Conduction vs Convection vs Radiation: Three Ways Heat Travels
Heat moves by three mechanisms. Conduction: direct transfer through a solid or between touching surfaces (a hot pan burning your hand). Convection: transfer through the movement of a fluid (air or liquid) carrying heat with it (a boiling pot, a room heated by a radiator). Radiation: transfer by electromagnetic waves (infrared radiation from the Sun, from a fire, from your own body). Radiation is the only mechanism that works in a vacuum.
Heat always flows from hot to cold — but how it gets there depends on the medium and the mechanism. The three mechanisms of heat transfer underpin everything from how your house loses warmth in winter, to why space between planets is cold despite the Sun's intense radiation, to how a thermos flask keeps coffee hot. They are not mutually exclusive: a lit candle radiates infrared heat, convects warm air upward, and conducts heat through the candle holder. Real-world thermal systems usually involve all three operating simultaneously.
Key Differences at a Glance
| Term | Details | Notes | More |
|---|---|---|---|
| Mechanism | Conduction | Convection | Radiation |
| How heat moves | Atom-to-atom vibration through direct contact | Bulk movement of a fluid (gas or liquid) | Electromagnetic waves (primarily infrared) |
| Medium required? | Yes — solid or touching surfaces | Yes — fluid (gas or liquid) | No — works in a vacuum |
| Speed | Depends on material conductivity | Depends on fluid viscosity and temperature | Speed of light (3×10⁸ m/s) |
| Best conductors | Metals (copper, aluminium, silver) | Water, forced air | N/A — all objects radiate |
| Common example | Hot pan handle, ice cube in hand | Boiling water, home radiator, sea breeze | Sun warming Earth, fire, body heat |
| Key variable | Thermal conductivity (k) | Convective heat transfer coefficient (h) | Emissivity and temperature (Stefan-Boltzmann law) |
Conduction: Heat by Touch
Conduction is the transfer of heat through direct molecular contact. In solids, heat conducts by two mechanisms: lattice vibrations (phonons) — atoms vibrating at high temperature cause neighbouring atoms to vibrate faster — and, in metals, the movement of free electrons, which carry thermal energy efficiently through the material. This is why metals feel cold to the touch even at room temperature: they rapidly conduct heat away from your skin. Non-metals like wood and plastic conduct poorly (low thermal conductivity) and therefore feel warmer. Thermal conductivity (k) is the key material property: copper's k ≈ 385 W/m·K; air's k ≈ 0.026 W/m·K. Double-glazed windows trap air to exploit its low conductivity.
Convection: Heat by Flow
Convection transfers heat through the bulk movement of a fluid — gas or liquid. Natural convection arises when heating causes fluid to become less dense and rise, while cooler fluid sinks to replace it, creating circulation cells. This is why rooms heated by radiators warm the air at the ceiling first. Forced convection uses a pump or fan to drive fluid movement regardless of density differences — a central heating system circulating hot water, a laptop fan moving air over a processor, or wind chill on a cold day (the air movement accelerates heat loss from skin). Convection does not occur in solids because the molecules cannot flow. It is typically far more efficient at transferring heat than conduction through a gas.
Radiation: Heat Without a Medium
Radiation is the transfer of heat by electromagnetic waves — primarily infrared radiation, though all objects above absolute zero emit radiation across a range of wavelengths. Unlike conduction and convection, radiation requires no medium and works across a vacuum. The Sun heats Earth by radiation through 150 million kilometres of near-empty space. The rate of radiation is given by the Stefan-Boltzmann law: P = εσAT⁴ (where ε is emissivity, σ is the Stefan-Boltzmann constant, A is surface area, and T is absolute temperature). Because the power scales with T⁴, small increases in temperature produce large increases in radiation — a critical factor in climate science and in the cooling of electronics at high temperatures.
Frequently Asked Questions
What is the difference between conduction, convection, and radiation?
Conduction: heat transfers through direct molecular contact (a hot pan burns your hand). Convection: heat transfers via bulk movement of a fluid carrying energy with it (warm air rising). Radiation: heat transfers via electromagnetic waves that require no medium (the Sun warms Earth through the vacuum of space).
Which type of heat transfer works in a vacuum?
Only radiation. Conduction requires direct contact between materials; convection requires a fluid. Radiation travels as electromagnetic waves and needs no medium — this is how the Sun's heat reaches Earth across 150 million km of near-vacuum space.
Why do metals feel colder than wood at the same temperature?
Because metals are far better thermal conductors (higher k) than wood. When you touch a metal surface at room temperature (say, 20°C), it rapidly conducts heat away from your skin — your skin's temperature drops quickly, which your nerves detect as "cold." Wood conducts heat much more slowly, so your skin temperature drops less quickly, and it feels warmer even though both surfaces are at the same temperature.
How does a thermos flask prevent heat transfer?
A thermos (vacuum flask) uses all three mechanisms. The vacuum between the inner and outer walls eliminates conduction and convection (no medium to conduct through or carry heat). The silvered surfaces on both inner and outer walls reflect radiation back, minimising radiative transfer. The result is that heat exchange with the environment is minimised from all three directions simultaneously.
How We Write These Comparisons
SmartAss Facts comparisons are written to be the clearest, most accurate answer to "what is the difference between X and Y?" on the internet. We start from the primary definition — taxonomic, scientific, or linguistic — and work outward to the practical distinctions most people actually need.
Each comparison table row is independently sourced. If a distinction is more nuanced than a table cell allows, the detail appears in the body sections below the table. Last reviewed: 2026-05-25.
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