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This article answers basic questions about how electricity works and answers basic questions about residential electrical wiring troubleshooting and installation. Readers of this article should also see ELECTRICAL DEFINITIONS. This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions.
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Sketch at page top courtesy of Carson Dunlop Associates.
Many years ago I [DF] asked an electrical engineer/programmer Daniel Martin and a physicist friend Trudy Kappel how electricity really works - what's really going on. The ensuing argument ended finally with "well ultimately, we really can't say, completely." And even with the recent confirmation of the Higgs Bosun, a key step in understanding modern physics, we still can't carry the explanation of electicity to a complete end. But it is possible to give some basic understanding of electrical principles and to use some simple analogies to help a more normal person understand "what's going on" when we turn on a light switch, start an electric motor, or touch a live electrical wire while standing on wet earth.
Working safely and efficiently with your home wiring and appliances is easier if you understand what electricity is and how it works. In addition to this article about how electricity works, readers should see our DEFINITIONS of ELECTRICAL TERMS and also AMPS & VOLTS DETERMINATION where we define terms and concepts like Volts, Amps, Resistance, and Watts.
Electricity is a Form of Energy
What Makes Electricity "Flow" in an Electrical Circuit
The short answer is "potential" which is a confusing synonym for "volts" or electrical pressure between an electrical power source (which is pushing at 120V or 240V) and earth (which is sitting at zero V).
The word potential is used to explain that the capacity to do work is present, but not that work is necessarily being performed.
Just above in this article we used a spigot and water pipe analogy to explain the flow of energy through a system, and we said that no water flows if the spigot is shut. We said that the water pressure when the spigot was shut was 50 psi of potential energy flow.
Water in the bucket in our sketch has a capacity to do work (move water, or exert pressure) but until water actually flows out of the bucket (say when it's tipped), no water is moving and no work is being performed. It's just a potential - a word we discuss further with our bucket analogy just below.
In our bucket analogy to explain electricity, shown here, when the bucket is just sitting on the table, the water it contains has the potential to do work (move to the floor), but nothing is happening. No current of water is flowing. As soon as we tip the bucket a current of water (like current in a river) spills out. (Onto the floor in this case, which my mother did not appreciate unless I also had a mop handy.)
Electrical potential or volts: similarly a live electrical wire that is connected to a "hot" electrical power source at just one end, and whose other wire end is not connected to anything (say an electrical switch is open) is not allowing any electrical current to flow. Maybe our wire is connected to a 120 Volt power source - it has 120 Volts of potential waiting to do something but no electrons and no current are flowing. 120 Volts of pressure is pushing at one end of the wire, but since there is no exit path out of the other wire end, no current flows. [Thanks to reader David Hoffman for careful editing - Ed.]
When we close an electrical switch that connects our wire through a light bulb, say, and onwards through wire to the earth, the 120 Volts of pressure or potential begin to successfully push electrons through the wire and electrical current flows through the wire to earth. Literally, earth. The ground. Outdoors. Dirt.
Why is electrical current flowing to earth in an electrical circuit? In our pipe and faucet analogy the water was pressing at 50 psi of potential current and when we opened the faucet to air pressure (which we pretended was at 0 psi), water began flowing from the 50-psi side of the system to the zero psi side.
Open circuit: no electrical current flows: In our electrical wire example, when one end of the wire is connected to a power source and the other end is not connected to anything (see the sketch), 120 Volts of potential current is pressing on the wire from the hot or "live" side of the circuit.
Closed electrical circuit: electrical current flows to ground or earth: When we close an electrical switch to let electricity flow through our wire to earth, the earth is at zero potential compared to the electrical wire "hot" source, so a current of electrical energy flows from the "hot" or energy source to the "zero" or "ground" side of the circuit. Yep. Mother earth serves as a zero potential to which we can connect electrical current.
Metal electrical wires: Metals such as copper, steel, and aluminum are called conductors because their atoms have many free electrons and so can conduct electricity efficiently. Wires made of copper, steel, or aluminum provide an ideal way to transport electrical energy with little loss of power.
Insulators: The atoms that make up materials such as rubber, plastic, paper, and wood have almost no free electrons. These materials are called insulators because they cannot conduct electricity. A safe and efficient way to move electric energy, then, is to enclose a wire made of copper, steel, or aluminum in some insulating material and then use the wires to carry electricity from the generateing plant to the final user.
Why do We Need Two Wires for an Electrical Circuit in a Building?
As long as the generating station at the source continues to operate, the shortage and surplus in the two wires will be maintained and electron movement will continue.
The phrase "current flow" (measured in Amps) is used to describe this electron movement. The rate of the current flow (that is, the number of electrons that pass a point in one second) is measured in units called amperes, or more commonly, amps. The device that measures this current flow is called an ammeter.
The pressure that exists to restore the electron balance (measured in Volts) depends upon how large the difference is between the surplus and the shortage.
The greater the difference, the higher the pressure. This pressure is called voltage and the units in which it is measured are called volts. The device for measuring voltage is called a voltmeter.
Why are Some Electrical Wires Called Hot, Neutral, or Ground Wires?
Hot or Live electrical wires refer to wires that are connected to a source of electrical power. By convention the hot or live wire in residential circuits is usually black or red - but be careful, someone may have mis-wired or used the wrong color wire. If you touch a live wire and are also connected to the earth (standing on it, touching a water pipe, etc) chances are good that you'll receive an electrical shock, potentially a fatal one.
But beware: the neutral wire and even a ground wire (discussed below) are carrying electrical current in some conditions, and are also potential sources of electrical shock, especially if the electrical wiring in a building is defective.
Ground means simply Mother Earth or something connected to earth, such as a cold water pipe in your home or a copper rod driven onto the ground outside your house near where electric power enters.
Literally, electricity in buildings flows from a power source to the earth. In some countries what is called "ground" in the U.S. is called "earth". buildings with safe electrical wire contain one or more local connections to earth, wired from the electrical panel, through a grounding electrode (a ground rod), to the earth. The white or neutral wire is connected to this ground in the electrical panel. (A second "ground" connection in electrical panels connects back to a remote ground or earth connection provided (somewhere) by the utility company.))
See Definitions of Electrical Ground, Grounding Electrode, Grounding Conductor, Grounded Conductor, Ground Wire, Neutral Wire, Ground Rod, for definitions of these confusing electrical terms.
The earth is such a huge volume of matter that a measurable surplus or shortage of electrons never exists. Earth or ground, therefore, is always electrically neutral.
Ground, and wires connected to ground, can accept electrons or give them up as necessary to cause current to flow between ground and a point at which a shortage or surplus exists.
While both grounding wires and neutral wires are connected to ground, there is a difference in the job each performs in electrical wiring. The job of the ground wire is to provide a path to ground for electric energy when faults occur in the primary power wiring or in electrical devices.
Grounding wires may have green insulation or be bare (no insulation).
Neutral wires: The job of the white wire (neutral) is to provide the normal path for return current flow to the source when no wiring faults exist. The term hot wire refers to the wire with black or red insulation. This is the wire that causes current to flow between it and the neutral wire (or grounding wire if a fault occurs.
Question: Which color is the "live" wire in U.S. electrical wiring convention? Is the black wire the "hot" wire?
Reply: Typically the hot wire is black, possibly red, or even white with ends taped to indicate a black-hot wire
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem.
That said, convention in the U.S. is that the black wire is hot and the white wire is neutral.
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