Heat (Thermal Energy)

Heat (also called thermal energy) is the energy a substance has because its particles (atoms and molecules) are moving. The faster the particles move, the more thermal energy the substance has, and the hotter it feels. Cold things are not "missing heat": their particles are just moving more slowly. Heat is one of the most familiar forms of energy. It cooks our food, heats our homes, drives weather and powers life itself. Yet understanding heat properly took scientists centuries.

  • What it isEnergy of particle motionIn any material
  • TemperatureAverage particle speedMeasured in Celsius or kelvin
  • Absolute zero-273.15 CAll motion essentially stops
  • Heat flowsFrom hot to coldUntil equal temperatures
  • Three waysConduction, convection, radiationHow heat moves
  • Specific heat of water4,180 J per kg per CVery high; water resists temperature change

What heat really is

Inside any object, particles are constantly moving: vibrating in solids, sliding around in liquids, flying about in gases. The faster the particles move, the more kinetic energy they collectively have. We call the total kinetic energy of all the particles in an object its thermal energy or heat content.

The temperature of an object measures the AVERAGE kinetic energy of its particles. A cup of boiling water at 100 C has higher average particle speeds than a glass of cold water at 10 C.

Heat and temperature are NOT the same thing. A small candle flame is at a very high temperature (around 1000 C), but it has little total thermal energy because there are not many particles in the flame. A big swimming pool at 25 C has lots more total thermal energy than a candle flame, even though it is much cooler.

Three ways heat moves

Heat can transfer from one place to another in three ways:

  • Conduction: heat passes through solids by particles bumping into their neighbours. A metal spoon left in hot tea gets hot all the way up the handle.
  • Convection: heat travels through liquids and gases by the warm fluid rising and cool fluid sinking. The reason radiators warm a whole room: the air near them heats up, rises, and circulates around.
  • Radiation: heat travels as infrared light, even through empty space. The Sun warms Earth through 150 million km of vacuum this way. So does the heat from a campfire, even without touching it.
Fact Heat always flows from hot to cold, never the other way. A hot cup of tea cools to room temperature, but a cool cup never warms up on its own. This is one of the most fundamental laws of physics, called the second law of thermodynamics. It tells us why time flows in one direction: things naturally move toward greater disorder and even heat distribution.

Temperature scales

  • Celsius (C): most common in the UK and Europe. Water freezes at 0 C, boils at 100 C.
  • Fahrenheit (F): used in the USA. Water freezes at 32 F, boils at 212 F.
  • Kelvin (K): scientists favourite. Starts at absolute zero (-273.15 C). Used in physics formulas because it has no negative numbers and represents actual particle energy.

To convert: K = C + 273.15. So 0 C = 273.15 K. Body temperature 37 C = 310 K. Room temperature 20 C = 293 K.

Conductors and insulators of heat

Some materials let heat pass through easily; others block it.

  • Good heat conductors: metals (copper, aluminium, iron, silver). Free electrons carry heat quickly. That is why metal cookware heats fast.
  • Poor heat conductors (insulators): wood, plastic, fabric, ceramics, air. Used for handles on pans, oven gloves, double-glazing.
  • Best insulators: aerogels, vacuum (no particles to carry heat). Vacuum flasks keep tea hot for hours by having a vacuum between the inner and outer walls.
Did you know? A vacuum flask (Thermos) blocks all three ways of moving heat. The vacuum between its inner and outer walls stops conduction and convection (no particles to carry heat). The reflective shiny silver coating reduces radiation by bouncing infrared back. The combination keeps a drink hot for hours. Modern flasks can keep tea boiling for over 12 hours, even though they look quite ordinary.

Specific heat

Different materials need different amounts of energy to raise their temperature. This is called specific heat capacity.

  • Water: 4,180 J per kg per C. Very high.
  • Iron: 449 J per kg per C. About 10 times lower.
  • Wood: 1,700 J per kg per C.
  • Air: about 1,000 J per kg per C.

This is why oceans take so long to warm up or cool down (high specific heat), buffering Earths climate. And why a metal handle on a hot pan heats up quickly: low specific heat plus high conductivity.

Heat in your home

Keeping your home warm uses huge amounts of energy. Some tips:

  • Insulate the roof: most heat is lost through poorly insulated lofts.
  • Insulate walls: cavity wall insulation or external insulation cuts wall losses.
  • Double-glazed windows: a gap of trapped air (or argon gas) between two panes blocks heat transfer.
  • Block draughts: cold air sneaking in around doors and windows is a big heat loss.
  • Curtains and rugs: add insulating layers.
  • Heat pumps: rather than burning fuel, these extract warmth from the outside air or ground using clever physics, much more efficient than direct heating.
Try this Touch various objects in your room: a metal door handle, a wooden table, a plastic toy, a soft cushion. They all feel different temperatures, but they have been in the room long enough to be at the same temperature. The difference is how fast each material conducts heat away from your skin. Metals feel cold because they conduct heat away from your hand quickly; cushions feel warm because they barely conduct your heat away at all.
Deeper dive: heat pumps and the strange physics of moving heat

One of the most important technologies for reducing carbon emissions is the heat pump. A heat pump is a clever device that pulls heat out of cold outdoor air (or ground) and dumps it inside a warm building, making the building warmer.

This sounds like it violates the rule that "heat flows from hot to cold". The trick is that the heat pump uses some electrical energy to do the moving, just like a fridge uses energy to pump heat OUT of its cold inside into the warmer kitchen.

Here is how a typical air-source heat pump works:

  1. A liquid refrigerant flows through a coil exposed to the outdoor air. Even on a cold day (down to about -15 C), the outdoor air contains some heat compared to the refrigerant.
  2. The refrigerant absorbs that small amount of heat and evaporates into a gas.
  3. A compressor (powered by electricity) squashes the gas, which dramatically raises its temperature.
  4. The hot, compressed gas flows through coils inside the house, releasing its heat to the indoor air and condensing back to liquid.
  5. The liquid passes through an expansion valve and is back to the start.

The beautiful result: for every 1 unit of electrical energy used by the compressor, about 3 units of heat are delivered into the house. A heat pump is therefore around 3 times more efficient than a direct electric heater. Compared to a gas boiler (which is typically 90 per cent efficient at burning gas, but you have to count the carbon emissions of the gas), a heat pump is much greener, especially when powered by renewable electricity.

The UK government is encouraging the switch to heat pumps as part of net-zero plans. Around 100,000 are installed each year in 2024-25, with a target of 600,000 a year by 2028. The physics is just clever movement of heat, the same as in your fridge, except in reverse.

For more, see conservation of energy and what is energy.