Flip a switch and a light turns on. Plug in a phone and it charges. Electricity powers almost everything in modern life, from refrigerators and computers to hospitals and trains. Yet electricity itself is invisible, and many people are unsure what it actually is. Is it a flow of something? Why do we talk about volts, amps and watts? Why does a balloon stick to the wall after you rub it on your hair? This explainer describes the basic ideas of electricity, from tiny charged particles to the power grid that delivers electricity to your home.
Electric charge
Everything around us is made of atoms. Each atom contains a nucleus with positively charged protons and neutral neutrons, surrounded by negatively charged electrons. Electric charge is a fundamental property of these particles, as basic as mass.
Two simple rules describe how charges behave:
- Opposite charges attract: positive and negative charges pull toward each other.
- Like charges repel: two positive charges or two negative charges push each other away.
Normally, atoms have equal numbers of protons and electrons, so they are electrically neutral. But electrons, especially the outer ones, can sometimes move from one atom or object to another. When an object gains extra electrons, it becomes negatively charged; when it loses electrons, it becomes positively charged.
Static electricity
When you rub a balloon on your hair, electrons transfer from your hair to the balloon. The balloon becomes negatively charged, and your hair positively charged. Your hair may stand up, because each strand carries the same positive charge and repels the others. The negatively charged balloon can stick to a wall, because it pushes electrons in the wall slightly away, leaving the wall's surface slightly positive and attracting the balloon.
This buildup of charge is called static electricity. When the charge suddenly jumps to another object, you feel a small shock or see a spark. Lightning is a giant example of static discharge, where charges built up in storm clouds jump to the ground or between clouds.
Electric current: charges in motion
Electric current is the flow of electric charge. In metal wires, the moving charges are electrons. Current is measured in amperes, or amps (A). One ampere represents a huge number of electrons, about 6.24 billion billion, passing a point every second.
Interestingly, electrons in a wire move quite slowly, drifting along at a fraction of a millimeter per second. Yet when you flip a switch, a light turns on almost instantly. That is because the wire is already full of electrons. When the circuit is completed, an electric field travels through the wire at nearly the speed of light, pushing all the electrons along at once, like water in a full hose that flows out the moment you open the tap.
By historical convention, current is described as flowing from positive to negative, even though electrons actually move from negative to positive. This convention was established before electrons were discovered.
Conductors and insulators
- Conductors allow electric charge to flow easily. Metals like copper, aluminum and silver are excellent conductors because their outer electrons can move freely between atoms. Salt water is also a conductor.
- Insulators resist the flow of charge. Rubber, plastic, glass and dry wood are good insulators. Electric wires are made of copper coated in plastic insulation.
- Semiconductors, such as silicon, are in between. Their conductivity can be precisely controlled, which makes them the basis of electronic devices like computer chips and solar cells.
Voltage: the push
For current to flow, something must push the charges. That push is voltage, also called electric potential difference, measured in volts (V). Voltage is like water pressure in a pipe: higher pressure pushes more water through.
A battery creates a voltage between its terminals through chemical reactions. A typical AA battery provides 1.5 volts, a car battery about 12 volts, and household outlets 120 volts in North America and 220 to 240 volts in much of the rest of the world.
Resistance
Resistance is the opposition to the flow of current, measured in ohms (Ω). Thin wires have more resistance than thick ones, long wires more than short ones, and some materials more than others. Resistance converts some electrical energy into heat. This is useful in toasters, electric heaters and old-style light bulbs, whose filaments glow because of high resistance. In power lines, resistance causes unwanted energy losses.
Ohm's law
The German physicist Georg Ohm discovered a simple relationship between voltage, current and resistance, now called Ohm's law:
Voltage = Current × Resistance
Using the water analogy: voltage is the pressure, current is the flow rate, and resistance is the narrowness of the pipe. For the same pressure, a narrower pipe allows less flow. For example, a 12-volt battery connected to a 6-ohm resistor produces a current of 2 amperes.
Power and energy
Electric power is the rate at which electrical energy is used or produced, measured in watts (W). It equals voltage multiplied by current. A 60-watt light bulb uses 60 joules of energy every second.
