Electric current

An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is the rate at which electric charge moves through a surface. The particles that move are called charge carriers, and they can be different types depending on what the conductor is. In electric circuits, the charge carriers are often electrons moving through a wire.

Electric current is measured in ampere, often called an "amp", which is one coulomb per second. This measurement is an important basic unit in science. We use a tool called an ammeter to measure electric current.

Electric currents create magnetic fields, which help power motors, generators, inductors, and transformers. In regular conductors, electric currents produce heat, which makes incandescent light bulbs glow. Changing electric currents also create electromagnetic waves, which we use in telecommunications to send information across distances.

Symbol

The symbol for electric current is I. This comes from a French phrase meaning "current intensity." A scientist named André-Marie Ampère used this symbol when he made important discoveries about electric current in 1820. The symbol traveled from France to Britain and became the standard way to show electric current. Some journals kept using a different symbol, C, until 1896 before switching to I.

The unit of electric current is named after Ampère.

Conventions

The usual way we think about the direction of electric current is called conventional current. It is simply the direction in which we imagine positive charges would move. In most wires and conductors, the particles that carry the electric current are called charge carriers. In metals, like the wires in our homes, the tiny parts of atoms called atomic nuclei stay in one place, while smaller particles called electrons move around and carry the current.

In some other materials, such as semiconductors, the charge carriers can be either positive or negative, depending on what they are mixed with. Sometimes, both positive and negative charge carriers can be moving at the same time, like in special liquids used in certain batteries called an electrolyte in an electrochemical cell.

Whether positive or negative charges are moving, the effect on the circuit is the same. Because of this, we need a common way to talk about the direction of current no matter what kind of charge carriers are involved. So, we say that negative charge carriers, like electrons, move in the opposite direction to what we call the conventional current direction in a circuit.

Ohm's law

Main article: Ohm's law

Ohm's law tells us that the flow of electricity, called current, through a wire depends on the difference in electric pressure, called voltage, between two points. The higher the voltage, the more current will flow. This relationship can be described with a simple formula: current equals voltage divided by resistance. This means that if the voltage goes up, the current goes up too, but if the resistance goes up, the current goes down.

Alternating and direct current

See also: War of the currents

In alternating current (AC), the flow of electric charge changes direction regularly. This is the type of electricity most homes and offices use. The shape of this flow is often a smooth up-and-down pattern, but some uses need different patterns.

Direct current (DC) is electricity that flows in just one direction. Things like batteries, solar cells, and some special machines can create direct current. AC can be turned into DC using a device called a rectifier. DC can travel through wires, but also through other materials or even empty space.

Occurrences

Electric current can be seen in nature, like during lightning, static electric discharge, and the solar wind that creates the polar auroras.

We also create electric current in everyday objects. It flows through metal wires, like those in power lines that bring electrical energy to our homes, and in the tiny wires inside electronic devices. Special currents called eddy currents happen when metals are near changing magnetic fields. In electronics, current can move through things like resistors, inside a battery, and even through the empty space in a vacuum tube. Our bodies also use tiny currents when neurons send signals for thinking and feeling.

Measurement

Current can be measured using an ammeter. One way to measure electric current is with a galvanometer, but this needs to break the electrical circuit, which can be tricky sometimes.

There are also ways to measure current without breaking the circuit. These methods detect the magnetic field made by the current. Some devices use different techniques, such as shunt resistors, Hall effect sensors, transformers (though they can’t measure DC), magnetoresistive sensors, Rogowski coils, and current clamps.

Resistive heating

Main article: Joule heating

Resistive heating happens when electricity flows through a material, making it warmer. This occurs because the moving electric current adds energy to the material, turning it into heat.

A scientist named James Prescott Joule studied this effect in 1841. He placed a wire in water and watched how much the water warmed up when electricity passed through the wire. He discovered that the amount of heat created depends on the strength of the electric current and the material's resistance to the flow of electricity.

