Topic 11 - static electricity

what is static electricty, charges

• a property of all matter

• positive and negative charges exist

  • if a body has the same amount of positive and negative charge, they cancel out, forming a neutral body (ie. protons and electrons in a neutral atom)

• like charges repel

• opposite charges attract

conducting electricity

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)

what is static electricity

• when two insulators are rubbed together it can be charged by friction

  • electrons are transferred from one object to the other

  • 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 occurs when enough charge builds up, and the objects are close but not touching

• the “spark” is when the 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

forces exerted

• the charged objects experiences a force - electrostatic force (of attraction/repulsion)

• greater charge = greater force (eg. a more positive object, a more negative object)

• 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

like charges repel, unlike charges attract

• a positively charged balloon next to a wall attracts electrons in the wall

  • this induction causes the balloon to stick to the wall

• comb charged induces the opposite charge in small pieces of paper, so picks them up

earthing

this allows electrons to flow to the earth, removing excess charge

• this allows materials to stay neutral

application of this

• insecticide sprays are sprayed from aircraft, and given a charge

• this means the spray droplets repel each other

• so the droplets spread evenly, and are attracted to the earth

  • if not charged, there is a risk that some droplets will blow away, or spray will fall unevenly

dangers of sparks

• if charge builds up and a spark forms when fuelling cars, it could ignite and cause a massive explosion

  • as fuel passes through a hose to the vehicle, a static charge can build up

  • when it is too large a spark might form

  • a resulting spark might ignite the fuel

  • the hoses are earthed to stop this occuring

electric fields

• like magnetic fields for magnets, electric fields are for charges

  • an electric field is the region where an electric charge experiences a force

  • they point in the direction a positive charge would go

    • ie. 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