61. Static Electricity

1. Friction and Electron Transfer

• The Process: When two materials are rubbed together, friction causes electrons to be rubbed off one material and onto the other.

• Insulators vs. Conductors:

  • In Conductors (like metals): Electrons can move easily, so they simply flow back to where they came from, preventing charge buildup.

  • In Insulators (like plastic or cloth): Electrons cannot flow back. This creates a static charge.

• Charge Type: * The material that gains electrons becomes negatively charged.

  • The material that loses electrons becomes positively charged.

• Crucial Rule: It is only the negative electrons that move. Positive charges are the result of losing electrons, not gaining protons.

2. Potential Difference and Sparks

As an object continues to gain or lose electrons, the size of its charge increases.

• Potential Difference: A potential difference (voltage) develops between the charged object and the Earth (or any earthed object, like a person), which is at 0V.

• Sparks: If the potential difference becomes large enough, electrons can "jump" across the gap to reach the earthed object. This jump is seen and felt as a spark.

• Example: You might get a small shock when touching a car. This is because the car's metal frame has built up charge from friction with the wind, and it discharges through your body to the ground.

3. Specific Material Examples

• Polyethene Rod and Cloth: Rubbing these together makes the rod negative and the cloth positive.

• Acetate Rod and Cloth: Rubbing these together makes the rod positive and the cloth negative.

4. Summary Table