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.
Key Formula
Q = m × c × ΔT
LaTeX: Q = mc\Delta T
| Symbol | Meaning | Unit |
|---|---|---|
| Q | Heat transferred | J |
| m | Mass of the substance | kg |
| c | Specific heat capacity | J kg⁻¹ K⁻¹ |
| ΔT | Change in temperature | K or °C |
Worked Example
Problem
How much heat is needed to raise the temperature of 2 kg of water from 20 °C to 80 °C? (Specific heat capacity of water = 4186 J kg⁻¹ K⁻¹)
Solution
Step 1: Identify variables: m = 2 kg, c = 4186 J kg⁻¹ K⁻¹, ΔT = 80 - 20 = 60 °C = 60 K. Step 2: Apply Q = mcΔT. Q = 2 × 4186 × 60 = 502 320 J.
Answer
Q = 502 320 J ≈ 502.3 kJ
Methods of Heat Transfer
| Method | Mechanism | Medium Required | Example |
|---|---|---|---|
| Conduction | Molecular vibration / electron diffusion | Yes (solid/liquid) | Metal rod heated at one end |
| Convection | Bulk fluid movement | Yes (fluid) | Boiling water, sea breeze |
| Radiation | Electromagnetic waves | No (vacuum OK) | Sunlight, infrared heaters |
Interactive Tools
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Related Terms
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.
Specific Heat Capacity
Specific heat capacity (symbol c) is the amount of heat energy required to raise the temperature of one kilogram of a substance by one degree Kelvin (or one degree Celsius). It is an intrinsic material property that reflects how strongly a substance resists temperature change when heat is added or removed. Water has an exceptionally high specific heat capacity (4186 J kg⁻¹ K⁻¹), which makes it an effective thermal buffer in climatic systems, industrial cooling, and biological organisms.
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.
From Old English "hætu" and Proto-Germanic "*haitiz" meaning warmth. The thermodynamic concept was formalised by Joseph Black in the 1760s, who distinguished heat from temperature and discovered latent heat. The caloric theory (18th century) was replaced by the kinetic theory through the work of Joule and Clausius in the mid-19th century.