Static Electricity

What is static electricity?

Static electricity is electric charge that builds up on the surface of an object and stays there ("static" means stationary). It is produced when two insulating materials are rubbed together, and you meet it every day: the crackle when you pull off a jumper, a balloon sticking to a wall, or a small shock when you touch a car door.

All matter is made of atoms, and atoms contain positively charged protons in the nucleus and negatively charged electrons moving around it. Normally an atom has equal numbers of each, so it is neutral. Static electricity is all about moving some of those electrons from one object to another.

Key terms Charge — a property of matter measured in coulombs (C); it can be positive or negative.

Electron — a tiny negatively charged particle; electrons are the only charges free to move when solids are rubbed.

Insulator — a material (e.g. plastic, rubber) that does not let charge flow through it, so charge stays put on its surface.

Charging by friction

When two different insulators are rubbed together, the friction transfers electrons from one surface to the other. The protons in the nuclei are locked in place and never move; only electrons are transferred.

The material that gains electrons becomes negatively charged (extra negative electrons).
The material that loses electrons becomes positively charged (it now has more protons than electrons).

A classic pair of examples used at IGCSE:

Notice that charge is never created or destroyed, only transferred. The two objects always end up with equal and opposite amounts of charge.

Watch out Never say protons move or that positive charge "flows" onto a rubbed insulator. A positive charge means electrons have been removed. Only electrons are transferred.

The law of charges

There are two kinds of charge, and they exert forces on each other without touching:

Like charges repel (two positives, or two negatives, push apart).
Unlike charges attract (a positive and a negative pull together).

You can show this by hanging a charged rod on a thread and bringing a second rod near it: rubbed in the same way they swing apart; rubbed oppositely they swing together.

The electric field

A charged object is surrounded by an electric field — a region where another charge feels a force. We draw the field as lines with arrows:

The arrows point in the direction of the force on a positive test charge.
So field lines point away from a positive charge and towards a negative charge.
Lines that are close together show a stronger field; the force gets weaker as you move further away.

This field idea explains "action at a distance": the rods do not touch, yet they still push or pull because each sits inside the other's field.

Exam tip When asked to explain attraction or repulsion, always mention electrons. For example: "The cloth lost electrons to the rod, so the rod is negative and the cloth is positive; unlike charges attract." Naming the electron transfer scores the marks.

Everyday effects and dangers of static

Charge can build to thousands of volts on an insulator. When the field becomes strong enough, electrons jump across the air gap as a spark, and this can be a nuisance or a serious hazard.

Sparks and shocks — walking on a nylon carpet charges you up; touching a metal door handle discharges you with a small spark.
Refuelling vehicles and aircraft — fuel rushing through a pipe rubs against it and builds up charge. A single spark could ignite the fuel vapour, so tankers are earthed (bonded) to the pump before fuel flows.
Lightning — ice and water droplets rub together inside storm clouds, separating charge until a huge spark jumps between cloud and ground.

Real world Refuelling a plane is one of the most important uses of earthing. A metal strap connects the aircraft and the fuel truck to the ground so charge flows safely away instead of building up. No charge difference means no spark, and no risk of igniting fuel vapour.

Useful applications of static

The same forces that cause sparks are put to work in many machines, all relying on charged particles being attracted to oppositely charged surfaces:

Earthing — removing charge

Earthing means connecting a charged object to the ground through a conductor (such as a metal wire). Electrons then flow between the object and the Earth until the object is neutral:

If the object is negatively charged, excess electrons flow to the Earth.
If the object is positively charged, electrons flow from the Earth onto the object.

Earthing is used as a safety measure wherever dangerous static could build up — fuel tankers, aircraft, and equipment handling flammable powders or vapours.

Worked example An acetate rod is rubbed with a cloth and becomes positively charged. Explain what happened and how to remove the charge.

The friction transferred electrons from the rod to the cloth. The rod lost electrons, so it has more protons than electrons and is positive; the cloth gained electrons and is negative.

To discharge the rod, connect it to earth with a conductor. Electrons flow from the Earth onto the rod until it has equal numbers of protons and electrons and is neutral again.