Basic Units of Electric Circuits
by Ron Kurtus (revised 23 October 2019)
The basic units of a simple electric circuit are the ampere, volt, and ohm.
A simple circuit typically consists of a voltage supply, metal wires that conduct the electric current, and one or more resistors that resist the conduction of the current. The current may be direct current (DC) or alternating current (AC), and there should be no additional devices that affect the current.
The unit for electric current—the ampere—is a basic International Standard (SI) unit. The voltage unit and resistance unit are derived from the ampere and other standard units. Unfortunately, the international committee of scientists made the definitions more complex than they need to be.
Questions you may have include:
- What is the definition of an ampere?
- What a volt?
- What is the unit of resistance?
This lesson will answer those questions. Useful tool: Units Conversion
The ampere (A) is a basic SI unit of electric current. It can be defined as the amount of electric charge or number of electrons that pass a point in a circuit in one second. One ampere equals 6.241*1018 electrons passing a point per second or one coulomb per second. (The coulomb (C) is the SI unit of electric charge.)
The official SI definition of an ampere is somewhat bizarre:
"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2*10−7 newton per meter of length."
Note: My view is that requiring a conductor of infinite length and "negligible" cross section is not practical and does not belong in a standard definition. Also, there are unstated implications of the relationship of force between two wire and current that should be expressed.
Since A is a basic SI unit, it is not expressed in terms of other units.
The volt (V) is the derived SI unit of electric potential or electromotive force that causes the electrons to move. Since a source of electricity creates energy, a volt can be defined as the potential difference between two points in an electric circuit that will impart one joule (J) of energy per coulomb (C) of charge that passes through it.
V = J/C
Voltage can also be stated as electric potential along a wire when an electric current of one ampere dissipates one watt (W) of power (W = J/s).
V = W/A
A volt can be stated in SI base units as 1 V = 1 kg m2 s−3 A−1 (one kilogram meter squared per second cubed per ampere).
Considering the official SI definition of an ampere, a volt is also equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb.
The ohm (Ω) is the unit of electric resistance in a circuit. It is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt (V), applied to these points, produces in the conductor a current of 1.0 ampere (A), provided the conductor is not the source of any electromotive force, such as in a battery.
Ω = V/A
Note that this is also the Ohm's Law equation.
Stating resistance in terms of basic SI units:
Ω = kg m2 s−3 A−2
Electric resistance is also a function of the cross section of the wire, as well as its temperature.
The ampere (A) is a basic SI unit consisting of the amount of electric charge or number of electrons that pass a point in a electric circuit in one second. The volt (V) is the electric potential causing electrons to move through a wire. It is a joule of energy per coulomb of charge. The ohm (Ω) is the unit of electric resistance equal to the 1 volt divided by 1 ampere.
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Resources and references
Unit of electric current (ampere) - National Institute of Standards and Technology (NIST)
Ampere - Wikipedia
Coulomb Force - Wolfram Science World
Coulomb - Wikipedia
Voltage - HyperPhysics
Electric Potential Difference - Physics Classroom
Volt - Wikipedia
Ohm - Wikipedia
Teach Yourself Electricity and Electronics by Stan Gibilisco; McGraw-Hill; (2001) $34.95 - Guide for professionals, hobbyists and technicians desiring to learn AC and DC circuits
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Basic Units of Electric Circuits