Topic 11 ~ Static Electricity

• Static electricity


  A property of all matter
• Positive and negative charges


  • If a body has the same amount of positive and negative charge, they cancel out, forming a neutral body (i.e. protons and electrons in a neutral atom)
• Like charges


  Repel
• Opposite charges


  Attract
• Insulators


  Do not conduct electricity, their electrons cannot flow throughout the material, they are fixed
• Conductors


  Can conduct electricity, their electrons can flow, and are not fixed (they are delocalised)
• Charging by friction


  1. Electrons are transferred from one object to the other

  2. Forming a positive charge on one object and a negative charge on the other
• If conductors were rubbed, electrons will flow in/out of them cancelling out any effect, so they stay neutral
• Insulators become charged because the electrons cannot flow
• A positive static charge forms on object which loses electrons
• A negative static charge forms on object which gains electrons
• Which object loses/gains electrons depends on the materials involved
• Sparking


  1. Charge builds up

  2. Objects are close but not touching

  3. Charge jumps through the air from the highly negative object to the highly positive object, to balance out the charges
• Lightning occurs when the charge difference between clouds and the Earth becomes so great, and a massive spark (lightning) jumps across to balance the charge
• Electrostatic force


  • Greater charge = greater force
  • Closer together = greater force (force is proportional to the inverse square of the distance)
  • It is a noncontact force, as force can be felt even when the objects are not touching
• Unlike charges


  Attract
• Attraction by induction


  • A positively charged balloon next to wall attracts electrons in the wall
  • Comb charged induces the opposite charge in small pieces of paper, so picks them up
• Earthing


  Allows electrons to flow to the earth, removing excess charge, allowing materials to stay neutral
• Applications of static electricity

  • Insecticide sprays are sprayed from aircraft, and given a charge so the spray droplets repel each other and are attracted to the earth
• Electric fields


  • The region where an electric charge experiences a force
  • They point in the direction a positive charge would go (away from positive charges, and towards negative charges)
  • They point to charges at right angles to the surface
  • Stronger the charge, the more field lines present and the stronger the force felt
  • Parallel plates have a uniform field