Experimental Design (OCR A Level Physics)

• Planning is an essential part of experimental physics
• Preliminary research can be very helpful when trying to design an experiment to investigate a particular theory or hypothesis
  • Preliminary means "to come before"

• Researching other similar studies or experiments can help with:
  • Posing a hypothesis and confirming the aims of the experiment
  • Choosing the appropriate apparatus
  • Using the correct techniques
  • Identifying variables
  • Controlling other variables
  • Recording and collecting data accurately
  • Processing and presenting data in a useful way
  • Identifying health and safety issues

• The choice of apparatus and techniques should be based on the science surrounding the issue being investigated
• For example, for measuring the instantaneous velocity of a trolley travelling down a ramp, it is crucial to have:
  • Light gates
  • A trolley
  • A ramp
  • A data logger
  • A metre ruler

• Once the preliminary research has been completed then preliminary studies can be conducted to further aid the experimental design
• These studies are very important for:
  • Identifying additional variables that affect the experiment
  • Finding the best way to control these variables
  • For example, when using Charles’ law to determine absolute zero, pressure must be kept constant

• Any experiment conducted without preliminary research or studies is likely to be invalid as the other variables that affect the results in the experiment will not have been identified and controlled

• Evaluating experimental methods is an important skill for a scientist and is appropriate to meet the expected outcomes of the experiment
• A good way to evaluate an experimental design is by repeating the experiment (using the instructions provided) and determining the reproducibility of the experiment ie. whether or not similar results can be achieved
  • This process is known as peer review

• When analysing and criticising the design of an experiment there are several key things to look out for:
  • Limitations
  • Accuracy
  • Precision

Limitations

• A limitation is any design flaw or fault that affects the accuracy of the experiment
• It is crucial that any limitations within an experiment are identified and removed/corrected in order to make the results and findings valid

Accuracy

• Accuracy - how close a reading/measurement is to its true value
  • Accuracy is affected by the presence of systematic errors

• Experiments should make use of the appropriate methods and equipment to ensure high levels of accuracy when making measurements
• When using measuring equipment, such as when measuring length, equipment with an appropriate measuring scale needs to be used
  • For example, accurately measuring a 5 mm object using a metre ruler would be very difficult
  • A micrometer or vernier callipers would be a much better choice as the measuring scale is of a higher resolution

Precision

• Precision - how similar repeat readings/measurements are to each other
  • Precision is affected by the presence of random errors

• Experiments are often repeated to ensure the reliability of results
• If the apparatus and measuring equipment in an experiment are not used correctly for each repeat, then there is likely to be a wide range of results
• Individuals conducting the experiment must use the same apparatus in the same way for each repeat of the experiment
• Readings that are tightly clustered together (a small range) are described as precise

The difference between accuracy and precision explained by using a dartboard as a metaphor

• Ideally, the design of an experiment should be evaluated at the preliminary stage, this way any corrections or adjustments can be made prior to conducting the actual experiment