Thermocouple Working Principle

A thermocouple is a device used for measuring temperature. It works on the principle of the Seebeck effect, which is a thermoelectric phenomenon. This effect states that when two different electrical conductors or semiconductors are joined at two junctions, and these two junctions are maintained at different temperatures, a voltage (electromotive force or EMF) is generated across the junctions.

In the given scenario, when heat is applied to the joined ends of the wires of a thermocouple, it means one junction is heated while the other is typically kept at a reference temperature (or at least at a different temperature). This temperature difference across the two junctions causes a small voltage to be generated.

Seebeck Effect and Voltage Generation

The Seebeck effect is fundamental to how a thermocouple operates. Here's a breakdown of what happens:

• A thermocouple consists of two wires made of dissimilar metals (e.g., copper and constantan, or iron and constantan) joined together at two points, forming two junctions.
• When heat is applied to one of these junctions (called the 'hot junction' or 'measuring junction') and the other junction (called the 'cold junction' or 'reference junction') is kept at a different temperature, a temperature gradient is established.
• Due to this temperature gradient, electrons in the metals at the hotter junction gain more thermal energy and diffuse towards the colder junction. Since the two metals are dissimilar, they have different rates of electron diffusion and different electron densities, leading to a net flow of charge.
• This net flow of charge creates an electric potential difference, or a small voltage, across the open ends of the thermocouple wires. The magnitude of this voltage is directly proportional to the temperature difference between the two junctions.
• The generated voltage can then be measured by a voltmeter, and this voltage reading can be correlated with the temperature difference to determine the unknown temperature.

Analyzing the Options

Let's examine why the other options are incorrect when heat is applied to the joined ends of the wires of a thermocouple:

• The wire contract: Heating generally causes materials to expand, not contract, due to increased thermal energy and molecular vibrations. Contraction upon heating is not a typical physical property, especially in the context of thermocouple function.
• The wires start to rotate: There is no physical principle or phenomenon by which applying heat to the joined ends of static wires would cause them to rotate. This is not related to thermoelectric effects or material properties.
• The wires separate: While extreme heat could melt or damage the wires, the normal operation of a thermocouple involves applying heat within its working range. Within this range, the wires are designed to remain joined to maintain the circuit and generate voltage; they do not separate. Separation would break the circuit and stop the voltage generation.

Therefore, based on the fundamental principle of the Seebeck effect and the operation of a thermocouple, the most accurate outcome when heat is applied to its joined ends is the generation of a small voltage.

The voltage ($\Delta V$) generated by a thermocouple can be approximated by the formula:

\(\Delta V = \alpha \cdot \Delta T\)

Where:

• \(\Delta V\) is the generated voltage.
• \(\alpha\) is the Seebeck coefficient (also known as thermoelectric sensitivity), which is specific to the pair of materials used.
• \(\Delta T\) is the temperature difference between the hot and cold junctions.