Electric charge

Electric charge is a basic property of matter that makes objects feel forces when they are near other objects with electric fields. This charge can be positive or negative. Charges that are the same push each other away, while charges that are different pull toward each other. When an object has no charge at all, we call it electrically neutral.

In any closed system, the total amount of charge never changes. Negative charge is carried by tiny parts of atoms called electrons, and positive charge is carried by parts called protons, which sit in the center of atoms. If an object has more electrons than protons, it has a negative charge. If it has fewer electrons than protons, it has a positive charge. And if the numbers are equal, the object is neutral.

Charge comes in tiny, fixed amounts called the elementary charge. This smallest charge belongs to particles like electrons and protons. Larger objects get their charge by gaining or losing many of these tiny pieces. Moving charges create magnetic fields, and together with electric fields, they are part of what makes up one of the four main forces in physics. The unit we use to measure electric charge is called the coulomb, named after a scientist who studied these forces.

Overview

Charge is a basic property of matter that causes it to push away or pull toward other charged matter. Electric charge can be positive or negative.

Small particles that make up matter, like electrons and protons, each have a charge. Usually, objects have the same number of positive and negative charges, so they feel no pull or push from other charges. However, if an object loses or gains electrons, it can become charged and either push away or pull toward other charged objects. Rubbing two different materials together, like amber with fur, can move electrons from one to the other, making one positively charged and the other negatively charged. This is called static electricity.

When charges move through metals, we call this flow electric current.

Unit

The SI unit for measuring electric charge is the coulomb, written as C. One coulomb is the amount of charge that moves through a wire carrying one ampere in one second. This unit was suggested in 1946 and officially approved in 1948.

The smallest amount of charge, called the elementary charge, is a fixed value in the SI system. It is exactly 1.602176634×10⁻¹⁹ C. This tiny amount of charge is carried by particles like the electron. Even though charge can seem to flow smoothly in large amounts, it is actually made up of many of these tiny pieces of charge.

History

See also: History of electromagnetic theory and Electricity § History

Long ago, people noticed some interesting things that we now understand using electric charge. They saw lightning, felt shocks from torpedo fish (or electric rays), saw St Elmo's Fire, and noticed that amber rubbed with fur could pull small objects toward it. The ancient Greek thinker Thales of Miletus wrote about the amber effect, but he thought it meant objects had a spirit inside them, not that they had electric charge.

Later, scientists began to study these effects more closely. In 1600, William Gilbert wrote a book called De Magnete where he used the word electrica to describe these phenomena. In the 1700s, scientists like Stephen Gray discovered that electric charge could move through materials, and Benjamin Franklin proposed that electricity was a single kind of invisible fluid. His ideas helped shape our modern understanding of positive and negative charges.

Role of charge in static electricity

Static electricity is the electric charge an object has and what happens when two objects touch and aren't balanced. When this happens, the objects can change their charge.

When you rub a piece of glass with a piece of resin and then pull them apart, they will attract each other. If you rub another piece of glass with another piece of resin and bring it close to the first pieces, something interesting happens:

• The two pieces of glass will push away from each other.
• Each piece of glass will pull toward each piece of resin.
• The two pieces of resin will push away from each other.

This pulling and pushing is an electric effect, and the objects are said to be electrified, or electrically charged. All electrified objects are either vitreously or resinously electrified. Scientists call vitreous electrification positive and resinous electrification negative.

Role of charge in electric current

Electric current is when electric charge moves through something. The tiny parts of atoms, like protons and electrons, carry this charge. When these parts move, we call it electric current.

There are many ways charge can move. Sometimes it’s electrons moving one way, or special places called holes acting like positive particles moving the other way. In liquids or plasmas, both positive and negative particles can move to create current. The way we usually talk about current direction is a bit different from how the actual particles move, but it helps us understand electricity more easily.

Conservation of electric charge

Main article: Charge conservation

The total electric charge of something that is not connected to anything else stays the same, no matter what happens inside it. This idea is a basic rule in physics. It means that when charge moves in or out of an area, the total amount of charge in that area changes in a predictable way. This helps us understand how electric charge behaves in different situations.

Relativistic invariance

Electric charge is a special property that stays the same no matter how fast something is moving. This means if a tiny part of matter, like a helium nucleus, has a certain electric charge, it will keep that same charge even if it is moving very fast. Scientists have tested this by comparing the charge of fast-moving helium nuclei to slower-moving deuterium nuclei and found the charges remain equal.

This shows that electric charge does not change with speed, which is an important idea in the study of how electric forces work.