- Collecting experimental data is only some of the work required when carrying out a practical
- When setting up a data table, space must always be left for some calculations
- The most common calculation is the mean for repeat readings
- The value of the mean is then used in for further calculations
- Anomalous readings should be ignored in this calculation
- The mean should have the same number of significant figures as the readings used to calculate it
- The experiment may require to calculate a variable which you can't directly measure
- E.g., the area of a wire
- In this case, other measurements are taken, which then by using an equation, the variable that is required can then be calculated
- E.g., area of a wire = πr2, so r is measured for the wire using a micrometer and substituted into this equation to calculate the corresponding area for each value of r
- Another example of this is finding the 'log' of a value
- One column in a data table should be for the measurement
- A column next to it should be the for the 'log' of that measurement
- In the Hooke's law experiment, the 'extension' cannot be measured direction, but can be calculated from the final and initial length, which can be measured
Hooke's Law Table of Results
A student wants to find the resistivity of a constantan wire. They set up the experiment by attaching one end of the wire to a circuit with a 6.0 V battery and the other with a flying lead and measuring the length with a ruler. Attaching the flying lead onto the wire at different lengths, they obtain the following table of results.
Calculate the the missing values from the table.
- The average current is calculated by
- The resistance is calculated using the equation
- All readings are to 3 significant figures, so all values calculated should also be to 3 s.f.
These calculations show why it's important not to draw your data tables too big, without leaving space for more columns and rows. Think carefully about what data you need to measure, but also what you may need to calculate in order to draw graphs in the future. Thinking ahead this way will reduce the change of drawing messy tables that you'll have to keep redoing in the exam!