Static electricity · Grade 9
Static electricity is charge that builds up on an insulator, usually by friction. When two materials rub, electrons move from one to the other: the one that gains electrons becomes negative, the one that loses them becomes positive. Like charges repel and unlike charges attract – this explains sparks, clingy clothes, and useful machines like paint sprayers and photocopiers.
Key facts
- Only electrons move when objects are charged by rubbing.
- Gaining electrons → negative charge; losing electrons → positive charge.
- Like charges repel; unlike charges attract. A charged object also attracts small uncharged ones (by induction).
- Charge on N electrons: Q = N × e, with e = 1.6 × 10⁻¹⁹ C.
- Charge is conserved: identical touching spheres share the total charge equally.
- Electric field lines point away from positive charges and towards negative charges.
Worked examples
Example 1
A comb gains 2.5 × 10¹¹ electrons. What is its charge in nC? (e = 1.6 × 10⁻¹⁹ C)
Answer: −40 nC
Example 2
Identical metal spheres carry +9 nC and −3 nC. They touch and are separated. Find the final charge on each and how many electrons moved. (e = 1.6 × 10⁻¹⁹ C)
Answer: +3 nC each; 3.75 × 10¹⁰ electrons