Sources of electricity

Every circuit an electrician works on starts at a source: a battery, a generator, a solar panel, a utility transformer fed by a distant power plant. Lesson 1 described voltage as the push that drives current. This lesson looks at where that push comes from. There are six ways to produce a voltage: friction, chemical action, magnetism, light, heat and pressure. Each underlies familiar equipment, from the static shock you get from a doorknob to the generators that supply the grid, the batteries in tools and UPS systems, rooftop solar panels, the thermocouples in a furnace and the sensors in a gas grill igniter. Understanding them helps you choose and troubleshoot equipment and recognize hazards such as static discharge and lightning.

Electric charge and electric fields

Lesson 1 introduced atoms, electrons and the coulomb. Two rules govern electric charge: like charges repel and unlike charges attract. Any object with more electrons than protons is negatively charged; one with fewer is positively charged. A charged object is surrounded by an electric field, a region where other charges feel a force. Voltage is a measure of the energy per unit charge in that field: one volt is one joule of energy per coulomb of charge.

where \(V\) is voltage in volts, \(W\) is energy in joules and \(Q\) is charge in coulombs. Every source of electricity does the same basic job: it uses some other form of energy to separate positive and negative charges, creating a voltage between two terminals. When a circuit connects the terminals, current flows and the energy is delivered to the load.

1. Friction: static electricity

Rubbing two different materials together transfers electrons from one to the other, leaving both charged. Walking across a carpet, sliding off a car seat or pouring dry powder through a pipe can build up thousands of volts. Because the charge has nowhere to go, it stays static until it finds a path, then discharges in a spark.

Static electricity is a nuisance in daily life, but it matters on the job:

• Electrostatic discharge (ESD) can damage electronic components such as circuit boards in drives, controls and computers at voltages far below what a person can feel. Technicians use wrist straps, grounded mats and antistatic bags when handling electronics.
• In hazardous locations, a static spark can ignite flammable vapors or dust. Fuel handling, grain elevators and paint booths use bonding and grounding to drain charge safely (ELEC-220).
• Lightning is static electricity on a massive scale: charge separated within storm clouds discharges to the ground at tens of thousands of amperes. Lightning protection systems and surge protective devices give it a controlled path.

2. Chemical action: cells and batteries

A cell produces voltage from a chemical reaction between two different electrodes in an electrolyte. One electrode gives up electrons more readily than the other, so electrons build up on one terminal and leave the other, producing a voltage that depends on the chemistry. A battery is two or more cells connected together. Primary cells (such as alkaline AA cells) are used once and discarded; secondary cells (such as lead-acid and lithium-ion) can be recharged by forcing current through them in reverse. Lesson 8 covers batteries in detail.

3. Magnetism: generators

Almost all the electricity on the grid is produced by magnetism. When a conductor moves through a magnetic field, or a magnetic field changes around a conductor, a voltage is induced in the conductor. This principle, discovered by Michael Faraday in 1831, is called electromagnetic induction. A generator spins coils of wire in a magnetic field (or spins magnets inside coils), inducing a voltage that alternates as the coil turns, producing alternating current (ELEC-210). Something must turn the generator: steam from burning fuel or nuclear fission, falling water, wind, or a diesel engine. Lesson 9 explains magnetism and induction in more detail.

The voltage induced depends on three things: the strength of the magnetic field, the speed of motion, and the number of turns of wire. Doubling any of them doubles the voltage. This is why generator output voltage is controlled by adjusting the field strength (the excitation) while the speed is held constant to keep the frequency at 60 Hz.

In the United States, natural gas generators produce roughly 40% of electricity, with nuclear, coal, wind, hydro and solar making up most of the rest; the mix varies greatly by region.

4. Light: photovoltaic cells

A photovoltaic (PV) cell converts light directly into electricity. It is made of a semiconductor, usually silicon, treated to create a junction between two layers. When light strikes the cell, its energy frees electrons, and the junction pushes them in one direction, producing a DC voltage of about 0.5 V to 0.6 V per cell. Cells are connected in series in a module (a solar panel) to produce useful voltages, commonly 30 V to 50 V per module, and modules are connected in series strings and parallel arrays.

A PV cell behaves differently from a battery. Its voltage stays fairly constant as light changes, but its current is nearly proportional to the light level, and it can never deliver more than its short-circuit current. PV arrays still produce dangerous DC voltages whenever light falls on them, so they cannot be switched off by opening a disconnect alone; codes require rapid shutdown systems on buildings and special procedures for working on arrays.

5. Heat: thermocouples

When two different metals are joined and the junction is heated, a small voltage appears: the thermoelectric (Seebeck) effect. A thermocouple uses this to measure temperature: one junction is placed where the temperature is to be measured, and the voltage, a few millivolts, indicates the temperature difference between it and the reference junction. Thermocouples are rugged and cover a wide range, so they are used in furnaces, ovens, kilns, engines and industrial processes. In gas appliances, a thermocouple heated by the pilot flame produces enough voltage to hold a safety gas valve open; if the flame goes out, the voltage disappears and the valve closes.

6. Pressure: piezoelectricity

Some crystals and ceramics, such as quartz, produce a voltage when they are squeezed or bent: the piezoelectric effect. The voltage can be large but the energy is small. Piezoelectric devices include spark igniters in barbecues and gas appliances, pressure and vibration sensors, microphones, ultrasonic transducers, and the quartz crystals that keep accurate time in clocks, meters and computers (where the effect works in reverse, with voltage making the crystal vibrate at a precise frequency).

Conductors, insulators and semiconductors

Lesson 1 introduced conductors and insulators. The difference comes from how tightly atoms hold their outer electrons. In metals, the outer electrons are free to move, so a small voltage drives a large current. In insulators, electrons are tightly bound, and only a very high voltage can force current through, usually by breaking down the material. Insulation is rated for a maximum voltage; above it, the insulation can puncture, which is why cables are marked with voltage ratings such as 300 V, 600 V or 1000 V.

Semiconductors, such as silicon, conduct poorly in their pure state, but adding tiny amounts of other elements (doping) controls their conductivity precisely. Semiconductors are the basis of diodes, transistors, PV cells, LEDs and all electronics, including the drives, controls and power supplies electricians install every day (ELEC-330).

DC and AC sources

Batteries, PV cells and thermocouples produce direct current (DC), which flows in one direction. Generators on the grid produce alternating current (AC), which reverses direction 60 times per second in North America. AC dominates power distribution because transformers can easily change its voltage (ELEC-210). DC is used in electronics, batteries, PV systems, vehicles, telecom and control circuits, and increasingly in data centers and high-voltage DC transmission. Power supplies and inverters convert between them. This course concentrates on DC circuits, where the rules are simplest; the same rules extend to AC with the additions covered in ELEC-210.