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Science of electricity

Science of electricity basics

Everything is made of atoms

To understand electricity, some basic information about atoms is helpful. Atoms are the building blocks of the universe. Everything in the universe is made of atoms—every star, every tree, and every animal. The human body is made of atoms. Air and water are made of atoms, too. Atoms are so small that millions of them would fit on the head of a pin.

Atoms are made of even smaller particles

The center of an atom is called the nucleus. The nucleus is made up of particles called protons and neutrons. Electrons spin around the nucleus in shells. If the nucleus was the size of a tennis ball, the atom would be the size of a sphere about 1,450 feet in diameter, or about the size of one of the largest sports stadiums in the world. Atoms are mostly empty space.

If the naked eye could see an atom, it would look a little like a tiny cluster of balls surrounded by giant invisible bubbles (or shells). The electrons would be on the surface of the bubbles, constantly spinning and moving to stay as far away from each other as possible. Electrons are held in their shells by an electrical force.

Diagram of an atom

The protons and electrons of an atom are attracted to each other. They both carry an electrical charge. Protons have a positive charge (+) and electrons have a negative charge (-). The positive charge of the protons is equal to the negative charge of the electrons. Opposite charges attract each other. An atom is in balance when it has an equal number of protons and electrons. The neutrons carry no charge and their number can vary.

The number of protons in an atom determines the kind of atom, or element, it is. An element is a substance consisting of one type of atom. The Periodic Table of Elements shows elements with their atomic numbers—the number of protons they have. For example, every atom of hydrogen (H) has one proton and every atom of carbon (C) has six protons.

Electricity is the movement of electrons between atoms

Diagram showing movement of electrons

Electrons usually remain a constant distance from the atom's nucleus in precise shells. The shell closest to the nucleus can hold two electrons. The next shell can hold up to eight. The outer shells can hold even more. Some atoms with many protons can have as many as seven shells with electrons in them.

The electrons in the shells closest to the nucleus have a strong force of attraction to the protons. Sometimes, the electrons in an atom's outermost shells do not have a strong force of attraction to the protons. These electrons can be pushed out of their orbits. Applying a force can make them shift from one atom to another. These shifting electrons are electricity.

Static electricity exists in nature

Lightning is a form of electricity. Lightning is electrons moving from one cloud to another or electrons jumping from a cloud to the ground. Have you ever felt a shock when you touched an object after walking across a carpet? A stream of electrons jumped to you from that object. This is called static electricity.

Have you ever made your hair stand straight up by rubbing a balloon on it? If so, you rubbed some electrons off the balloon. The electrons moved into your hair from the balloon. The electrons tried to get far away from each other by moving to the ends of your hair. They pushed against or repelled each other and made your hair move. Just as opposite charges attract each other, like charges repel each other.

Magnets and electricity

magnetic field around a bar magnet
Magnetic field around a bar magnet
diagram showing magnetic repulsion
diagram showing magnetic attraction

Source: National Energy Education Development Project (public domain)

The spinning of the electrons around the nucleus of an atom creates a tiny magnetic field. The electrons in most objects spin in random directions, and their magnetic forces cancel each other out.

Magnets are different because the molecules in magnets are arranged so that their electrons spin in the same direction. This arrangement and movement creates a magnetic force that flows out from a north-seeking pole and from a south-seeking pole. This magnetic force creates a magnetic field around a magnet.

Have you ever held two magnets close to each other? They don't act like most objects. If you try to push the two north poles or two south poles together, they repel each other. But if you put a north pole and a south pole together, the magnets will stick together because the north and south poles attract each other. Just like protons and electrons—opposites attract in magnets.

Magnetic fields can be used to make electricity

The properties of magnets are used to make electricity. Moving magnetic fields pull and push electrons. Metals such as copper and aluminum have electrons that are loosely held. Moving a magnet around a coil of wire, or moving a coil of wire around a magnet, pushes the electrons in the wire and creates an electrical current. Electricity generators essentially convert kinetic energy (the energy of motion) into electrical energy.

Batteries, circuits, & transformers

Batteries produce electricity

image of battery and light bulb

Source: Adapted from National Energy Education Development Project (public domain)

An electrochemical battery produces electricity with two different metals in a chemical substance called an electrolyte. One end of the battery is attached to one of the metals, and the other end is attached to the other metal. A chemical reaction between the metals and the electrolyte frees more electrons in one metal than it does in the other.

