What is resistance? - Definition from WhatIs.com
Part of the Electronics glossary:

Also see current, voltage, and Ohm's Law.

Resistance is the opposition that a substance offers to the flow of electric current.  It is represented by the uppercase letter R.  The standard unit of resistance is the ohm, sometimes written out as a word, and sometimes symbolized by the uppercase Greek letter omega.  When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt exists, then the resistance of that component is one ohm.

In general, when the applied voltage is held constant, the current in a direct-current (DC) electrical circuit is inversely proportional to the resistance.  If the resistance is doubled, the current is cut in half; if the resistance is halved, the current is doubled.  This rule also holds true for most low-frequency alternating-current (AC) systems, such as household utility circuits.  In some AC circuits, especially at high frequencies, the situation is more complex, because some components in these systems can store and release energy, as well as dissipating or converting it.

The electrical resistance per unit length, area, or volume of a substance is known as resistivity.   Resistivity figures are often specified for copper and aluminum wire, in ohms per kilometer.

Opposition to AC, but not to DC, is a property known as reactance.  In an AC circuit, the resistance and reactance combine vectorially to yield impedance.

This was last updated in April 2005
Posted by: Margaret Rouse

Related Terms

Definitions

  • aluminum battery

    - Aluminum batteries run longer on a charge than lithium ion batteries and are safer in use. Stanford researchers have created an aluminum-graphite battery for mobile devices, the electric grid and f... (WhatIs.com)

  • optoisolator (optical coupler or optocoupler)

    - An optoisolator (also optical coupler or optocoupler) is a component that uses light to transfer electrical signals between circuits to keep them electrically isolated from each other, preventing s... (SearchNetworking.com)

  • ampere hour (Ah or amp hour)

    - An ampere hour (abbreviated Ah, or sometimes amp hour) is the amount of energy charge in a battery that will allow one ampere of current to flow for one hour. (WhatIs.com)

Glossaries

  • Electronics

    - Terms related to electronics, including definitions about electrical components and words and phrases about computers, laptops parts, digital cameras, televisions and home appliances.

  • Internet applications

    - This WhatIs.com glossary contains terms related to Internet applications, including definitions about Software as a Service (SaaS) delivery models and words and phrases about web sites, e-commerce ...

Ask a Question. Find an Answer.Powered by ITKnowledgeExchange.com

Ask An IT Question

Get answers from your peers on your most technical challenges

Ask Question

Tech TalkComment

Share
Comments

    Results

    Contribute to the conversation

    All fields are required. Comments will appear at the bottom of the article.