How to answer ‘design an experiment’ questions in physics A level exams.

Andrew LawsonUncategorizedLeave a Comment

Design A Physics Experiment Questions:

Questions that ask you to design an experiment to measure, calculate or prove something come up quite regularly in physics A level papers. They sometimes focus on one part of the experiment, others ask you to write a brief summary of the whole experiment.

There is a method for doing these questions. What I will explain here is how to design the whole experiment, from start to finish. Then all you need to do is make sure you read the question very very carefully. Only write about the parts of the experiment the question specifically asks about.

Every experiment, from design, to performing it, to processing the data, through to analysing the results can be broken down into the following stages.

1) Think about the relevant theory and identify an equation.
2) Identify what in the equation you are going to calculate.
3) Identify any known constants in the equation, such as the speed of light in a vacuum, c has the value 3.0×108 ms-1.
4) Identify what in the equation will be held constant throughout the experiment. All of these will need to be measured. For each one, figure out a way to accurately measure them.
5) What is left in the equation will be what you are trying to calculate, and the dependent and independent variable.
6) The independent variable will be manually altered, and the value measured, and noted down.
7) The value of the dependent variable will then be measured, and noted down.
8) Repeat 7) and 8) for as many data points as needed.
9) Repeat readings/the experiment if possible.
10) Consider safety and accuracy for all measurements/readings.
11) Work out how to process your data to calculate the unknown you are trying to find a value for. Typically this will involve producing a straight line graph, and measuring the gradient and/or y-intercept.
12) Process your data in a table.
13) Plot the graph.
14) Calculate the gradient/y-intercept, and process as needed to find the value you are after.
15) Calculate the rough error in the reading.

The question wording will tell you which bits of this method to focus on. The number of marks give you an indication as to the detail you need to go into. Typically 1 mark means at least 1 point made in your write up.

Here is an example question from OCR A physics paper 3, from the sample question paper sent out in 2015, when the new specification you are all sitting was first introduced. It was worth 6 marks.

Note how the question gives you the equation to use, and explains to you the maximum EMF is directly proportional to the frequency of the rotation of the magnet.
Directly proportional means EMF=constant x rotation frequency.
So we will have to set the rotational frequency of the magnet (independent variable), and measure the maximum EMF (dependent variable). If you forget which is which, say a sentence like this – ‘The maximum EMF depends on the frequency I set’. The maximum EMF depends…..the maximum EMF is the dependent variable.
Remember y=mx + c, the equation of a straight line.
As it’s EMF = c x freq, a graph of EMF (y-axis) against freq (x-axis) should be a straight line and through the origin to be directly proportional. So that’s what we have to show.

So the experiment is to vary and measure the rotation speed.
Say how you do this:
– alter the motor speed control.
– measure the period of oscillation from the EMF signal on the oscilloscope.
o move the wave on the oscilloscope screen so a peak lies right next to the gradations for accuracy (gradations are the markings on the screen you use to measure distances).
o peak to peak horizontal distance on the wave on the oscilloscope screen, multiplied by the s/cm (seconds per cm) gives you the period of the rotation.
Then measure the maximum EMF.
– Ensure the trough lies along a line on the oscilloscope screen.
– Measure the distance from the trough to a peak, and convert using V/cm scale setting.
Repeat for a number of rotation speeds.
Repeating individual readings is difficult, as getting exactly the same rotation speed will be impossible, and unnecessary. The best thing to do is to gather as many points as needed, at least 6.

Process the data:
– Convert the period to a frequency f=1/T.
– Plot a graph of V against f.

– A straight line graph through the origin is required to validate Faraday’s law.

Here’s the mark scheme…….

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