Energy transfer by heating

In a nutshell

Thermal energy always transfers from a hot region to a cooler region. If there is no temperature difference, then heat energy cannot transfer. Heat transfers by conduction, convection and radiation. You can slow down heat transfers by using insulators or speed up heat transfers using conductors.

Definitions

Key word	Definition
Thermal equilibrium	When objects or substances are touching and are the same temperature, and no thermal energy can be transferred
Dense	How close particles are together. A more dense object will have particles closer together than an object which is less dense
Absolute zero	Where particles have no thermal energy store, and are not vibrating. This is at a temperature of $-273\ \degree C$
Insulators	Materials which have a lower rate of thermal energy transfer
Conductors	Materials which have a higher rate of thermal energy transfer

Temperature difference in thermal energy transfer

Thermal (heat) energy is always transferred from a hotter region to a colder region.

Example

If you put your hand on a table, heat from your hand will transfer to the table. Your hand feels cooler because its thermal energy store has decreased. It has been transferred to the table.

Two objects at the same temperature, which are touching each other, are in thermal equilibrium. This means that heat cannot be transferred, because there is no temperature difference.

Types of thermal energy transfer

There are three types of thermal energy transfer: conduction, convection and radiation. The type of transfer can depend on whether the energy transfer is happening in solids, liquids or gases.

Conduction

Conduction happens in solids. Heated particles vibrate more than cooler particles. The more particles vibrate, the more kinetic energy they have. The faster moving particles pass on their kinetic energy to other particles next to them. This continues as heat is transferred.

Example

Explain how heat is transferred from a pan on a cooker, to a metal handle.

Particles at the bottom of the pan, will start to vibrate more. This will make their neighbouring particles also vibrate more. This keeps happening until the vibrations reach the handle.

Convection

Convection happens in fluids (liquids and gases). Warmer areas in fluids rise as they are less dense. This is because warmer particles have more energy. The cooler regions are pushed down by the warmer regions, where they will also become heated and the process repeats.

Example

Explain how water is heated in a hot pan.

Water at the bottom of the pan will heat up. The warmer water from the bottom rises into the cooler water on the top. The cooler area is pushed down to the bottom of the pan, where it starts to heat up. This causes a convection current as warm water is always moving into the cooler water.

Radiation

Anything that has a temperature that is above absolute zero ( $-273\ \degree C$ ) radiates heat in the form of 'invisible' infrared waves. Radiation does not needs particles to travel through, unlike conduction and convection. Therefore, radiation can travel across a vacuum, like space.

Example

How does energy transfer from a flame (on a cooker) to a pan?

The flames from a cooker transfer heat via infra-red radiation to the pan.

Science; Energy; KS3 Year 7; Energy transfer by heating

1.	Conduction
2.	Convection
3.	Radiation

Insulators

Insulators are used to slow down thermal energy transfers. Some materials allow heat to transfer quicker than others, so a good insulator means heat transfer is slow. Insulators are used when you want to keep something hot, or cold.

Energy transfer	Example of reducing energy transfer
Conduction	Using a pan with a plastic handle will reduce conduction from the pan to your hand. This is because plastic is an insulator and makes conduction difficult to happen.
Convection	In double glazed windows, air is trapped between two pieces of glass in a small gap. This means it is harder for air to flow and convection currents to happen.
Radiation	To keep a drink warm, there are special cups with shiny insides. The shiny surface reflects infrared light back into cup, keeping the drink warm.

Other insulating materials include wool, polystyrene, glass and oil.

Materials which are good conductors, and allow energy transfers to happen quickly are copper, concrete, steel, iron and dirty water. This is why cooking pans are made of metals, as they allow heat to transfer quickly.