Learn about heat

Understand physical heat and the things we experience as heat

Start with temperature and heat transfer, then follow the science into materials, buildings, electronics, cooking, spicy food, the human body, vehicles, Earth, space and the physiological experiences behind everyday uses of the word heat.

Fundamentals

What Is Heat?

Heat is energy transferred across a system boundary because of a temperature difference. It is a transfer process rather than a substance stored inside an object.

Heat vs Temperature

Temperature describes thermal state. Heat describes energy transferred because of a temperature difference. The two quantities are related, but they are not interchangeable.

Thermal Equilibrium and Why Temperatures Settle

Two systems are in thermal equilibrium when they can exchange energy but no net heat transfer occurs between them because their temperatures are equal.

Heat Flux: Heat Transfer Per Unit Area

Heat flux expresses the rate of thermal energy transfer through a unit area, usually in watts per square metre.

Latent Heat and Phase Change

Latent heat is the energy absorbed or released during a phase change without a corresponding temperature change under the idealized constant-pressure condition.

Newton's Law of Cooling

Newton’s law of cooling models convective heat transfer as proportional to the temperature difference between a surface and its surroundings.

How heat moves

Conduction: Heat Transfer Through Matter

Conduction transfers thermal energy through matter because neighbouring regions are at different temperatures.

Convection: How Heat Moves Through Fluids

Convection transfers heat between a surface and a moving fluid. The motion may arise naturally from buoyancy or be driven by a fan, pump or other external force.

Thermal Radiation

Thermal radiation transfers energy through electromagnetic waves emitted by matter because of its temperature.

The Stefan-Boltzmann Law

The Stefan-Boltzmann law states that ideal blackbody radiant emission is proportional to the fourth power of absolute temperature.

Blackbody Radiation and Thermal Emission

A blackbody is an ideal surface that absorbs all incident electromagnetic radiation and emits the maximum possible thermal radiation for its temperature.

Emissivity and Thermal Radiation

Emissivity describes how effectively a real surface emits thermal radiation compared with an ideal blackbody at the same temperature.

Material properties

Thermal Conductivity

Thermal conductivity describes a material’s ability to conduct heat in response to a temperature gradient.

Specific Heat Capacity

Specific heat capacity is the energy required per unit mass to change a material’s temperature by one degree under specified conditions.

Thermal Resistance

Thermal resistance expresses how strongly a material layer, interface or thermal path opposes heat transfer.

Thermal Diffusivity

Thermal diffusivity compares a material’s ability to conduct heat with its ability to store thermal energy.

Thermal Expansion in Solids

Most solids change dimensions when temperature changes because average atomic spacing changes with thermal energy.

Thermal Mass: How Materials Store Heat

Thermal mass describes a material or assembly’s capacity to absorb, store and later release thermal energy.

Buildings and energy

How Insulation Slows Heat Transfer

Insulation reduces unwanted heat transfer by creating layers that resist conduction and, depending on the system, also suppress convection or radiation.

U-Value and R-Value in Building Heat Transfer

R-value measures thermal resistance, while U-value measures thermal transmittance. For a complete assembly using consistent units, U is the reciprocal of total R.

Thermal Bridging in Walls and Building Envelopes

A thermal bridge is a region where heat can bypass the more resistive parts of an assembly through a more conductive path.

How Buildings Lose Heat

Buildings lose heat through their envelope and through air exchange. The rate depends on temperature difference, area, thermal transmittance and airflow.

Room Heating Load: What a Simple Estimate Can and Cannot Tell You

A room heating-load estimate begins with heat escaping through exposed surfaces and air exchange under a chosen indoor-outdoor design condition.

Dew Point, Humidity and Condensation

Dew point is the temperature to which air must be cooled, at approximately constant pressure and water-vapour content, for saturation to occur.

How Heat Pumps Move Heat

A heat pump uses work to move thermal energy from a colder source to a warmer sink rather than converting electricity directly into heat at the point of use.

Solar Heat Gain Through Windows

Solar heat gain through glazing depends on incident solar energy, glazed area and how much of that energy the glazing system admits indoors.

Heat Loss From Insulated Pipes

Heat moving radially through pipe insulation follows cylindrical geometry, so resistance depends on the logarithm of outer-to-inner radius.

Electronics and thermal design

Junction Temperature and Electronics Cooling

Junction temperature is the temperature at the active semiconductor junction inside a device. It depends on power dissipation and the thermal path from the junction to the surroundings.

Heat Sinks and Thermal Resistance

A heat sink increases the area available for heat transfer from a component to the surrounding fluid, usually air.

Thermal Interface Materials

Thermal interface materials reduce the thermal resistance created by microscopic air gaps between contacting solid surfaces.

