Heat energy
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.
Heat energy is in volcanoes and ice
Heat energy is all around us – in volcanoes and in ice. All matter contains heat energy.
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.
Particles in collision
Tiny particles in solids, liquids and gases are always in motion. It is the motion of particles that creates a form of energy called thermal (heat) 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 modes of heat transfer
This image illustrates ways in which heat energy is transferred:
hands are warmed by radiation
water in the pot is heated by convection
the pot handle is heated by conduction.
Original diagram by nandalalsarkar, 123RF Ltd.
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.
Convection
Convection transfers heat through the air. As the air heats, it rises. Cooler air below moves in, heats up and rises. As the risen air cools, it becomes denser and falls. This circular flow of air creates a convection current.
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.
Conduction
Conduction moves heat through a material. It keeps a fire going by spreading the heat through solid material.
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.
Radiation
Radiation is the heat that we feel coming from a fire. It warms the air using heat waves that radiate out from the fire in all directions until it is absorbed by objects.
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.
Related content
To help understand more about heat, particularly in relation to fire, see these articles:
Activity ideas
There are a number of activities that support student learning. Hands-on activities include:
Drama in the microworld – using drama to model atoms, molecules, heat transfer and combustion.
The great candle experiment – the oft-used inverted jar in a saucer of water but without the common misconceptions.
The flying teabag – investigating convection.
In Alternative conceptions about fire discover some common misunderstandings about fire and keep them in mind while teaching – and address them as they come up.