The metal that frees more electrons develops a positive charge, and the other metal develops a negative charge. If an electrical conductor, or wire, connects one end of the battery to the other, electrons flow through the wire to balance the electrical charge.

An electrical load is a device that uses electricity to do work or to perform a job. If an electrical load—such as an incandescent light bulb—is placed along the wire, the electricity can do work as it flows through the wire and the light bulb. Electrons flow from the negative end of the battery through the wire and the light bulb and back to the positive end of the battery.

Electricity travels in circuits

Electricity must have a complete path, or electrical circuit, before the electrons can move. The switch or on-off button on all electrical devices closes (turns on) or opens (turns off) an electrical circuit in the device. Turning—or switching—off a light opens a circuit and electrons cannot flow through the light. Turning on a light closes a circuit, which allows electricity to flow from one electric wire, through the light bulb, and then through another wire.

An incandescent light bulb produces light as electricity flows through a tiny wire in the bulb, which gets very hot and glows. An incandescent light bulb burns out when the tiny wire inside the bulb breaks, which opens the circuit.

image of battery and light bulb

Source: Adapted from National Energy Education Development Project (public domain)

Transformers help to move electricity efficiently over long distances

To solve the problem of sending electricity over long distances, William Stanley developed a device called a transformer. A transformer changes the voltage of electricity in a conductor or power line. High-voltage transmission lines, such as those that hang between tall metal towers, carry electricity over long distances to where it is needed. Higher voltage electricity is more efficient and less expensive for long distance electricity transmission. Lower voltage electricity is safer for use in homes and businesses. Transformers increase (step up) or reduce (step down) voltages as electricity moves from power plants to homes and businesses.

Measuring electricity

Electricity is measured in Watts and kilowatts

A mechanical electricity meter
Picture of a residential electricity meter.

Source: Stock photography (copyrighted)

A smart electricity meter
Picture of a smart meter.

Source: Stock photography (copyrighted)

Electricity is measured in units of power called Watts, named to honor James Watt, the inventor of the steam engine. A Watt is the unit of electrical power equal to one ampere under the pressure of one volt.

One Watt is a small amount of power. Some devices require only a few Watts to operate, and other devices require larger amounts. The power consumption of small devices is usually measured in Watts, and the power consumption of larger devices is measured in kilowatts (kW), or 1,000 Watts.

Electricity generation capacity is often measured in multiples of kilowatts, such as megawatts (MW) and gigawatts (GW). One MW is 1,000 kW (or 1,000,000 Watts), and one GW is 1,000 MW (or 1,000,000,000 Watts).

Electricity use over time is measured in Watthours

A Watthour (Wh) is equal to the energy of one Watt steadily supplied to, or taken from, an electric circuit for one hour. The amount of electricity that a power plant generates or an electric utility customer uses is typically measured in kilowatthours (kWh). One kWh is one kilowatt generated or consumed for one hour. For example, if you use a 40-Watt (0.04 kW) light bulb for five hours, you have used 200 Wh, or 0.2 kWh, of electrical energy.

Utility companies measure and monitor electricity use with meters

Electric utilities measure the electricity consumption of their customers with meters that are usually located on the outside of the customer's property where the power line enters the property. In the past, all electricity meters were mechanical devices that a utility employee had to read manually. Eventually, automated reader devices became available. These meters periodically report electricity use to utilities from mechanical meters with an electronic signal. Now, many utilities use electronic smart meters, which provide wireless access to the meter's power usage data, to measure electricity consumption in real-time. Some smart meters can even measure the electricity use of individual devices and allow the utility or customer to control electricity use remotely.

How electricity is generated

An electric generator is a device that converts a form of energy into electricity. There are many different types of electricity generators. Most of world electricity generation is from generators that are based on scientist Michael Faraday’s discovery in 1831 that moving a magnet inside a coil of wire makes (induces) an electric current to flow in the wire. He made the first electricity generator called a Faraday disk, which operates on this relationship between magnetism and electricity and which led to the design of the electromagnetic generators that we use today.

Electromagnetic generators use an electromagnet—a magnet produced by electricity—not a traditional magnet. A basic electromagnetic generator has a series of insulated coils of wire that form a stationary cylinder—called a stator—surrounding an electromagnetic shaft—called a rotor. Turning the rotor makes an electric current flow in each section of the wire coil, which becomes a separate electric conductor. The currents in the individual sections combine to form one large current. This current is the electricity that moves from generators through power lines to consumers. Electromagnetic generators driven by kinetic (mechanical) prime movers account for nearly all of U.S. electricity generation.

