Editing Resistor (section)

===Power dissipation===
[[File:Resistor warming thermal video.webm|thumb|Resistor warming caused by electrical current captured by thermal camera]]
At any instant, the power ''P'' (watts) consumed by a resistor of resistance ''R'' (ohms) is calculated as:
<math display="block">
P = I V = I^2 R =  \frac{V^2}{R}
</math>
where ''V'' (volts) is the voltage across the resistor and ''I'' (amps) is the [[Ampere|current]] flowing through it. Using [[Ohm's law]], the two other forms can be derived. This power is converted into heat which must be dissipated by the resistor's package before its temperature rises excessively.<ref name=arrl1968 />{{rp|p.22}}

Resistors are rated according to their maximum power dissipation.  Discrete resistors in solid-state electronic systems are typically rated as {{frac|10}}, {{frac|8}}, or {{frac|4}} watt. They usually absorb much less than a watt of electrical power and require little attention to their power rating.

[[File:Danotherm HS50 power resistor.jpg|thumb|An aluminium-encased power resistor rated for dissipation of 50 W when mounted on a heat-sink]]
Power resistors are required to dissipate substantial amounts of power and are typically used in power supplies, power conversion circuits, and power amplifiers; this designation is loosely applied to resistors with power ratings of 1 watt or greater. Power resistors are physically larger and may not use the preferred values, color codes, and external packages described below.

If the average power dissipated by a resistor is more than its power rating, damage to the resistor may occur, permanently altering its resistance; this is distinct from the reversible change in resistance due to its [[temperature coefficient]] when it warms. Excessive power dissipation may raise the temperature of the resistor to a point where it can burn the circuit board or adjacent components, or even cause a fire. There are flameproof resistors that will not produce flames with any overload of any duration.

Resistors may be specified with higher rated dissipation than is experienced in service to account for poor air circulation, high altitude, or high [[operating temperature]].

All resistors have a maximum voltage rating; this may limit the power dissipation for higher resistance values.<ref>{{cite web |url=https://seielect.com/news/20170821_-_Resistor_Data_Sheet_Information.pdf |title=Specifications and How to Interpret Them|publisher= Stackpole Electronics|access-date=July 6, 2021}}</ref> For instance, among {{frac|4}} watt resistors (a very common sort of [[through-hole technology|leaded]] resistor) one is listed with a resistance of 100&nbsp;MΩ<ref>{{Cite web |title=Through Hole Resistor, 0.1 Gohm, RGP Series, 250 mW, ± 5%, Axial Leaded, 750 V |url=https://nl.farnell.com/te-connectivity/rgp0207chj100m/res-100m-5-250mw-axial-thick-film/dp/2805251 |url-status=dead |archive-url=https://web.archive.org/web/20210709190647/https://nl.farnell.com/te-connectivity/rgp0207chj100m/res-100m-5-250mw-axial-thick-film/dp/2805251 |archive-date=2021-07-09 |access-date=2023-10-07 |website=nl.farnell.com}}</ref> and a maximum rated voltage of 750&nbsp;V. However even placing 750&nbsp;V across a 100&nbsp;MΩ resistor continuously would only result in a power dissipation of less than 6&nbsp;mW, making the nominal {{frac|4}} watt rating meaningless.

[[File:USSR power resistor VZR 12W.JPG|thumb|VZR power resistor 1.5&nbsp;kΩ 12&nbsp;W, manufactured in 1963 in the Soviet Union]]