Practise analysing charging-by-friction investigations, choosing variables, spotting anomalous data and calculating valid means from results tables.
A student uses this apparatus to investigate static electricity.
The positively charged ball is suspended vertically on a long length of string.
The student extends the investigation by measuring the reading on the scale for rods made from different materials.
As a control variable, the student charges each rod by rubbing it with a cloth the same number of times.
Give two other control variables for the student's investigation.
1 ............
2 ............