The method and the results are on the static electricity lab. Do the analysis yourself first, then check the answers below.
Quick answer: Static electricity is electric charge that builds up on insulators when electrons are transferred by rubbing. These answers go with the Physics 9-10 lab on static electricity: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
The model answers use the sample table. Your own counts will vary with humidity, the cloth and how evenly you rub, but you should still see more bits lifted at first and then a levelling off.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P2, HS-PS2-4
Plot (10, 2.3), (20, 5.0), (30, 8.3), (40, 11.0), (50, 12.3). The curve rises steeply at first and then flattens near 12 bits.
Question 2 Describe the trend
The number of bits rises from 2.3 at 10 rubs to 12.3 at 50 rubs. It is roughly proportional at first, so more rubbing gives more charge, but it increases more slowly at the end.
Question 3 Why it levels off
As charge builds up it leaks into the air, and more charge on the strip repels further charge. Once the gain and the loss balance, rubbing no longer adds charge, so the strip reaches a maximum.
Question 4 Control variables
Same strip and cloth, same rubbing speed and pressure, same height of 2 cm and same time over the paper. Changing any of these would alter the charge or the force and spoil the comparison.
Question 5 Humidity
Damp air contains water that conducts, so charge leaks off the strip quickly. Fewer bits would be lifted; the result would be lower for every number of rubs.
Question 6 Reliability
The three repeats differ by at most 2 bits at any number of rubs, which is a spread of about 8-40 % at low counts. Averaging three trials gives a fairly reliable mean, but more repeats would help.
Question 7 Improvement
Use a gold-leaf electroscope or a charge sensor, or lift bits of foil of a known mass, so that the measure is continuous instead of a count.
