Energy transfer

What you need to know

Reflections and Exam tips

Energy Basics

Before you start, you must recall the following from early secondary school:

• The particle theory of matter
• Matter is made up of particles, their arrangement differs in solids (close together, only vibrate), liquids(more spaces and more kinetic energy) and gases (large spaces, free movement)
• Particles in these 3 states of matter are always moving- have kinetic energy
• Gas particles have most kinetic energy

Heating

Heat moves through materials and space (vacuum) through 4 main ways:

• Conduction
• Convection
• Radiation
• Evaporation

1. Conduction

This happens mostly in solids because their particles are close together (in contact). The particles (atoms, molecules) vibrate or move about (electrons) to pass energy to their neighbours.

This does not however mean that all solids are good conductors of heat as some are:

Bad Conductors (Good insulators)- their atoms and electrons are tightly held together and can not move to pass/transfer energy around e.g. plastics, rubber. In these all the electrons are involved in bond formation and are not freely available to conduct heat

Good Conductors of heat (poor insulators)- Their atoms can vibrate to pass energy around. In addition, very good conductors of heat like metals have a 'sea of free electrons' which can move around transferring heat.

2. Convection

Convection mainly happens in liquids and hases. Convection is when particles of a liquid or gas move around in circles (convection currents) carrying heat around. Hot particles rise to the top carrying heat with them, when they lose the heat, they sink to the bottom to heat up again.

Think of a few examples

3. Evaporation

When a liquid is heated, particles gain kinetic energy hence convection happens. Some particles gain enough energy to escape from the surface of the liquid, this is evaporation. Evaporation may happen at any temperature; the liquid doesn't need to be boiling. As particles escape, they carry energy away with them which means they remove heat energy from where they are coming from. This cools down the source. Examples of this evaporative cooling include sweating.

4. Radiation (Infra-red Radiation)

We have seen that conduction, convection and evaporation all require particles to transfer heat.

How then does heat from the sun cross space (a vacuum) to reach the Earth?

Radiation does not need any particles but it is when heat travels as Electromagnetic waves- in simple terms as waves not particles. Hot objects are always giving off Electromagnetic Radiation, the hotter an object is, the more electromagnetic radiation it gives off.

Objects can also abosrb Electromagnetic radiation; ever worn a black shirt in summer or been in a black car in summer? How did it feel?

As a rule:

• Dull, dark surfaces are good at both absorbing and emitting radiation
• Shiny white surfaces are poor emitters and absorbers of radiation.

Reflect on these:

Fish and chips takeaways (fast foods) are wrapped in several layers of paper.

Some Chinese takeaway foods (fastfoods) come in shiny aluminum containers.

Car radiators are painted dull black.