Topic 11: Static Electricity

• How an Object becomes Charged:

  • two insulators (cloth/balloon) are neutral at first - equal amount of protons and electrons
  • when the insulators are rubbed together, the friction transfers electrons as they are attracted to the protons - friction gives energy for them to move/jump across
  • cloth becomes positively charged - more positive than negative
  • balloon becomes negatively charged - more negative than positive
• Induction of Insulators:

  A negative balloon can stick to a wall because the negative charge repels the electrons in the wall.

  This leaves the wall surface positively charged as the protons are attracted to the negatively charged balloon.

  The separated charges in the wall are called induced charged and the forces of attraction are stronger than gravity
• Frame Painting:

  • paint spray nozzle with positively charged paint
  • negatively charged frame
  • droplets of paint repel each other but are attracted to the frame
  • paint cloud is formed around the frame in all directions
  • less paint is needed and even coating is created

  This is similar to insecticide/pesticide as sprayers work by electrostatic induction (spray is charged, plant is neutral)
• Electrostatic Precipitator in Chimneys:

  • wires at centre are positive to repel positive smoke particles into earthed metal plates
  • smoke particles turn positively charged by a grid or air ionised by centre wires
  • mechanical hammer knocks off smoke into a bin to be made into house bricks
• Friction between 2 insulators causes a build up of charge.

  Eventually, there would be enough charge that can jump across a gap - this is a spark.

  The spark can ignite a fuel/vapour and cause a fire
• Earthing removes excess charge from an object to prevent a build up of charge so no spark will occur.

  To earth an object, a conducting wire such as copper is used:

  • electrons in a negatively charged object will be transferred into the ground
  • electrons in the ground will be transferred into a positively charged object
• As fuel passes through a fuel pipe, friction between the fuel and pipe will cause a build up of charge.

  Before fuelling, the tanker and plane are connected together with a conducting copper wire to prevent a build up of charge (earthed)
• Lightning Conductor:

  • negative charged cloud induces a positive charge at conductor tip
  • positive protons are repelled up and negative electrons are attracted down
  • electric current flows through the wire and travels down to earth (earthed)
  • repelling positive ions neutralises lightning flashes
• Field lines always go from positive to negative and are at right angles to the charge.

  The field is strongest close to the surface and gets weaker the further away
• Electric fields ionise nearby air so electrons can travel through the ionised air which causes a spark.

  Ionisation - able to conduct electricity
• Field lines are equally spaced and parallel to each other. Between two flat parallel metal plates, the shape field lines take is called a uniform shield