Physicists use absolute zero instead of the concept of "cold" to define the temperature scale in thermodynamics because absolute zero represents a well-defined, fundamental reference point that can be universally understood and measured.
Temperature is a measure of the average kinetic energy of particles in a substance. It is difficult to define "cold" in an absolute sense because it is a subjective perception related to the absence or decrease of heat. Different materials and systems have different thresholds for what is considered cold, making it a relative and subjective concept.
On the other hand, absolute zero is defined as the lowest possible temperature that a system can reach, where the particles have minimal or no kinetic energy. It is denoted as 0 Kelvin (K) or -273.15 degrees Celsius (°C) on the Celsius scale. At absolute zero, all molecular motion theoretically ceases. This reference point provides a clear and universally applicable starting point for measuring temperatures.
Using absolute zero as the foundation of the temperature scale allows for a consistent and objective measurement system. It provides a fixed reference point that enables accurate comparisons and calculations in thermodynamics and other branches of physics. Additionally, the Kelvin scale, which is based on absolute zero, is widely used in scientific research and engineering because it avoids negative temperatures and simplifies certain calculations involving temperature differentials and energy transfers.
In summary, physicists use absolute zero instead of "cold" to define the temperature scale because it provides a well-defined, universally applicable reference point for measuring and comparing temperatures, allowing for objective and consistent analysis in thermodynamics and other scientific disciplines.