Electricity and DC & AC Power Supply: The Basics

If you want to understand how electricity works, this guide is for you. Keep reading to learn the basic principles of electricity and the difference between DC & AC power supply.

Principles of Electricity

In the most basic terms, electricity is the movement of electrons. The movement of electrons creates electric current or charge, which is harnessed to do work like power a lightbulb.

More specifically, electric current is made up of free electrons that transfer from one atom to the next as they flow through a wire or metal conductor. So the more free electrons a material has, the better it conducts.

There are three key elements of electrical charge:

• Voltage: the difference in charge between two points
• Current: the rate at which charge is flowing
• Resistance: a material’s tendency to resist the flow of charge (current)

There are ways to measure this electrical charge. One of these is coulombs per second. A current of one coulomb per second equals one ampere.

Even materials that conduct electricity, like wires, resist the flow of electrons. Electrical resistance is measured in ohms, named after the Bavarian scientist Georg Ohm. The resistance in a conductor is determined by its size, its material, e.g. copper or aluminum, and its temperature. A conductor’s resistance increases as its length increases or its diameter decreases. As temperature increases, a conductor’s resistance generally increases.

The pressure needed to make one coulomb per second flow through a conductor with a resistance of one ohm is one volt.

Ohm’s Law explains the relationships between voltage (E), resistance (R), and current (I). To understand how these concepts work, we need to introduce the idea of circuits. A circuit is a closed loop that allows charge to move from one place to another, and it allows us to control the charge and harness it to do work.

If there are two circuits with equal voltage, the current will be proportionately greater in the circuit of lower resistance. In circuits of equal resistance, the current flowing will be directly proportional to the voltage applied. Current is directly proportional to voltage. Current is inversely proportional to resistance, meaning a higher resistance will result in less current, or vice versa.

So here is the formula for Ohm’s Law:

I (Amperes) = E (Volts) / R (Ohms)

Or:

E = IR

Or:

R = E/I

While current is measured in amperes, one of the most common electrical measurements you’ll use is the watt, a unit of electrical power.

W (Watts) = E (Volts) x I (Amperes)

Appliances often have a wattage listed on them, but since some electrical power is absorbed, an appliance is never 100% efficient. Watts are measured in joules per second, but the joule, a unit of electrical energy, is so small, it is not generally used. Today it is measured in the kilowatt hour. A kilowatt is 1,000 watts of electrical power, and kilowatt hours are used to measure electricity bills.

Another term you should know is ampacity. Ampacity is the amount of current a conductor can handle before its temperature exceeds accepted limits, and ampacity is affected by many external factors.

Conductors & Insulators

1. Conductors

Conductors are made of atoms whose outer, or valence, electrons have relatively weak bonds to their nuclei (the nucleus is the central core of an atom). When a bunch of metal atoms are together, they share their electrons with each other, creating a “swarm” of electrons not associated with a certain nucleus. So a very small electrical force can make the electron swarm move. Copper, gold, silver, aluminum—and saltwater—are good conductors.

The movement of free electrons in an atom is usually random, but for example, if a piece of copper is connected to an external electrical supply, the movement of electrons becomes ordered. The orderly movement of electrons is electrical current or charge.

Electrons have a negative charge and do most of the work in electric circuits, but positively charged ions (ions are positively or negatively charged particles) can carry electric current as well. For example, when table salt or sodium chloride, NaCl, dissolves in water, it breaks into Na+ and Cl- ions that move in opposite directions in response to electric force, and carry current.

Charged objects, or ions, move in response to electric and magnetic forces. These forces come from electric and magnetic fields, which in turn come from the position and motion of other charges. Like magnets, unlike poles (or opposites) attract, and like poles repel.

There are also poor conductors. Tungsten, a metal used for light bulb filaments, is a relatively poor conductor because its electrons are less prone to move. It is used in light bulbs because of its high melting point and resistance to heat. Carbon, in the form of diamonds, is also a poor conductor.

2. Insulators

Insulators are materials whose outer electrons are tightly bound to their nuclei, so a small electrical force is not able to pull these electrons free. When electric force is applied, the electron clouds around the atom stretch and deform, but the electrons do not break free. Glass, plastic, stone, and air are insulators. But a high enough force can still break the electrons of an insulator away. When this happens, it is called breakdown, and it’s what happens to air molecules when you see a spark.

DC & AC Power Supply

1. DC (Direct Current)

Direct current provides a constant voltage or current. Batteries are an example of DC, which generate current from a chemical reaction inside the battery. Once a battery is dead, it can no longer generate current. DC is defined as “unidirectional,” meaning the current only flows in one direction. Examples of DC electronics include cell phones, flashlights, and electric vehicles.

2. AC (Alternating Current)

Alternating current describes a flow of charge that changes direction periodically. The voltage level reverses along with the current. AC is used to deliver power to houses, office buildings, etc., and most homes are set up to receive AC.

AC can be produced using an alternator, a type of electrical generator designed to produce AC. A loop of wire is spun inside a magnetic field, which creates current along the wire. The rotation of the wire can be caused by many different things: a wind turbine, a steam turbine, flowing water, etc. Because the wire spins and enters a different magnetic polarity periodically, the voltage and current alternates on the wire.

Home and office outlets are almost always AC because generating AC across long distances is relatively easy. At high voltages, less energy is lost in electrical power transmission. Higher voltages mean lower currents, and lower currents generate less heat in the power line due to resistance. Transformers are used to easily convert AC to and from high voltages.

AC also powers electric motors. Motors and generators are the same devices, but motors convert electrical energy into mechanical energy. Dishwashers, refrigerators, and more run on AC.

An AC generator with a device called a “commutator” can produce DC, and a device called a “rectifier” can also convert AC to DC.

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