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Most of us use the word 'heat' to mean something that feels warm, but science defines heat as the flow of energy from a warm object to a cooler object.

Actually, heat energy is all around us - in volcanoes, in icebergs and in your body. All matter contains heat energy.

Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases. Heat energy can be transferred from one object to another. The transfer or flow due to the difference in temperature between the two objects is called heat.

For example, an ice cube has heat energy and so does a glass of lemonade. If you put the ice in the lemonade, the lemonade (which is warmer) will transfer some of its heat energy to the ice. In other words, it will heat up the ice. Eventually, the ice will melt and the lemonade and water from the ice will be the same temperature. This is known as reaching a state of thermal equilibrium.

Moving particles

Matter is all around you. It is everything in the universe - anything that has both mass and volume and takes up space is matter. Matter exists in different physical forms - solids, liquids and gases.

All matter is made of tiny particles called atoms, molecules and ions. These tiny particles are always in motion - either bumping into each other or vibrating back and forth. It is the motion of particles that creates a form of energy called heat (or thermal) energy that is present in all matter.

The particles in solids are tightly packed and can only vibrate. The particles in liquids also vibrate but are able to move around by rolling over each other and sliding around. In gases, the particles move freely with rapid, random motion.

Transferring heat energy - particles in collision

At higher temperatures, particles have more energy. Some of this energy can be transmitted to other particles that are at a lower temperature. For example, in the gas state, when a fast moving particle collides with a slower moving particle, it transfers some of its energy to the slower moving particle, increasing the speed of that particle.

With billions of moving particles colliding into each other, an area of high energy will slowly transfer across the material until thermal equilibrium is reached (the temperature is the same across the material).

Changing states by heat transfer

Faster moving particles 'excite' nearby particles. If heated sufficiently, the movement of particles in a solid increases and overcomes the bonds that hold the particles together. The substance changes its state from a solid to a liquid (melting). If the movement of the particles increases further in the liquid, then a stage is reached where the substance changes into a gas (evaporation).

Three ways of transferring heat energy

All heat energy, including heat generated by fire, is transferred in different ways:

Convection transfers heat energy through gases and liquids. As air is heated, the particles gain heat energy allowing them to move faster and further apart, carrying the heat energy with them. Warm air is less dense than cold air and will rise. Cooler air moves in below to replace the air that has risen. It heats up, rises, and is again replaced by cooler air, creating a circular flow called a convection current. These currents circle and heat the room.

Conduction transfers heat energy in solids. The moving particles of a warm soild material can increase the heat energy of the particles in a cooler solid material by transferring it directly from one particle to the next. Since particles are closer together, solids conduct heat better than liquids or gases.

Radiation is a method of heat transfer that does not require particles to carry the heat energy. Instead, heat is transferred in infrared waves (part of the electromagnetic spectrum). Heat waves radiate out from hot objects in all directions, travelling at the speed of light, until they hit another object. When this happens, the heat energy carried by the waves can be either absorbed or reflected.

Fire illustrates the three different methods of heat transfer. For example, the firebox will heat up due to convection. The air above the fire will be warm due to convection. You can warm your hands near to the flames due to radiant heat transfer.

An effect of heat - expansion

When gases, liquids and solids are heated, they expand. As they cool, they contract or get smaller. The expansion of the gases and liquids is because the particles are moving around very fast when they are heated and are able to move further apart so they take up more room. If the gas or liquid is heated in a closed container, the particles collide with the sides of the container, and this causes pressure. The greater the number of collisions, the greater the pressure.

Sometimes when a house is on fire, the windows will explode outwards. This is because the air in the house has been heated and the excited molecules are moving at high speed around the room. They are pushing against the walls, ceiling, floor and windows. Because the windows are the weakest part of the house structure, they break and burst open, releasing the increased pressure.

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Most of us use the word 'heat' to mean something that feels warm, but science defines heat as the flow of energy from a warm object to a cooler object. Actually, heat energy is all around us - in volcanoes, in icebergs and in your body. All matter contains heat energy.

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The differences between conduction, convection and radiation are explained. Radiation is described as the transfer of heat energy by electromagnetic waves without involving particles. Conduction is described as the flow of heat from one solid to another, using a hot knife as an example. Convection is described as the movement of particles of gases and liquids away from a heat source to form currents.

Students could use this clip to look at the differences between conduction, convection and radiation. They could then be asked to think of other examples of when they take place.

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Conduction, convection and radiation - KS3 Physics - BBC Bitesize

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The differences between conduction, convection and radiation are explained. Radiation is described as the transfer of heat energy by electromagnetic waves without involving particles. Conduction is described as the flow of heat from one solid to another, using a hot knife as an example.

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Heat can be transferred from one place to another by three methods: conduction in solids, convection of fluids (liquids or gases), and radiation through anything that will allow radiation to pass. The method used to transfer heat is usually the one that is the most efficient. If there is a temperature difference in a system, heat will always move from higher to lower temperatures.

A thermal infrared image of a coffee cup filled with a hot liquid. Notice the rings of color showing heat traveling from the hot liquid through the metal cup. You can see this in the metal spoon as well. This is a good example of conduction.
CONDUCTION:

Conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. Some substances conduct heat more easily than others. Solids are better conductor than liquids and liquids are better conductor than gases. Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e.g. when you wash your hands in warm or cold water.

CONVECTION:

In liquids and gases, convection is usually the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continous circulation pattern and heat is transfered to cooler areas. You see convection when you boil water in a pan. The bubbles of water that rise are the hotter parts of the water rising to the cooler area of water at the top of the pan. You have probably heard the expression "Hot air rises and cool air falls to take its place" - this is a description of convection in our atmosphere. Heat energy is transfered by the circulation of the air.

This thermal infrared image shows hot oil boiling in a pan. The oil is transfering heat out of the pan by convection. Notice the hot (yellow) centers of rising hot oil and the cooler outlines of the sinking oil. Image courtesy of K.-P. Möllmann and M. Vollmer, University of Applied Sciences Brandenburg/Germany.

A thermal infrared image of the center of our galaxy. This heat from numerous stars and interstellar clouds traveled about 24,000 light years (about 150,000,000,000,000,000 miles!) through space by radiation to reach our infrared telescopes.

RADIATION:

Both conduction and convection require matter to transfer heat. Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object. For example, we feel heat from the sun even though we are not touching it. Heat can be transmitted though empty space by thermal radiation. Thermal radiation (often called infrared radiation) is a type electromagnetic radiation (or light). Radiation is a form of energy transport consisting of electromagnetic waves traveling at the speed of light. No mass is exchanged and no medium is required.

Objects emit radiation when high energy electrons in a higher atomic level fall down to lower energy levels. The energy lost is emitted as light or electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels. All objects absorb and emit radiation. ( Here is a java applet showing how an atom absorbs and emits radiation) When the absorption of energy balances the emission of energy, the temperature of an object stays constant. If the absorption of energy is greater than the emission of energy, the temperature of an object rises. If the absorption of energy is less than the emission of energy, the temperature of an object falls.

Heat & Temperature HOME PAGE|What is Heat? |What is Temperature?|Heat vs Temperature|Heat Transfer|Detecting Heat|Measuring Temperature| What Do We Learn From Heat?|Classroom Activities|Standards K-4|Standards 5-8|Standards 9-12

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Heat can be transferred from one place to another by three methods: conduction in solids, convection of fluids (liquids or gases), and radiation through anything that will allow radiation to pass. The method used to transfer heat is usually the one that is the most efficient.