[L1] 1 Overview [L2] 1) Definition [L4] - Broadly refers to a temperature range below 90K, narrowly below 4.2K. [L5] * Based on the boiling point of liquid oxygen (90K). [L5] * Based on the boiling point of liquid helium (4.2K). [L2] 2) Features [L4] - Quantum effects can be observed macroscopically without interference from thermal motion. [L5] * Thermal motion of atoms almost disappears, and quantum effects obscured by thermal motion are observed as macroscopic physical phenomena. [L4] - Phenomena not seen at room temperature, such as superfluidity and superconductivity, occur. [L5] * Superfluidity: A liquid state with zero viscosity and surface tension, where no vortices exist when flowing. [L1] 2 Realization of Cryogenics [L2] 1) Principle [L4] - Generally uses adiabatic expansion to cool and liquefy gas. However, efficiency deteriorates as it approaches absolute zero. [L4] - Generally, cryogenics creates low temperatures by liquefying helium using the Joule-Thomson effect. [L4] - The adiabatic demagnetization method is effective for conditions below 4.2K. [L2] 2) Method: Joule-Thomson Effect [L4] - Utilizes the phenomenon where temperature changes when compressed gas is ejected through a narrow insulated hole (nozzle) (Throttling process). [L5] * Refrigeration System: Uses the principle where liquid refrigerant expands and turns into gas while passing through a nozzle, lowering the temperature. [L5] * Hydrogen temperature rises, but other gases cool down. [L5] * Heat in an object is generated by collisions of molecules; compressing induces high collisions, and rapid expansion lowers the temperature. [L4] - Used for cooling or liquefying gases like helium and cooling air conditioner refrigerants. [L4] - Joule-Thomson Coefficient: Used as an indicator to quantitatively represent the Joule-Thomson effect. It is calculated by the following formula: [L5] * µ = (∂T/∂P)_H [L5] µ : Joule-Thomson Coefficient [L5] T : Temperature [L5] P : Pressure [L5] H : Enthalpy [L5] * If the Joule-Thomson coefficient is positive, the temperature decreases; if negative, the temperature increases.