complete oscillations will be measured using a stopwatch and then divided by 20

    • This reduces the significance of human reaction time error in starting and stopping the watch, leading to a more precise value for the period

  • A fiducial marker (e.g., a pin held in a clamp) will be placed at the equilibrium position of the pendulum

    • The timing will start and stop when the centre of the bob passes this marker, ensuring a consistent point of measurement for each swing

Worked Example

Designing an electrical resistance investigation

Research question:

  • "What is the relationship between the length of a constantan wire and its electrical resistance?"

Identifying and justifying variables:

  • Independent variable:

    • The length L of the wire, measured in metres (m), between the voltmeter probes

  • Dependent variable:

    • The resistance R of the wire, calculated using R space equals space V over I from voltmeter and ammeter readings

  • Control variables:

    • The cross-sectional area of the wire must be constant (use the same piece of wire throughout)

    • The temperature of the wire must be kept constant by using a low current to avoid significant heating

    • The material of the wire must be the same (constant resistivity)

Justifying the methodology:

  • A range of lengths from 0.10 m to 0.80 m will be investigated to provide a wide spread of data.

    • A micrometer screw gauge will be used to measure the diameter of the wire at several points to ensure it is uniform and to calculate the cross-sectional area accurately

  • For each length, the power supply will be switched on only for a short time while the readings are taken.

    • This prevents the wire from heating up, which would increase its resistance and introduce a systematic error

Worked Example

Designing a specific heat capacity investigation

Research question:

  • "How can the specific heat capacity of a metal block be determined experimentally?"

Identifying and justifying variables:

  • Independent variable:

    • There is no independent variable in this determination experiment; the goal is to find a single value for a property of a material

  • Dependent variable:

    • The key measurements are the mass of the metal block (kg), the mass of water (kg), the initial temperature of the water (°C), the initial temperature of the heated block (°C), and the final equilibrium temperature of the water and block (°C)

  • Control variables:

    • Heat loss to the surroundings is the most critical variable that needs to be controlled

    • The water used should be from the same source

    • The same thermometer and balance must be used for all measurements

Justifying the methodology:

  • The experiment will be conducted using the method of mixtures

    • This involves heating the metal block in a beaker of boiling water for a sufficient time (e.g., 5 minutes) to ensure it reaches a uniform and measurable initial temperature

  • To minimise the primary source of systematic error (heat loss to the surroundings), the cool water will be placed in an insulated polystyrene cup

    • A lid will be used, and the block will be transferred from the boiling water to the calorimeter as quickly and safely as possible to minimise cooling during the transfer

  • The final temperature will be recorded as the maximum temperature reached by the water after the block has been added and the mixture has been gently stirred

    • This is because the system will begin to cool back to room temperature as soon as thermal equilibrium is reached

Examiner Tips and Tricks

  • Detail is key:

    • A vague method cannot be replicated and will not score well.

    • Instead of "Measure the length," write:

      • Measure the length of the pendulum from the bottom of the clamp to the centre of the bob using a metre ruler (±0.001 m)

  • Safety is not an afterthought:

    • Your design must include a dedicated safety section.

    • Be specific.

    • Instead of "Be careful," write:

      • A crash mat should be placed under the pendulum bob in case it detaches from the string during oscillation.

      • Ensure the retort stand has a heavy base to prevent it from toppling.

  • Justify your choices:

    • For top marks, you must explain why you chose a specific piece of apparatus or a certain range of values.

    • This shows the assessor you are thinking like a physicist.

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    Author
    Reviewer
    Katie M

    Author: Katie M

    Expertise: Physics Content Creator

    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.

    Caroline Carroll

    Reviewer: Caroline Carroll

    Expertise: Physics & Chemistry Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.