Static electricity

Electric charge

Electric charge is a basic property of matter. There are two kinds of electric charge: positive and negative. Objects with the same kind of charge repel each other, while objects with opposite charges attract. This simple rule explains many electrical effects.

Atoms contain negatively charged electrons and positively charged protons. Normally, an object has equal amounts of positive and negative charge, making it electrically neutral. If electrons are transferred from one object to another, one object can become negatively charged and the other positively charged.

Charging by friction

Static electricity often appears when materials rub together. For example, rubbing a balloon on hair can transfer electrons between the hair and balloon. The balloon may become negatively charged, while the hair becomes positively charged. Opposite charges attract, so the hair may stand up toward the balloon.

The word static means not flowing. In static electricity, charge builds up in one place rather than moving continuously through a circuit. Eventually, the charge may suddenly move, producing a spark.

Attraction and repulsion

Charged objects exert forces on each other. The force between charges is stronger when the charges are larger and weaker when they are farther apart. A simplified statement of Coulomb's law is:

F propto rac{q_1q_2}{r^2}

This means electric force depends on the charges $q_1$ and $q_2$ and the distance $r$ between them. The exact form includes a constant, but the main idea is that electric force gets weaker quickly with distance.

Electric forces can be much stronger than gravitational forces at the atomic scale. However, because positive and negative charges often balance, large objects usually appear electrically neutral.

Conductors and insulators

A conductor is a material in which electric charge can move easily. Metals are good conductors because some electrons can move freely through the material. An insulator is a material in which charge does not move easily. Rubber, plastic, glass, and dry air are common insulators.

Static charge often remains on insulators because it cannot easily spread out. On conductors, excess charge can move and redistribute. If a charged conductor touches the ground, charge may flow into or out of Earth. This process is called grounding.

Electric fields

An electric field is a way of describing how a charge affects the space around it. Another charge placed in that field will experience a force. Electric field lines are a visual model. They point away from positive charges and toward negative charges.

You cannot see electric fields directly, but you can observe their effects. A charged balloon sticking to a wall, dust attracted to a screen, and sparks from a doorknob all involve electric fields.

Lightning

Lightning is a dramatic example of static electricity. During storms, charge separation can occur within clouds and between clouds and the ground. When the electric field becomes strong enough, air breaks down and allows a sudden flow of charge. The result is a bright flash and rapid heating of air, which creates thunder.

The big idea

Static electricity comes from the buildup and interaction of electric charges. Like charges repel, opposite charges attract, and charges can be transferred between materials. Conductors allow charge to move easily, while insulators do not. Electric fields help explain how charges exert forces even without touching.