Electricity bills measure energy in kilowatt-hours (kWh). One kilowatt-hour is the energy used by a 1,000-watt device running for one hour, such as a small electric heater, or a 10-watt LED bulb running for 100 hours.
| Quantity | Unit | Analogy |
|---|---|---|
| Charge | coulomb (C) | amount of water |
| Current | ampere (A) | flow rate of water |
| Voltage | volt (V) | water pressure |
| Resistance | ohm (Ω) | narrowness of the pipe |
| Power | watt (W) | rate of doing work |
| Energy | kilowatt-hour (kWh) | total work done |
Circuits
Electricity flows only when there is a complete path, called a circuit, from the source, through the device and back to the source. A switch opens or closes the circuit. In a series circuit, components are connected one after another, so the same current flows through all of them; if one fails, the whole circuit stops. In a parallel circuit, components are on separate branches, so each gets the full voltage and can work independently. Homes use parallel circuits, so turning off one light does not turn off the others.
Direct current and alternating current
- Direct current (DC) flows in one direction. Batteries and solar cells produce DC, and electronic devices use it internally.
- Alternating current (AC) reverses direction many times per second: 60 times per second (60 Hz) in North America and 50 times per second (50 Hz) in most other countries. Power grids use AC because its voltage can be easily changed with transformers.
In the late nineteenth century, Thomas Edison promoted DC, while Nikola Tesla and George Westinghouse championed AC. AC won for power distribution because it could be transmitted efficiently over long distances, a contest sometimes called the "war of the currents."
How electricity is generated
Most electricity is generated using the principle of electromagnetic induction, discovered by Michael Faraday in 1831: moving a magnet near a coil of wire, or moving a wire through a magnetic field, creates an electric current.
Power plants use this principle in generators. Something must turn the generator:
- In fossil fuel and nuclear power plants, heat boils water into steam, which spins turbines.
- In hydroelectric plants, falling water spins turbines.
- In wind turbines, the wind spins the blades.
Solar panels are an exception: they convert sunlight directly into electricity using the photovoltaic effect, without moving parts.
The power grid
Electricity travels from power plants to homes through the power grid:
- Step-up transformers raise the voltage to very high levels, often hundreds of thousands of volts. High voltage reduces current, which reduces energy lost as heat in the wires.
- Transmission lines carry electricity over long distances.
- Substations use step-down transformers to reduce the voltage.
- Distribution lines carry electricity through neighborhoods.
- A final transformer near your home reduces the voltage to the level used by household outlets.
Electrical safety
Electricity can be dangerous. Current flowing through the body can disrupt the heart's rhythm and cause burns. Even household voltages can be lethal. Basic safety rules include:
- Never touch electrical devices with wet hands or near water.
- Do not overload outlets or use damaged cords.
- Use devices like circuit breakers, fuses and ground-fault circuit interrupters, which cut off power when something goes wrong.
- Stay far away from downed power lines and call the utility company.
- Leave electrical repairs to qualified electricians.
Questions readers ask
What is electricity? Electricity is the presence and movement of electric charges, usually electrons flowing through a conductor as current.
What is the difference between voltage and current? Voltage is the push that drives charges; current is the rate at which charges flow.
Why do power lines use high voltage? High voltage allows the same power to be transmitted with lower current, which reduces energy lost as heat.
What is the difference between AC and DC? DC flows in one direction; AC reverses direction many times per second.
Why does static electricity give me a shock? Charge builds up on your body, for example from walking on a carpet, and suddenly jumps to a conductor like a metal doorknob.
In short
Electricity is the behavior of electric charges. Static electricity is charge that builds up on objects; electric current is charge in motion, usually electrons flowing through wires. Voltage provides the push, resistance opposes the flow, and Ohm's law ties them together. Power measures how fast energy is used, and electricity bills count kilowatt-hours. Generators turn motion into electricity using magnetism, and the grid uses transformers to deliver it efficiently over long distances. Understanding these basics makes the invisible force behind modern life a little more visible.