Different substances hold onto their electrons with different degrees of attraction. By rubbing two different substances off each other, the substance with a weaker attraction will lose some of its electrons to the other substance. Rubbing polythene or Perspex rods with a cotton cloth causes the polythene rod to gain electrons from the cloth, whereas the Perspex loses electrons to it.
Polythene: Negative, Perspex: Positive
Memory Aid: Remembering the made up words Polythenegative and Perspexcellent may help remind you of the type of charge on each plastic.
DEMONSTRATION 15.1 Demonstration of Forces between Charges
Repulsion
Polythene
Sling String
Repulsion
Polythene (a) Perspex
Sling String
Attraction
Perspex (b) Fig 15.2
Apparatus Perspex and polythene rods, retort and clamp, string and sling
Procedure Set up apparatus with charged polythene rod in sling as illustrated in Fig 15.2(a). Bring a charged polythene rod close to the rod in the sling and note what happens to it. Repeat the process, bringing a charged Perspex rod close to the charged polythene rod in the sling as illustrated in Fig 15.2(b).
Repeat the process with a charged Perspex rod in the sling, bringing charged polythene and Perspex rods close to it as illustrated in Fig 15.2(c).
Observation When two polythene or two Perspex rods are brought close together, they repel; when polythene is brought close to Perspex, they attract.
Conclusion Like charges repel. Unlike charges attract.