Turbine driven generators

Most of U.S. and world electricity generation is from electric power plants that use a turbine to drive electricity generators. In a turbine generator, a moving fluid—water, steam, combustion gases, or air—pushes a series of blades mounted on a rotor shaft. The force of the fluid on the blades spins/rotates the rotor shaft of a generator. The generator, in turn, converts the mechanical (kinetic) energy of the rotor to electrical energy. Different types of turbines include steam turbines, combustion (gas) turbines, hydroelectric turbines, and wind turbines.

Diagram of an electric turbine generator - Spinning rotor turning rotor shaft to generate electricity in an electromagnetic generator.

Steam turbines are used to generate the majority of the world’s electricity and they accounted for about 48% of U.S. electricity generation in 2019. Most steam turbines have a boiler in which a fuel is burned to produce hot water and steam in a heat exchanger, and the steam powers a turbine that drives a generator. Nuclear power reactors use nuclear fuel rods to produce steam. Solar thermal power plants use solar energy to produce steam. Of the top 10 U.S. electric power plants in 2019, 9 have steam turbines powered by nuclear energy, coal, and natural gas.

Combustion gas turbines, which are similar to jet engines, burn gaseous or liquid fuels to produce hot gases to turn the blades in the turbine.

Steam and combustion turbines can be operated as stand-alone generators in a single-cycle or combined in a sequential combined-cycle. Combined-cycle systems use combustion gases from one turbine to generate more electricity in another turbine. Most combined-cycle systems have separate generators for each turbine. In single-shaft combined cycle systems, both turbines may drive a single generator. Learn more about different types of combined-cycle power plants. In 2019, combined-cycle power plants supplied about 33% of U.S. electricity generation.

Combined-heat-and-power (CHP) plants, which may be referred to as cogenerators, use the heat that is not directly converted to electricity in a steam turbine, combustion turbine, or an internal combustion engine generator for industrial process heat or for space and water heating. Most of the largest CHP plants in the United States are at industrial facilities such as pulp and paper mills, but they are also used at many colleges, universities, and government facilities. CHP and combined-cycle power plants are the most efficient ways to convert a single fuel into useful energy.

Hydroelectric turbines use the force of moving water to spin turbine blades to power a generator. Most hydroelectric power plants use water stored in a reservoir or diverted from a river or stream. These conventional hydroelectric power plants accounted for about 7% of U.S. electricity generation in 2019. Pumped-storage hydropower plants use the same types of hydro turbines that conventional hydropower plants use, but they are considered electricity storage systems (see below). Other types of hydroelectric turbines called hydrokinetic turbines are used in tidal power and wave power systems. Learn more about different types of hydropower turbines.

Wind turbines use the power in wind to move the blades of a rotor to power a generator. There are two general types of wind turbines: horizontal axis (the most common) and vertical-axis turbines. Wind turbines were the source of about 7% of U.S. electricity generation in 2019.

Geothermal power plants use geothermal resources to drive electrical generators.

Ocean thermal energy conversion (OTEC) systems use a temperature difference between ocean water at different depths to power a turbine to produce electricity. There is a demonstration OTEC system in Hawaii.

Other types of generators

There are many different types of electricity generators that do not use turbines to generate electricity. The most common in use today are solar photovoltaic (PV) systems and internal combustion engines.

Solar photovoltaic cells convert sunlight directly into electricity. They are used to power devices as small as wrist watches and can be connected together in panels that are connected together in arrays to power individual homes or form large power plants. PV power plants are now one of the fastest growing sources of electricity generation around the world.

Internal combustion engines, such as diesel engines, are used all around the world for electricity generation including in many remote villages in Alaska. They are also widely used for mobile power supply at construction sites and for emergency or backup power supply for buildings and power plants. Diesel-engine generators can use a variety of fuels including petroleum diesel, biomass-based liquid fuels and biogas, natural gas, and propane. Small internal combustion engine generators fueled with gasoline, natural gas, or propane are commonly used by construction crews and tradespeople and for emergency power supply for homes.

Other types of electricity generators include fuel cells, Stirling engines (used in solar thermal parabolic-dish generators), and thermoelectric generators.

  • The share of total U.S. utility-scale electricity generation in 2019 by major types of electricity generators
  • steam turbines58%
  • combustion turbines24%
  • hydroelectric turbines7%
  • wind turbines7%
  • solar photovoltaic systems2%
  • all other types2%