Thermal Resistance Networks

Thermal resistance networks turn complex heat-flow paths into combinations of temperature differences and resistances when the simplifying assumptions are appropriate.

Heat Exchangers and Energy Balance

A heat exchanger transfers thermal energy between fluid streams while usually keeping the fluids physically separate.

Electrical Heating and Joule Heating

Joule heating occurs when electrical energy is dissipated as thermal energy in a resistive conductor or component.

Human and environmental heat

Humidity, Evaporation and Cooling

Evaporation removes heat because liquid water requires substantial latent energy to become vapour. The surrounding air’s humidity affects how readily that vapour can be accepted.

Evaporative Cooling and Wet-Bulb Temperature

Direct evaporative cooling lowers air dry-bulb temperature by using sensible heat from the air to evaporate water, increasing humidity in the process.

Thermal Comfort: More Than Air Temperature

Thermal comfort depends on the balance between the body’s heat production and heat exchange with the environment, not on air temperature alone.

Heat Index: What It Represents and What It Does Not

Heat index combines air temperature and relative humidity to estimate how hot conditions may feel under specific assumptions used by the National Weather Service.

WBGT and Occupational Heat Stress

Wet-Bulb Globe Temperature combines measurements related to humidity, radiant heat and air temperature to support heat-stress assessment in work and activity settings.

Biological Heat: Metabolism and Thermoregulation

Living organisms generate heat through metabolism and regulate temperature by changing heat production, blood flow and heat exchange with the environment.

Food and cooking heat

How Heat Moves Through Food During Cooking

Cooking combines heat transfer at the food surface with conduction and moisture movement inside the food. Different cooking methods change the boundary conditions rather than eliminating the underlying physics.

How Cookware Materials Affect Heat Transfer

Cookware behaviour depends on the complete pan construction, including thermal conductivity, thickness, heat capacity, geometry and how heat enters from the hob or flame.

Why Water Boils at a Lower Temperature at High Altitude

Boiling occurs when a liquid’s vapour pressure reaches the surrounding pressure. Lower atmospheric pressure at higher altitude therefore lowers water’s boiling temperature.

Heat, Browning and the Maillard Reaction

The Maillard reaction is a network of reactions involving reducing sugars and amino compounds that contributes to browning and flavour development during many forms of cooking.

Spicy heat and heat perception

The Scoville Scale and the Science of Spicy Heat

Scoville Heat Units describe chilli pungency rather than temperature. Capsaicin and related capsaicinoids create a burning sensation by activating sensory pathways that also respond to noxious heat.

Why Chilli Peppers Feel Hot

Chilli feels hot because capsaicin stimulates heat- and pain-sensitive sensory neurons. The sensation resembles thermal burning even though capsaicin does not make the food physically hotter.

Heat Perception: Why Temperature and Sensation Can Disagree

What feels hot or cold depends on the rate and location of heat exchange and on sensory signalling, so perceived temperature does not always track a thermometer reading directly.

Why Metal Feels Colder Than Wood at the Same Temperature

Metal often feels colder because it can draw heat from warm skin faster than wood under the same conditions, causing the skin at the contact point to cool more rapidly.

Mechanical, chemical and combustion heat

Frictional Heating: Why Rubbing Surfaces Get Hot

Frictional heating occurs when mechanical energy is dissipated into internal energy as surfaces slide, deform or interact at microscopic contact points.

Chemical Heat: Exothermic and Endothermic Reactions

Chemical reactions can release thermal energy to their surroundings or absorb thermal energy from them, depending on the energy difference between reactants and products.

Combustion Heat: Flames, Energy Release and Temperature

Combustion can release chemical energy rapidly, producing hot gases and transferring heat by convection and radiation.

Automotive Heat: Engines, Brakes, Batteries and Cooling Systems

Vehicles generate and move heat through combustion or electrical losses, friction, braking, batteries, motors and environmental exposure.

Earth and space heat

Geothermal Heat: Where Earth’s Internal Heat Comes From

Earth’s interior remains hot because of residual heat from planetary formation and continuing heat production from radioactive decay, among other processes.

Heat in Space: Stars, Planets and Spacecraft

In space, thermal radiation becomes central because convection is absent in vacuum, while spacecraft also generate internal heat and absorb radiation from the Sun and planets.

Emotion and physiological warmth

Why Strong Emotions Can Make You Feel Hot

Feeling hot during a strong emotion can reflect changes in autonomic activity, skin blood flow and sweating. The expression has a physiological basis in some situations, but emotional “heat” is not a thermodynamic quantity.

Blushing and the Sensation of Facial Warmth

Blushing involves increased blood flow in superficial facial skin and can produce visible redness and a sensation of warmth during social or emotional arousal.