Temperature
Overview
Temperature (온도, Temperature) is a physical quantity representing the average kinetic energy of molecular motion in a substance, quantitatively expressing the degree of hotness or coldness of an object. According to the zeroth law of thermodynamics, two bodies in thermal equilibrium have the same temperature, which forms the theoretical basis for temperature measurement. Temperature plays a key role not only in daily life but also in almost all natural science fields such as science, engineering, medicine, and meteorology.
Main Content
Definition and History of Temperature
The concept of temperature has existed since ancient Greek times, but modern temperature scales developed in the 17th–18th centuries. Galileo Galilei (1592) invented the first thermometer, and Fahrenheit (1724) and Celsius (1742) proposed temperature scales, respectively. Thermodynamic temperature was established as the concept of absolute temperature by Kelvin (1848), which is directly linked to molecular kinetic energy.
Temperature Measurement Units
- Celsius (°C): A common scale based on the freezing point (0°C) and boiling point (100°C) of water.
- Fahrenheit (°F): Based on the freezing point (32°F) and boiling point (212°F) of water, used in the United States and elsewhere.
- Kelvin (K): An absolute temperature scale, where 0 K is absolute zero (-273.15°C), where molecular motion ceases. Standard in scientific research.
- Rankine (°R): An absolute temperature scale based on Fahrenheit, where 0°R is absolute zero.
Principles and Instruments of Temperature Measurement
Temperature measurement utilizes temperature-dependent properties of materials (thermal expansion, electrical resistance, radiant energy, etc.).
- Mercury/Alcohol Thermometer: Uses thermal expansion of a liquid.
- Thermocouple: Measures thermoelectric voltage generated at the junction of two different metals.
- Resistance Temperature Detector (RTD): Based on the principle that the electrical resistance of a metal (e.g., platinum) changes with temperature.
- Thermistor: Uses resistance changes in semiconductors, offering high sensitivity.
- Infrared Thermometer: Measures infrared radiation emitted from an object for non-contact temperature measurement.
- Optical Pyrometer: Uses the color and radiance of an object at very high temperatures (e.g., furnaces).
Temperature and the Laws of Thermodynamics
- Zeroth Law of Thermodynamics: Defines the transitivity of thermal equilibrium, forming the basis for temperature measurement.
- First Law of Thermodynamics: Conservation of energy, the relationship between heat and work.
- Second Law of Thermodynamics: Increase in entropy, the direction of heat flow from high to low temperature.
- Third Law of Thermodynamics: The entropy of a perfect crystal at absolute zero is zero.
Temperature and States of Matter
Temperature determines the phase (solid, liquid, gas, plasma) changes of matter. For example, water freezes at 0°C and boils at 100°C. Above the critical temperature, a gas cannot be liquefied, and superconductivity occurs at specific low temperatures.
Temperature and Life
Living organisms can only survive within a specific temperature range. The normal human body temperature is about 36.5–37.5°C, and enzyme activity is sensitive to temperature. Extremophiles (thermophiles, psychrophiles) adapted to extreme environments (hydrothermal vents, Antarctica) also exist.
Temperature and Climate
The Earth's average temperature is about 15°C, and global warming due to increased greenhouse gases has become a serious environmental issue. Climate variability such as El Niño and La Niña is also closely related to changes in sea surface temperature.
Recent Trends
As of 2024–2025, temperature-related research and technology are developing in the following directions.
- Ultra-precise Temperature Measurement: Nanometer-scale temperature measurement technology using quantum sensors (nitrogen-vacancy centers, NV centers) is advancing, utilized for real-time mapping of temperature distribution within living organisms.
- Cryogenic Technology: Millikelvin (mK) level cryogenic cooling technology is being advanced for stabilizing qubits in quantum computers, and NASA's James Webb Space Telescope operates below 7 K.
- Climate Modeling: AI-based climate models predict regional temperature changes at high resolution, and 2024 is likely to be recorded as the hottest year on record (due to El Niño effects).
- Temperature Control Materials: Smart clothing and building materials using phase change materials (PCMs) are being commercialized to improve energy efficiency.
- Space Temperature Exploration: Development of heat- and cold-resistant materials for probes operating in extreme temperature environments on the Moon and Mars (lunar surface: 120°C during the day, -170°C at night) is active.
Related Topics
- [[Thermodynamics]]
- [[Absolute zero]]
- [[Thermal expansion]]
- [[Climate change]]
- [[Thermometer]]
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