What is the process by which heat transfers from one particle to another when the particles collide?

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.

Heat Transfer

Heat can be transferred by conduction, convection and radiation. It is only transferred from hotter things to cooler things.

Heat is transferred in order to equalise the temperatures of the object and its environment

Examples:

• A cup of coffee will cool down because it is giving out heat energy into the surroundings
• A cold drinks can (taken out of the fridge) will warm up because it is taking in heat energy from the surroundings

Conduction

In a hot solid, particles vibrate more. They collide with the particles next to them and set them vibrating. The kinetic energy is transferred from particle to particle. Metals are the best conductors. Solids are better than liquids. Gases are very poor conductors. They are insulators.

Atoms in a substance are always vibrating.If the substance gets hotter, the atoms vibrate more. The heat energy is given to the atoms, which makes them move about faster

Every time they collide with another atom, the heat energy is transferred. This is how heat travels through a solid

The video below shows how heat transfer occurs through conduction

Convection

In a hot fluid (gas or liquid) the particles have more kinetic energy so they move more. They spread out and the fluid becomes less dense. The hot fluid rises above the denser cold fluid forming a convection current.

• Hot air rises in cold air
• Hot water rises in cold water
• This is called convection

When hot air rises, colder air has to move in to replace it. Convection cannot happen in solids, as the atoms aren't able to move around

The video below explains how heat transfer occurs by convection

Radiation

All objects emit and absorb infrared radiation. The higher the temperature the more they emit.

When objects absorb this energy their temperature increases.

Radiation will travel through a vacuum – it does not need a medium (material) to pass through:

• Dark and matt surfaces are good absorbers and emitters of infrared radiation.
• Light and shiny surfaces are poor absorbers and emitters of infrared radiation.
• Light and shiny surfaces are good reflectors of infrared radiation.

The video below explains about how heat transfer occurs by Radiation

Insulation

When we insulate our homes we reduce the heat lost, we use less fuel and it costs less. Still air is a good insulator, so materials with air trapped in them are often used:

• In cavity walls the air gap between the walls stops conduction.
• In cavity wall insulation the cavity is filled with foam or mineral wool.
• Loft insulation using layers of fibreglass or mineral wool
• Reflective foil on walls reflects infrared radiation.
• Draught-proofing stops hot air leaving and cold air entering the house.

All these improvements cost money to buy and install, but they save money on fuel costs. You can work out the payback time which is the time it takes before the money spent on improvements is balanced by the fuel savings, and you begin to save money:

Payback time (in years) = cost of insulation ÷ cost of fuel saved each year

If the price of the fuel increases, the payback time will be less.

Conductors and insulators

This video opens by asking the question 'How many wires go into a kettle?' before moving to a cut-away diagram of a three-core mains cable with an animation of electrons moving in the live and neutral wires. The ability of free/delocalised electrons to conduct electricity is then discussed.