Static Electricity

• Static electricity


  Charges which are not free to move, e.g. in insulating materials
• Build-up of static is caused by friction

  1. Negatively charged electrons are scraped off one material and dumped on the other

  2. This leaves the materials electrically charged, with a positive static charge on one and an equal negative static charge on the other

  3. The direction of electron transfer depends on the two materials involved
• Classic examples


  • Polythene and acetate rods being rubbed with a cloth duster
• Only electrons move, never positive charges
• A positive static charge is always caused by electrons moving away elsewhere
• As electric charge builds on an object

  The potential difference between the object and the earth increases
• Sparks occur when the potential difference gets large enough

  1. Electrons can jump across the gap between the charged object and the earth

  2. They can also jump to any earthed conductor that is nearby
• Lightning is just a really big spark
• Electrostatic forces


  • Like charges repel
  • Opposite charges attract
  • The forces get weaker the further apart the two things are
• Observing electrostatic attraction/repulsion


  1. Suspend a rod with a known charge from a piece of string

  2. Place an object with the same charge nearby to see repulsion

  3. Place an object with the opposite charge nearby to see attraction
• Electrostatic forces are non-contact forces (the objects don't need to touch)