On a cold morning, a metal railing feels icy while a wooden bench at the same temperature feels only cool. A spoon in hot soup grows warm all the way to the handle. The Sun heats your face from 93 million miles away, through the emptiness of space. Everyday experiences like these are explained by the physics of heat and temperature. The two words are often used as if they meant the same thing, but they describe different ideas. This explainer sets out what heat and temperature really are, how heat travels from one place to another and why our sense of hot and cold can be misleading.
Everything is made of moving particles
All matter is made of atoms and molecules that are constantly moving. In a solid, they vibrate around fixed positions. In a liquid, they move past each other while staying close together. In a gas, they fly around freely, bouncing off each other and the walls of their container.
The energy of this motion is called thermal energy. The faster the particles move on average, the hotter the substance is. Heating a substance makes its particles move faster; cooling it slows them down.
What is temperature?
Temperature measures the average kinetic energy of the particles in a substance, in other words, how fast they are moving on average. A cup of boiling water has a higher temperature than a bathtub of warm water because its molecules are, on average, moving faster.
Temperature does not depend on how much of a substance there is. A teaspoon of boiling water and a pot of boiling water have the same temperature.
What is heat?
Heat is energy that flows from one object to another because of a difference in temperature. It always flows naturally from the hotter object to the colder one, until both reach the same temperature, a state called thermal equilibrium.
Unlike temperature, the total thermal energy in an object depends on its size. A bathtub of warm water contains much more thermal energy than a cup of boiling water, even though it is at a lower temperature. That is why a bathtub of warm water could melt more ice than a cup of boiling water.
In everyday speech, we say an object "has a lot of heat," but strictly speaking, heat is energy in transit. Once it has flowed into an object, it becomes part of that object's thermal energy.
Temperature scales
Three temperature scales are in common use:
| Scale | Water freezes | Water boils (at sea level) | Used mainly in |
|---|---|---|---|
| Fahrenheit (°F) | 32 °F | 212 °F | United States |
| Celsius (°C) | 0 °C | 100 °C | most of the world and in science |
| Kelvin (K) | 273.15 K | 373.15 K | science |
To convert from Celsius to Fahrenheit, multiply by 9/5 and add 32. To convert from Fahrenheit to Celsius, subtract 32 and multiply by 5/9. One degree Celsius is the same size as one kelvin, but the Kelvin scale starts at absolute zero.
The two scales meet at one point: −40 °F equals −40 °C.
Absolute zero
As a substance cools, its particles move more and more slowly. In theory, there is a lowest possible temperature, at which particles would have the minimum possible motion. This is called absolute zero: 0 kelvin, −273.15 °C or −459.67 °F.
Absolute zero can never be reached exactly, but scientists have cooled atoms to within billionths of a degree of it. At such extremely low temperatures, matter behaves in strange ways. Some materials become superconductors, conducting electricity with no resistance, and some gases form a Bose-Einstein condensate, a state of matter in which many atoms behave like a single quantum object.
There is no corresponding upper limit for temperature in everyday terms. The core of the Sun reaches about 27 million °F (15 million °C), and particle collisions in accelerators briefly create temperatures of trillions of degrees.
How heat moves
Heat travels from one place to another in three ways: conduction, convection and radiation.
Conduction
Conduction is the transfer of heat through direct contact. When one end of a metal spoon sits in hot soup, the fast-moving particles at that end collide with their neighbors, passing on energy. The energy spreads along the spoon until the handle becomes warm.
Some materials conduct heat much better than others:
- Good conductors include metals like copper, aluminum and silver. In metals, free electrons carry thermal energy quickly through the material.
- Poor conductors, called insulators, include wood, plastic, glass, air, wool and foam. They slow down heat transfer.
That is why pots are made of metal, while their handles are often made of plastic or wood, and why winter clothes trap air in their fibers.
