Static electricity

Question 1

What happens when two insulators rub together?

Answer 1

1) Electrons move from one surface to the other due to friction.
2) The object losing electrons becomes positively charged.
3) The object gaining electrons becomes negatively charged.

Charge can not flow away because insulators do not allow electrons to move freely.

Question 2

How do insulating materials become charged by induction?

Answer 2

A charged object brought near an insulator causes:
1) Electrons to shift slightly within the material.
2) One side becomes slightly positive, the other slightly negative.
3) No contact is needed.

Question 3

How can static electricity be useful?

Answer 3

1) Paint sprayers: nozzle of sprayer is connected to electricity supply. This means the droplets all get the same kind of static charge so they spread out evenly. The object being painted is given the opposite charge to the paint. The paint is attracted to the object being painted and less paint is wasted.
2) Photocopiers: static charge attracts toner to the paper.
3) Insecticide sprays: nozzle of sprayers is connected to electricity supply. This means the droplets all get the same kind of static charge. Droplets repel each other so the spray spreads out evenly.

Question 4

What are the dangers of static electricity?

Answer 4

1) Sparks can ignite fuel vapours.
2) Aircraft must be earthed during refuelling. This is because if the charge is not removed from an aeroplane or tanker, then a spark may occur when the nozzle of the fuel tanker touches the aeroplane. The spark is an uncontrolled discharge of electrons, and it can ignite fuel vapours and cause an explosion. Earthing is used to ensure the safe movement of electrons off the surface to avoid a dangerous build-up of charge. An earthing or bonding wire is connected between the aeroplane or fuel tanker and ground before refuelling starts.
3) Lightning is a large-scale static recharge.

Question 5

What is the electrical field like around a point of charge?

Answer 5

1) Radial field lines.
2) Direction shows force on a positive test charge.

Question 6

What is the electrical field like around parallel plates?

Answer 6

1) Uniform field.
2) Straight, evenly spaced lines.
3) Stronger field if plates are closer or voltage is higher.