Static Electricity

Aim

• Show that rubbing two insulators together transfers electrons between them, leaving one with a net negative charge and the other with an equal net positive charge
• Identify which rods, when rubbed with a cloth, end up with the same sign of charge and which end up oppositely charged

Variables

• Independent variable: the material of the rod being tested (polythene, acetate, perspex, glass, ebonite, etc.)
• Dependent variable: whether the test rod attracts or repels a known, similarly charged reference rod, when both have been rubbed with the same kind of cloth
• Control variables:
  • the type of cloth used (e.g. duster, silk, or woollen pad; pick one and keep it)
  • the length of time spent rubbing each rod
  • the length of each rod
  • the room kept still and free from draughts

Apparatus

Method

1. Grip a polythene rod in the middle so your fingers cannot leak charge from the ends, and stroke both ends firmly with the chosen cloth. Polythene gains electrons from the cloth, so it ends up negatively charged
2. Hang the rod horizontally inside the nylon cradle from the wooden stand. Do not touch the charged ends, because your skin would let charge leak away
3. Take one of the test rods and rub it with a fresh cloth in the same way
4. Without making physical contact, bring one charged end of the test rod close to one end of the suspended polythene rod
5. Observe whether the polythene rod rotates towards the test rod (attraction) or rotates away from it (repulsion)
6. Record the observation, then repeat steps 3–5 with the other test rods

Analysis

• Charging is by electron transfer. Atoms cannot lose their protons by rubbing, so when an insulator becomes charged it must be because electrons have moved from one surface to the other:
  • The material that gains electrons becomes negatively charged
  • The material that loses electrons becomes positively charged
  • The total charge before and after rubbing is the same (charge is conserved), and the two surfaces simply end up with equal and opposite amounts of it
• Reading the results:
  • If the suspended rod repels the test rod, the two rods carry the same sign of charge
  • If the suspended rod attracts the test rod, the two rods carry opposite signs of charge
• Typical school-lab outcomes:
  • Polythene rubbed with a duster → gains electrons, charges negative
  • Acetate (or perspex) rubbed with a duster → loses electrons, charges positive
  • Two polythene rods rubbed the same way repel each other; a polythene rod and a freshly rubbed acetate rod attract each other

Sources of error and improvements

• Random errors:
  • Draughts in the room can swing the suspended rod by themselves; close doors and windows and let the rod settle before each test
  • If the rotation is hard to see, rub the rod for longer to deposit more charge and produce a clearer deflection
• Improvement, making the experiment quantitative. Instead of just reading attract/repel, count the number of small paper discs each freshly rubbed rod can pick up from a tray; a graph or bar chart of the results then shows which rod has gained the most charge