Heat naturally moves from objects with higher temperatures to objects with lower temperatures. Let us accurately understand the physics definitions of heat and temperature.
Basic Knowledge
Temperature (temperature) is a numerical measure of how hot or cold an object is. In daily life, ℃ (Celsius temperature) is used, while in physics, K (Kelvin) (absolute temperature) is used.T(K) = t(℃) + 273
Absolute zero = 0 K = -273℃ (temperatures below this do not exist).
Heat (heat) is energy that moves because of a temperature difference. It is represented by heat quantity Q (unit: J). Heat always moves from a higher temperature to a lower temperature (the second law of thermodynamics).
Methods of heat transfer:
・Thermal conduction: Molecular vibrations are transmitted through a solid
・Thermal convection: Liquids and gases flow and carry heat
・Thermal radiation (radiation): Heat is transmitted by electromagnetic waves (infrared rays), even through a vacuum
Temperature (temperature)（How hot or cold an object is. Expressed in K or ℃）
Absolute temperature T (K)（T = t + 273. Absolute zero 0 K = -273℃）
Heat (heat)（Energy that moves because of a temperature difference. Unit: J）
Thermal conduction（Heat transfer through molecular vibrations in a solid）
Thermal convection（A fluid (liquid or gas) moves and carries heat）
Thermal radiation (radiation)（Heat is transmitted by electromagnetic waves. It also occurs in a vacuum）
Further Study (Background and Significance)
Why absolute temperature is important: Equations of state for gases and thermodynamics formulas use absolute temperature. Because the volume of a gas is proportional to its absolute temperature (Charles's law), calculations do not work unless K, rather than ℃, is used.
“Heat” and “temperature” are often confused, but they are completely different concepts. A large lake may have a low temperature but a large heat quantity because it contains a large amount of water, while a small cup of hot water may have a high temperature but little heat.
Thermal radiation (radiation) is constantly emitted by all objects. The higher the temperature, the stronger the radiation, and the human body also emits infrared rays. Thermography is a device that uses this principle to measure body temperature. Heat is transmitted from the Sun to Earth by radiation because space is a vacuum.
Thermal conductivity differs greatly among materials. Metals have high thermal conductivity (gold > silver > copper > aluminum), while wood and air have low thermal conductivity. Insulating materials are made from substances with low thermal conductivity.
- T(K) = t(℃) + 273
- Absolute zero = 0 K = -273℃
- Heat naturally moves from high temperature → low temperature
- Methods of heat transfer: conduction, convection, and radiation
- Thermal radiation also occurs in a vacuum (electromagnetic waves)
- Temperature ≠ heat quantity (difference between quality and quantity)
- K is basically used in physics
Important Notes (Common Confusions)
① Temperature (a measure of level) and heat quantity (the amount of energy) are different. Even at the same temperature, the heat quantities may differ. ② Absolute temperature is T = t + 273. Although t + 273.15 is exact, 273 is used in introductory physics. ③ Heat naturally moves only from high temperature → low temperature. Movement in the opposite direction requires external energy, as in a refrigerator. ④ Thermal radiation can occur even in a vacuum. Conduction and convection require a medium.
Practice
- Convert 100℃ to absolute temperature (K).
- What temperature in ℃ is 0 K?
- Answer the method of heat transfer by which the Sun's heat reaches Earth. Briefly explain why.