The Effects of Temperature on Resistance

Electrons

• The characteristic of a conductive material that makes its resistance low is the presence of free electrons in its atomic structure. These electrons are able to migrate in response to an externally applied electric field to produce electric current.

Conductors and Temperature

• The normal random motion of the electrons tends to interfere with current flow and this motion is proportional to the amount of energy in the atoms which relates directly to the ambient temperature of the material. Thus, at lower temperatures most conductors have less resistance as the temperature rises. This is true for all common metals used for electrical components and wire such as copper, silver, aluminum and gold.

Semiconductors

• Semiconductors are a class of materials that only partially conduct electricity because they have far fewer electrons available for conducting electric current. Although the motion of their electrons responds to temperature in the same way metals do, this factor is not as significant as the large increase in the number of free electrons as they get hotter. The resistance of these materials, therefore, goes down instead of up as the temperature increases. Common semiconductor materials are the silicon and germanium found in transistors and integrated circuits, but carbon also exhibits this characteristic.

Temperature Coefficient

• Most materials exhibit a fairly linear change in resistance vs. temperature that engineers describe with the constant called the temperature coefficient. It is positive in good conductors because the resistance rises with the temperature, and in semiconductor materials whose resistance changes in the opposite direction the coefficient is negative.

Practical Uses

• Many electric circuit elements and the wires that connect them are made of metals with positive coefficients. To counteract the changes in operating characteristics as they heat up and keep the circuits operating correctly, they contain devices called thermistors with negative temperature coefficients to counteract this effect. Thermistors also serve as the sensing elements in electronic thermometers because of their predictable change in resistance as the temperature varies.

Superconductivity

• When the temperature of a conductor nears absolute zero it reaches a state of superconductivity where its resistance theoretically becomes zero. Researchers are very interested in this phenomenon as it suggests the possibility of electrical energy storage and transmission completely without losses.