Electromagnetism

Main article: Electromagnetism

Electromagnet

Main article: Electromagnet

When electricity flows through a coil of wires, it can act like a magnet. But when the electricity stops, the coil stops being a magnet right away. Electricity creates something called a magnetic field around the wire, which you can think of as invisible lines circling the wire while the electricity is flowing.

Electromagnetic induction

Main article: Electromagnetic induction

Magnetic fields can also create electricity. If you change a magnetic field near a wire or conductor, it can make electricity start flowing in that wire, as long as the wire provides a path for the electricity to move.

Radio waves

Main article: Radio waves

Further information: Radio-frequency current

When electricity flows in a special shape at very high speeds, it can create radio waves. These waves travel at the same speed as light and can make electricity flow in wires far away from the original source.

Conduction mechanisms in various media

Main articles: Electrical conductivity and Charge transport mechanisms

In metals, electric current flows because of tiny particles called electrons moving from one place to another. In other materials, charged particles like ions can also create electric current. Scientists use a special idea called "conventional current" to describe the direction of current, which is the same as if positive charges were moving, even though in metals the actual particles moving are electrons, which are negative.

In a vacuum, beams of ions or electrons can form currents. In some materials, both positive and negative particles move at the same time to create current. For example, in electrolytes like saltwater, both positive and negative ions move to make up the current. In ice and certain solids, the current is made only of moving ions.

Metals

In metals, some electrons are free to move around easily. When a metal is connected to a battery or another source of electricity, these free electrons move toward the positive end, creating an electric current. This movement happens very fast, and the number of moving electrons shows how strong the current is.

Electrolytes

Main article: Conductivity (electrolytic)

In liquids that conduct electricity, called electrolytes, the current is made of charged particles called ions. For example, in a solution with sodium and chloride ions, the sodium ions move toward the negative end, and the chloride ions move toward the positive end when electricity is applied.

Gases and plasmas

In ordinary air and gases, electricity usually doesn’t flow well. But if the electricity is strong enough, it can cause the gas to break apart into a mix of free electrons and ions, creating a plasma. This plasma can conduct electricity and create sparks or lightning.

Vacuum

In a perfect vacuum with no particles, electricity usually doesn’t flow. But if metal surfaces are heated or have a strong electric field, they can release electrons into the vacuum, allowing electricity to flow.

Superconductivity

Main article: Superconductivity

Some materials can conduct electricity with no resistance at all when they are cooled below a certain temperature. This is called superconductivity. It was first discovered in 1911. In these materials, magnetic fields are pushed out, which is different from normal conductors.

Semiconductor

Main article: Semiconductor

In semiconductors, electricity can flow, but not as easily as in metals. Sometimes, it’s helpful to think of the current as moving positive particles called “holes.” These materials have just the right amount of ability to let electricity flow, more than insulators but less than metals.

Current density and Ohm's law

Main article: Current density

Current density tells us how much electric charge moves through a certain area in a certain time. It is measured in units called amperes per square meter.

In many metals, when the frequency of the electric current is low, something called Ohm's law helps us understand how electricity behaves. Ohm's law says that the amount of current is directly related to the difference in electric potential, or voltage, between two points. This relationship depends on the material's resistance, which measures how much it opposes the flow of electricity. For alternating currents, especially at higher frequencies, the current does not spread out evenly in the wire. Instead, it tends to stay closer to the surface, which can make the wire seem to have more resistance.

Speed

Drift speed

Main article: Drift speed

Charged particles in a conductor, like metals, move in many directions at once, similar to gas particles. To create a flow of electric current, these particles need to move together with an average speed called drift speed. In most metals, electrons are the particles that carry the charge. They bounce around but generally move opposite to the electric field. This drift speed is usually quite slow. For example, in a copper wire carrying a normal amount of electric current, the electrons move only about a millimeter each second.

Wavefront speed

Main article: Speed of electricity

When an electric current changes, it creates waves of energy that travel outside the conductor at a very high speed. This speed is a large part of the speed of light and is much faster than the drift speed of the electrons inside the wire. In power lines, these energy waves travel through the space between the wires from the source to where the electricity is used, even though the electrons in the wires themselves only move back and forth a tiny amount.