Electrostatics

• Explain how an insulator can be charged by friction, through the transfer of electrons

  • When insulating materials rub together, negatively charged electrons are transferred from one material to another.
  • The electrons, as the materials are insulators, are not free to move - so the build-up of charge is static electricity.
• The material gaining electrons becomes negatively charged and the material losing electrons is left with an equal positive charge
• Like charges repel and unlike charges attract

  These forces get weaker the further apart they are
• Explain common electrostatic phenomena in terms of

  movement of electrons, including

  a) shocks from everyday objects

  • As electric charge builds up on an object, the potential difference between the object and the earth increases
  • If the potential difference gets large enough, electrons can jump across the gap between the charged object and the earth, causing a spark
  • They can also jump to any earthed conductor nearby
• Explain common electrostatic phenomena in terms of

  movement of electrons, including

  b) lightning

  • In a storm, clouds move over each other, electrons are transferred between them and cause them to become charged
  • The ground is neutral, so the negative charges in the clouds jump to meet the positive charges on the ground, creating a large spark
• Explain common electrostatic phenomena in terms of

  movement of electrons, including

  c) attraction by induction such as a charged balloon attracted

  to a wall

  • Rubbing a balloon against a jumper will transfer electrons to the balloon, making it negatively charged
  • The balloon will stick to a wall that has a neutral charge
  • This is because the negative charges on the balloon repel the negative charges on the surface of the wall
  • This leaves a positive charge on the surface of the wall, which attracts the negatively charged balloon
• Explain common electrostatic phenomena in terms of

  movement of electrons, including

  c) attraction by induction such as a charged comb picking up small pieces of paper

  • When you run a comb through your hair, electrons will be transferred to the comb, making it negatively charged
  • The negative charges of the comb repel the negative charges on the surface of the pieces of paper, making them positively charged
  • The pieces of paper are now attracted to the comb and will stick on it
• What is attraction by induction?

  The negative charges on the surface of a neutrally charged object will be repelled by a negatively charged object

  This leaves the surface of the neutral object to be positively charged

  The negatively charged object will attract to the positively charged object
• Explain how earthing removes excess charge by movement of electrons

  • Earthing provides an easy route for static charges to travel into the ground
  • This means no charge can build up to give a shock or spark
  • Electrons flow down the conductor if the charge is negative (repel and will flow up the conductor if the charge is positive (attract)
  • This is to cancel the charge
• Explain how paint sprayers use electrostatic charges

  • Paint droplets are charged as they pass through plates
  • Droplets become positively charged, removing the electrons off the droplets
  • The droplets repel to get an even, fine spray
  • The droplets stick to the negatively charged object
• Explain how insecticide sprayers use electrostatic charges

  • The nozzle of the spray gun is negatively charged
  • Electrons are put onto droplets as they pass through the nozzle
  • The droplets become negatively charged
  • The droplets repel and cover a large area
  • The droplets induce a positive charge on the surfaces of leaves by repulsion
  • Droplets attracted and stick to the surface of the leaf
• Describe some of the dangers of sparking in everyday situations, including fuelling cars, and explain the use of earthing to prevent dangerous build-up of charge

  • Friction between fuel and hose causes them to be charged (hose pulls electrons from fuel), which could lead to a spark and an explosion
  • A bonding wire allows electrons to flow back into fuel as it is a good conductor of electrons
  • Fuel and hose become discharged
• Define an electric field

  the region where an electric charge experiences a force
• Describe the shape and direction of the electric field around a point charge.

  In a negative point charge, the field lines are pointing inwards.

  In a positive point charge, the field lines are pointing outwards.

  The field is strongest where the field lines are close together.
• Describe the shape and direction of the electric field between two parallel plates.

  The electric field between two parallel plates is uniform

  The strength and direction of the field is the same in all places between the two points.
• Explain how the concept of an electric field helps to explain the phenomena of static electricity

  Charge builds up on a surface and electric field strength increases with more rubbing (friction)

  Forces of attraction increases with increasing overlap of electric fields

  Force eventually strong enough to pull electrons off the surface - causing a spark