What occurs at the junction of a thermocouple when heat is applied?

When heat is applied at the junction of a thermocouple, voltage generation occurs due to the Seebeck effect. This phenomenon arises from the fact that a thermocouple is made of two different metals joined at one end, known as the measuring or hot junction. When this junction experiences a temperature difference relative to the other junction (the cold junction), it generates a small voltage that is proportional to that temperature difference.

The underlying principle is that the different metals have different electron mobility and thermoelectric properties. When one junction is heated, it causes charge carriers in each metal to move differently, resulting in an electromotive force (EMF) or voltage. This voltage can then be measured and is indicative of the temperature at the hot junction.

In this context, it is important to note that the other choices do not represent the correct phenomena associated with thermocouples under heat. The generation of a voltage is a key principle in their operation, making it central to their application in temperature measurement.