What is the junction of copper and iron wire called when heated to produce a voltage?

The correct answer is a thermocouple. A thermocouple is a device that consists of two dissimilar metal wires, commonly copper and iron, joined at one end. When this junction is heated, it generates a voltage due to the Seebeck effect, which is a phenomenon where a voltage is produced in response to a temperature difference between the junction and the other ends of the wires. This makes thermocouples useful for temperature measurement and control in various applications, as they can convert thermal energy into electrical energy.

In contrast, thermoplastic refers to materials that become soft when heated and hard when cooled, which is not related to electrical properties. A resistor is a component that opposes the flow of electric current, but it does not produce voltage through heating dissimilar metals. A transistor is a semiconductor device used to amplify or switch electronic signals and is not directly associated with heating junctions of dissimilar metals to generate voltage. Each of these terms serves a different purpose within the realm of electrical and electronic components, highlighting the specific role of a thermocouple in generating voltage from heat.