Heat
Heat (Q) is energy transferred between a system and its surroundings because of a temperature difference, by conduction, convection or radiation. It is a process quantity, not a state function: a body possesses internal energy, not heat. Its SI unit is the joule; the calorie (4.184 J, thermochemical) is a historical unit.
Curious
For a long time, heat was thought to be an invisible fluid called “caloric” that flowed from hot to cold bodies. In 1798 Benjamin Thompson (Count Rumford) noticed that boring cannon barrels produced seemingly endless heat from friction, which a conserved fluid could not explain. In the 1840s James Joule showed that mechanical work and heat are interchangeable forms of energy (see First Law).
Heat travels in three ways: conduction (through direct contact, as along a metal spoon), convection (carried by moving fluid, as in boiling water) and radiation (as electromagnetic waves, as from the Sun). How much the temperature of a substance rises depends on its specific heat capacity; water's is unusually large, which is why oceans moderate climate.
Exploring
| Substance | Specific heat capacity (J kg−1 K−1, ~25 °C) |
|---|---|
| Water (liquid) | ~4,181 |
| Ice (near 0 °C) | ~2,100 |
| Aluminium | ~897 |
| Iron | ~449 |
| Copper | ~385 |
Deep Dive
Formal status. In the first law, ΔU = Q − W, heat and work are both modes of energy transfer, distinguished by mechanism: work involves macroscopic, organised displacement against a force, while heat involves disordered microscopic energy exchange driven by a temperature difference. Because δQ is an inexact differential, only for reversible processes can it be expressed as T dS (see Entropy).
Transfer laws. Conduction follows Fourier's law, q = −k∇T, with thermal conductivity k; in metals electrons carry most heat, linking thermal and electrical conductivity through the Wiedemann–Franz law (see Metals). Radiation from a black body follows the Stefan–Boltzmann law, P = σAT4 (see Red Giants for its use in stars). Convection depends on fluid flow and is usually treated with empirical correlations.
Heat capacity and quantum theory. Classical equipartition predicted that solids have molar heat capacity 3R (Dulong–Petit), which holds at room temperature for many elements but fails at low temperature; Einstein's 1907 quantum treatment and Debye's 1912 model explained the fall (see Third Law). Water's large heat capacity reflects hydrogen bonding, which stores energy in molecular rotations and bond rearrangements.
Concept Relationship Map
Requires: temperature, first law
Enables: specific heat, latent heat, heat transfer, entropy
Commonly confused with: temperature; internal energy
Sources
- Thompson, B. (1798). Philosophical Transactions of the Royal Society, 88, 80–102.
- Joule, J.P. (1850). Philosophical Transactions of the Royal Society, 140, 61–82.
- Schroeder, D.V. (2000). An Introduction to Thermal Physics. Addison-Wesley.