Static electricity and electric fieldsIB MYP Physics: Revision notes

All matter contains atoms with positive protons in the nucleus and negative electrons around it. Normally an object is neutral, because the positive and negative charges balance.

When two different insulating materials are rubbed together, friction pulls some electrons off one material and onto the other. Only electrons move, because protons are fixed in the nucleus.

• The material that gains electrons becomes negatively charged.
• The material that loses electrons becomes positively charged.

For example, rubbing a polythene rod with a woollen cloth transfers electrons from the cloth to the rod. The rod is negative and the cloth is positive. The two objects always end up with equal and opposite charges.

There are only two types of electric charge: positive and negative.

• Like charges repel (positive and positive, or negative and negative).
• Unlike charges attract (positive and negative).

This is why a charged rod can pick up small scraps of paper. The charged rod makes the nearby side of the paper the opposite charge, so the paper is attracted. The force gets stronger as the objects get closer.

In a conductor (such as a metal) some electrons are free to move, so charge can flow through it. In an insulator (such as polythene, glass or rubber) the electrons cannot move freely, so charge stays where it is. This is why insulators can be charged by friction and keep their charge, while a metal held in the hand loses its charge at once.

Earthing means connecting a charged object to the ground with a conductor. Electrons flow to or from the Earth, which is so large that it can supply or take any amount of charge, and the object becomes neutral. Earthing is used to stop dangerous charge building up.

A gold-leaf electroscope has a metal cap, a metal stem and a thin gold leaf, held in an insulated case. It detects whether an object is charged.

When a charged rod touches or is brought near the cap, charge spreads down to the stem and leaf. The stem and leaf have the same charge, so they repel and the leaf rises. The more charge, the bigger the angle.

To find the type of charge, charge the electroscope with a known sign. Then bring the unknown object near the cap. If the leaf rises further, the charges are the same sign. If it falls, they are opposite.

An electric field is a region where a charged object feels a force. The stronger the field, the bigger the force.

We draw field lines with arrows to show the direction of the force on a positive charge.

• Around a positive point charge, lines point straight outwards.
• Around a negative point charge, lines point straight inwards.
• Between two oppositely charged parallel plates, the lines are straight, parallel and evenly spaced, pointing from the positive plate to the negative plate. The field is uniform.

Lines closer together show a stronger field.

Uses of static electricity

• Photocopiers: charged areas of a drum attract black powder (toner), which is transferred to paper.
• Paint spraying: charged droplets repel each other and spread out, then are attracted to an oppositely charged object, giving an even coat with less waste.
• Electrostatic precipitators: smoke particles are charged and attracted to plates, so chimneys release less dust.

Hazards

• Lightning is a huge spark caused by charge building up in clouds. Tall buildings use a metal lightning conductor to carry the charge safely to earth.
• Sparks when refuelling could ignite fuel vapour, so tankers and aircraft are earthed with metal cables.