Temperature
Temperature is a scalar physical quantity that measures the average kinetic energy of the particles in a substance, indicating how hot or cold the substance is. It is a fundamental thermodynamic property that determines the direction of heat flow between objects in thermal contact — heat always flows from a higher-temperature body to a lower-temperature body. Temperature is measured using thermometers and is expressed in units of Kelvin (SI), Celsius, or Fahrenheit, and it plays a central role in all thermodynamic processes including phase transitions, chemical reactions, and heat engines.
Key Formula
T(Kelvin) = T(Celsius) + 273.15
LaTeX: T_{K} = T_{C} + 273.15
| Symbol | Meaning | Unit |
|---|---|---|
| T_K | Temperature in Kelvin | K |
| T_C | Temperature in Celsius | °C |
Worked Example
Problem
Convert a body temperature of 37 °C to Kelvin and to Fahrenheit.
Solution
Step 1: Convert to Kelvin using T_K = T_C + 273.15. T_K = 37 + 273.15 = 310.15 K. Step 2: Convert to Fahrenheit using T_F = (T_C × 9/5) + 32. T_F = (37 × 1.8) + 32 = 66.6 + 32 = 98.6 °F.
Answer
310.15 K and 98.6 °F
Comparison of Temperature Scales
| Reference Point | Kelvin (K) | Celsius (°C) | Fahrenheit (°F) |
|---|---|---|---|
| Absolute Zero | 0 | -273.15 | -459.67 |
| Freezing point of water | 273.15 | 0 | 32 |
| Normal body temperature | 310.15 | 37 | 98.6 |
| Boiling point of water | 373.15 | 100 | 212 |
| Surface of the Sun (approx.) | 5778 | 5505 | 9941 |
Interactive Tools
PhET Energy Forms and Changes
Interactive simulation showing how temperature relates to particle motion and heat transfer.
Open ToolKhan Academy: Temperature and Heat
Conceptual introduction to temperature, heat, and thermal equilibrium.
Open ToolWolframAlpha Temperature Converter
Instantly convert between Kelvin, Celsius, and Fahrenheit.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Heat (thermodynamics)
Heat is the transfer of thermal energy between two objects or systems due to a temperature difference; it always flows spontaneously from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached. Unlike temperature (a state property), heat is a process quantity — it only exists as energy in transit, not stored within a body. Heat transfer occurs via three mechanisms: conduction (direct molecular contact), convection (fluid movement), and radiation (electromagnetic waves), and it is measured in joules (J) in the SI system.
Thermal Energy
Thermal energy is the total internal kinetic energy associated with the random translational, rotational, and vibrational motion of all particles (atoms and molecules) within a substance. It is a state property stored within a system, proportional to both the temperature and the number of particles present. Thermal energy is the source of heat flow when a temperature difference exists, and it underpins all thermodynamic processes including phase changes, chemical reactions, and the operation of heat engines.
Ideal Gas Law
The Ideal Gas Law is an equation of state for a hypothetical ideal gas, combining the empirical gas laws of Boyle, Charles, and Gay-Lussac into a single relationship between the pressure, volume, amount, and absolute temperature of a gas. It assumes gas molecules have negligible volume and no intermolecular forces, making it an excellent approximation for real gases at low pressures and high temperatures. It is foundational to thermodynamics, chemistry, and engineering, used in everything from weather balloon calculations to industrial gas storage and the analysis of respiratory physiology.
From Latin "temperatura" meaning "a mixing in due proportion", derived from "temperare" (to mix, moderate). The concept was quantified by Galileo Galilei around 1593 with the first thermoscope, and formal temperature scales were introduced in the 18th century by Fahrenheit (1724), Celsius (1742), and Kelvin (1848).