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# Thermal energy transfer

## Thermal conduction by atomic collisions

Conduction is the transfer of thermal energy through a material from one particle to another.

If you hold a piece of ice in your hand, it will begin to melt. This is because thermal energy is transferred from your hand to the ice by conduction.

Particles in solids that are in hotter regions vibrate faster. These particles collide with their neighbours and transfer some of their energy. These particles then also begin to vibrate faster.

In this way energy is transferred across the material.

Solids conduct heat better than liquids or gases. The particles inside solids are packed more closely together so the particles collide more often. This leads to a faster transfer of energy.

The diagram shows a piece of wood. Heat is transferred from left to right by the collisions between atoms.

## Conduction by free electron diffusion

Conduction by free electron diffusion is the transfer of thermal energy through a metal by the movement of free electrons.

Metals have many free electrons (electrons which are not bound to an atom). When a part of the metal is heated, some electrons gain energy. These electrons travel at high speeds around the metal.

The free electrons collide with stationary atoms in other parts of the metal and transfer some of their energy.

In this way thermal energy is transferred across the metal. This is a very fast process.

When cooking with a saucepan that has a metal handle you will notice the handle gets hot quite quickly.

If the handle is made from wood or plastic it takes a lot longer for the handle to heat up.

The diagram shows a piece of metal. Heat is transferred from left to right by the diffusion of electrons.

## Thermal conductors and insulators

A good thermal conductor is a material that transfers thermal energy quickly.

All metals (e.g. iron, copper or gold) are thermal conductors. This is because they transfer thermal energy by free electron diffusion.

A thermal insulator is a material that conducts thermal energy very slowly.

Most non-metals like plastic or wood are thermal insulators. This is because insulators transfer thermal energy by atomic collisions only.

A perfect thermal insulator transfers no thermal energy at all.

A vacuum is a perfect insulator as there are no particles to conduct the heat.

These rolls of woollen blankets are very good thermal insulators.

## Rate of thermal conduction

The rate of thermal conduction refers to the speed of thermal energy transfer by conduction through a material between two points.

The rate of conduction (in degrees Celsius per second) through a material can be calculated by the formula:$$\Tblue{\text{rate of conduction}} = \dfrac{\Tred{\text{change in temperature}}}{\Tviolet{\text{time taken}}}$$

If you heat one end of a copper wire, the other end becomes $\Tred{5^{\circ} \text{C}}$ hotter in $\Tviolet{50 \us}$.

So the rate of conduction between these two points is $\Tblue{0.1 ^{\circ} \text{C}/\us}.$

The rate of conduction through good thermal conductors is higher than through thermal insulators.

The rate of conduction through a perfect thermal insulator is zero.

Iron is a good thermal conductor, so the heat is transferred through the bar at a fast rate.

## Convection definition

Convection is the movement of thermal energy from one place to another by the movement of fluids (liquids and gases).

A saucepan full of water is heated on a stove. The water particles at the bottom of the pan gain energy from the heat of the stove. These particles then move around and spread the energy.

Convection involves the transfer of particles through the fluid. For this reason, convection cannot take place within solids as the particles are in fixed positions.

In thermal conduction, heat is transferred as high-energy particles collide with low-energy particles transferring some of their energy.

The movement of a fluid due to convection is often called a convection current.

A set of convection currents inside a beaker of water that is heated from below.

## Convection mechanism

Convection happens naturally because of gravity:

Pockets of denser fluid tend to fall because they are heavier while pockets of less dense fluid tend to rise because they are lighter.

1. A pocket of fluid which is warmer than its surroundings expands and becomes less dense. The pocket begins to rise.
2. The pocket is replaced by colder, denser fluid. This pocket of fluid is then also heated and starts to rise.
3. As a pocket of fluid rises it begins to cool. It contracts and becomes denser again. The pocket starts to fall back down.

In this way, heat is transported throughout the fluid.

Remember that a fluid can be a liquid or a gas.

A radiator in a home heats the air around it. This hot air then rises and some colder air takes its place. The radiator then heats the new colder air and the cycle continues.

The radiator can heat all of the air in the room in this way.

A set of convection currents inside a beaker of water that is heated from below.

Thermal radiation is the transfer of thermal energy from one place to another by means of electromagnetic radiation.

All objects radiate thermal energy in all directions.

The sun radiates lots of visible light.

Hotter objects like a glowing piece of wood in a fire radiate more energy than colder objects like ice.

Unlike conduction and convection, thermal radiation does not require a material to transfer energy or to move. Objects can radiate energy across a vacuum.

When you heat a stone in a fire and then take it out, the stone will radiate. Getting close to the stone you will feel this radiation as warmth.

There is very little contact between particles from the Sun and the Earth. Because radiation does not require a material, the Sun can still provide energy on Earth. (Not to scale)

## Absorbers and emitters of thermal radiation

Some materials emit (give off) thermal radiation better than others.

Objects that are dark or rough are good emitters and absorbers of radiation.

Objects that are shiny, smooth or white are poor emitters and absorbers of radiation.

A black car and a white car are parked next to each other in the sun. The black car will be hotter than the white car as it is a better absorber of radiation.

Objects that have greater surface area transfer energy faster.

A larger radiator is able to heat up a room faster than a smaller one even if they are both at the same temperature.

The grey teapot will cool down faster than the white one as it is a better emitter of radiation.

## Summary of conduction, convection and radiation

A summary of the three ways in which thermal energy can be transferred.

Conduction

• Energy transferred between particles by collisions
• Fastest when free electrions can move through the material to transfer energy
• Does not work in a vacuum
• Works in any medium, but is fastest in solids, particularly metals

Convection

• Energy transferred through a fluid
• Convection currents move the energy through the fluid
• Does not work in solids or in a vacuum