Final answer:
The response of a first-order system like a thermocouple to temperature fluctuations is characterized by exponential decay. Thermometers reach thermal equilibrium as described by the Zeroth Law of Thermodynamics.
Step-by-step explanation:
The characteristic response of a first-order system, such as a thermocouple immersed in a liquid to monitor its temperature fluctuations, is exponential decay in temperature fluctuations. This means that when exposed to a temperature gradient, the thermocouple will initially show a rapid change in temperature followed by a slower approach to the final temperature of the liquid. The rate of temperature change will decrease exponentially over time until thermal equilibrium is reached.
Thermometers, including thermocouples, reach thermal equilibrium with their surroundings due to the Zeroth Law of Thermodynamics. This law states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. Therefore, a thermometer measures the temperature of an object by actually measuring its own temperature once equilibrium has been established.