electrostatics

When two insulators move relative to each other…

Causes friction between the two → results in both objects becoming electrically charged

Can conductors become charged through friction?

Yes, but will only retain charge if they are insulated from their surroundings. If not insulated, then any charge that builds up will leak away

Charging by induction

If a balloon is rubbed with a cloth so that via friction electrons are transferred to the balloon, giving the balloon an overall negative charge, when you bring this balloon close to a neutral object, charging by induction occurs.

The excess electrons in the balloon repel the electrons in the neutral object, so the surface of the neutral object closest to the balloon is positive.

How to permanently charge an object initially charged by induction

If you earth one side of the charge object with excess of electrons, that accumulated negative charge can leak away, so the object is now permanently positively charged.

If two insulators rub together to create friction, which way do the electrons go?

The electrons move towards the material with atoms that attract electrons more strongly.

Can objects charged by friction get rid of excess charge by earthing?

Only if the potential difference between the earth and object is large enough

Repulsion and attraction

Two charged objects placed near each other exert an electrostatic force on each other.

The larger the charges → larger the force

The larger the distance of separation → the smaller the force

Sparking

When two charged objects have air between them, sparks can be discharged by a spark between them. This happens when either the charge becomes large enough or when the distance between objects becomes small enough

Sparking risks and mitigation

Risk of fires is possible. E.g. risk of explosion when aircraft refuels

Risk can be eliminated by earthing. If two objects, that would otherwise cause each other to be charged by friction, are connected by a earthing wire, then electrostatic charging cannot take place

Earthing

When an object is earthed, electrons can flow to and from the earth to balance out negative charges (this is when electrons will flow to the earth) and positive charges (this is when electrons will flow from the earth). Only electrons flow