Convection
Convection is the transfer of heat by the movement of fluids, liquids and gases. When part of a fluid is heated, it expands, becomes less dense and rises. Cooler, denser fluid sinks to take its place. This creates a circulating flow called a convection current.
Convection explains why:
- Water in a pot heats evenly as it circulates.
- Hot air rises and warm rooms are often warmer near the ceiling.
- Sea breezes form during the day, as air over warm land rises and cooler air from the sea flows in.
- Weather systems and ocean currents move heat around the planet.
- Earth's mantle slowly churns over millions of years.
Radiation
Radiation is the transfer of heat by electromagnetic waves, mainly infrared radiation. Unlike conduction and convection, radiation does not need a medium. It can travel through empty space. That is how the Sun's energy reaches Earth.
All objects emit thermal radiation. The hotter they are, the more they emit and the shorter the wavelengths. You can feel radiation from a campfire or a hot stove even without touching it. Thermal cameras detect infrared radiation, revealing differences in temperature.
Dark, matte surfaces absorb and emit radiation well, while light or shiny surfaces reflect it. That is why light-colored clothing keeps you cooler in the sun and why emergency blankets are shiny.
Why metal feels colder than wood
On a cold day, metal and wood outside are at the same temperature, but metal feels much colder. This is because your skin does not sense temperature directly; it senses how quickly heat flows into or out of your body.
Metal is a good conductor, so it quickly draws heat away from your warm skin, making it feel cold. Wood is a poor conductor, so it draws heat away slowly, and your skin stays warmer. On a hot day, the opposite happens: metal in the sun can feel burning hot because it quickly transfers heat into your skin.
Specific heat capacity
Different substances need different amounts of energy to change their temperature. The specific heat capacity of a substance is the amount of energy needed to raise the temperature of a given mass by one degree.
Water has an unusually high specific heat capacity. It takes a lot of energy to warm it up, and it releases a lot of energy as it cools. This explains why:
- Coastal regions have milder climates than inland areas.
- Oceans store vast amounts of heat and moderate Earth's climate.
- Sand on a beach becomes much hotter than the water on a sunny day.
- Water is used to cool car engines and power plants.
Changes of state
When a substance melts, boils or freezes, energy is absorbed or released without changing its temperature. This energy, called latent heat, goes into breaking or forming the bonds between particles. That is why ice water stays at 32 °F (0 °C) until all the ice has melted, and why boiling water stays at 212 °F (100 °C) at sea level, no matter how high the flame. Evaporation absorbs latent heat, which is why sweating cools the body.
Keeping heat in and out
Understanding heat transfer helps design better homes and products:
- Insulation in walls and roofs traps air, reducing conduction and convection.
- Double-glazed windows have a layer of gas between the panes to reduce heat loss.
- Vacuum flasks use a vacuum to block conduction and convection, and shiny surfaces to reduce radiation.
- Heat sinks on computer chips use metal fins to conduct heat away and increase surface area for convection.
Questions readers ask
What is the difference between heat and temperature? Temperature measures the average energy of particles' motion; heat is energy that flows because of a temperature difference.
Why does heat rise? Strictly, it is hot air or liquid that rises, because warming makes fluids expand and become less dense.
How does heat travel through space? By radiation, through electromagnetic waves that do not need a medium.
What is absolute zero? The lowest possible temperature, 0 kelvin or −273.15 °C, at which particles have the minimum possible motion.
Why do some materials feel colder than others at the same temperature? Good conductors like metal draw heat from your skin faster than insulators like wood.
In short
Temperature measures how fast the particles in a substance are moving on average, while heat is energy flowing from hotter to colder objects. Heat travels by conduction through direct contact, convection through moving fluids and radiation through electromagnetic waves, even across empty space. Materials differ in how well they conduct heat and how much energy they need to warm up, with water standing out for its large heat capacity. These ideas explain why metal feels cold, why coasts are mild, why ice water stays at freezing and how insulation keeps us comfortable.