<P> where P is the power (energy per unit time) converted from electrical energy to thermal energy, R is the resistance, and I is the current through the resistor . </P> <P> Near room temperature, the resistivity of metals typically increases as temperature is increased, while the resistivity of semiconductors typically decreases as temperature is increased . The resistivity of insulators and electrolytes may increase or decrease depending on the system . For the detailed behavior and explanation, see Electrical resistivity and conductivity . </P> <P> As a consequence, the resistance of wires, resistors, and other components often change with temperature . This effect may be undesired, causing an electronic circuit to malfunction at extreme temperatures . In some cases, however, the effect is put to good use . When temperature - dependent resistance of a component is used purposefully, the component is called a resistance thermometer or thermistor . (A resistance thermometer is made of metal, usually platinum, while a thermistor is made of ceramic or polymer .) </P> <P> Resistance thermometers and thermistors are generally used in two ways . First, they can be used as thermometers: By measuring the resistance, the temperature of the environment can be inferred . Second, they can be used in conjunction with Joule heating (also called self - heating): If a large current is running through the resistor, the resistor's temperature rises and therefore its resistance changes . Therefore, these components can be used in a circuit - protection role similar to fuses, or for feedback in circuits, or for many other purposes . In general, self - heating can turn a resistor into a nonlinear and hysteretic circuit element . For more details see Thermistor #Self - heating effects . </P>

A resistance of 20k ohm has a conductance of