Starting with charge
Everything around you is made of atoms, and atoms contain tiny particles with electric charge. Electrons carry negative charge, protons carry positive charge, and neutrons carry none. Opposite charges attract and like charges repel. The structure of atoms is covered further in the article on subatomic particles.
Most objects have about equal numbers of positive and negative charges, so they are electrically neutral. Rubbing a balloon on hair moves some electrons from one surface to the other. The balloon gains extra negative charge, the hair is left with a deficit, and the two attract. That is static electricity, which is charge that builds up and stays in place. A lightning strike is a dramatic version, in which accumulated charge suddenly discharges through the air.
What an electric current is
An electric current is a flow of charge. In a metal wire, some electrons are only loosely held by their atoms, and they can drift through the material when pushed. A steady flow of those electrons is an electric current.
Current is measured in amperes, usually shortened to amps. One amp means a particular, very large number of charges passing a point each second. One interesting detail is that individual electrons drift quite slowly through a wire. A light turns on almost instantly because the whole wire is already full of charges, and pushing on one end sets all of them moving together, like pushing on a tube already full of marbles.
By historical convention, diagrams show current flowing from positive to negative, the opposite of the actual electron movement in a metal. The convention was established before electrons were discovered, and it works fine for calculations.
Voltage: what pushes the charges
Charges need a push to move steadily, and voltage supplies it. Voltage, measured in volts, is the energy given to each unit of charge as it moves through a source such as a battery, or the energy used when it passes through a device. A higher voltage means a stronger push.
A water analogy is helpful, though imperfect. Imagine water in pipes:
- Voltage is like water pressure.
- Current is like the amount of water flowing past a point each second.
- Resistance is like a narrow section of pipe that makes it harder for water to flow.
A battery, like those discussed in battery storage, works like a pump: it uses chemical energy to maintain a voltage across its two terminals.
Resistance and Ohm’s law
Resistance, measured in ohms, describes how strongly a material opposes the flow of current. Copper has low resistance and is used for wires. Materials such as rubber and glass have extremely high resistance and are called insulators. A thin wire has more resistance than a thick one of the same material.
For many common components, voltage, current and resistance are related by Ohm’s law:
voltage = current × resistance
A worked example: a 12-volt supply connected across a 6-ohm resistor produces a current of 12 ÷ 6 = 2 amps. The electrical power, the rate at which energy is converted, is voltage × current, which is 12 × 2 = 24 watts. In a resistor that energy appears as heat, which is how toasters and electric heaters work.
Ohm’s law is not a universal rule of nature. It describes many materials well, but some components, such as diodes and light-emitting diodes, do not follow a simple proportional pattern.
Circuits: why a loop matters
For current to flow continuously, charges need a complete, unbroken path from the source, through the device and back to the source. This is a circuit. A switch simply opens or closes a gap in the loop. When it is open, there is no complete path and the current stops.
Two common ways to connect devices:
- Series: the components are connected one after another in a single path. The same current flows through all of them, and if one breaks, the whole loop is broken.
- Parallel: the components are on separate branches. Each branch gets the full voltage, and one branch failing does not stop the others. Household wiring uses parallel connections for this reason.
Direct and alternating current
A battery supplies direct current, in which charges flow in one direction. The mains electricity from a wall socket is alternating current, in which the direction reverses many times a second. The rate depends on the country; it is commonly 50 or 60 times per second. Alternating current is widely used for power grids largely because its voltage can be changed efficiently with transformers, which reduces losses over long distances.
Electricity can come from many places, including burning fuel, spinning turbines, and technologies like solar panels, which convert sunlight into electricity. Overviews of where electricity originates are given in the articles on energy sources and renewable energy.
A note on safety
Household electricity can cause serious injury or death. It is not safe to experiment with mains wiring or to touch damaged cords and sockets. If you want to explore circuits hands-on, use low-voltage battery-powered kits and follow the guidance of a qualified person.
Why it matters
Electric current is also closely tied to magnetism: a current produces a magnetic field, and a changing magnetic field can drive a current. That relationship is the basis of generators, motors and transformers, and it is why electricity and magnetism are treated as two sides of one subject.