List of Topics

SfC Home > Physics > Electricity >

 

Basic Units of Electrical Circuits

by Ron Kurtus (9 December 2013)

A simple electrical 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:

This lesson will answer those questions. Useful tool: Units Conversion



Ampere

The ampere (A) is a basic SI unit of electrical 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 electrical 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.

Sine A is a basic SI unit, it is not expressed in terms of other units.

Volt

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.

Ohm

The ohm (Ω) is the unit of electrical 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

Electrical resistance is also a function of the cross section of the wire, as well as its temperature.

Summary

The ampere (A) is a basic SI unit consisting of the current per second passing a point in an electrical circuit. The volt (V) is the electrical 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 electrical resistance equal to the 1 volt divided by 1 ampere.


Excel in what you do


Resources and references

Ron Kurtus' Credentials

Websites

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

DC and AC Electricity Resources

Physics Resources

Books

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


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


Share

Click on a button to bookmark or share this page through Twitter, Facebook, email, or other services:

 

Students and researchers

The Web address of this page is:
www.school-for-champions.com/science/
electricity_basic_units.htm

Please include it as a link on your website or as a reference in your report, document, or thesis.

Copyright © Restrictions


Where are you now?

School for Champions

Physics topics

Basic Units of Electrical Circuits




Electricity topics

Static Electricity

Electrical Circuits

DC Electricity

AC Electricity

Also see



Let's make the world a better place

Be the best that you can be.

Use your knowledge and skills to help others succeed.

Don't be wasteful; protect our environment.

You CAN influence the world.





Live Your Life as a Champion:

Take care of your health

Seek knowledge and gain skills

Do excellent work

Be valuable to others

Have utmost character

Be a Champion!



The School for Champions helps you become the type of person who can be called a Champion.