Energy sources

The picture reveals the source of almost all of our energy - the sun. This might be obvious for solar cells, but what's the link between the sun and other energy resources? 


Key Concepts

Fuels

A fuel is any substance that contains chemical or nuclear energy that can be harnessed to produce heat energy:

  • Fossil fuels (oil, coal and gas) contain chemical energy. They can be combusted (burned in oxygen) to produce thermal energy.
  • Nuclear fuels contain nuclear energy. Nuclear processes, such as radioactive decay, produce a net output of thermal energy.

The Sun is powered by nuclear fuels. Under the dense, high temperature conditions caused by immense gravitational forces, hydrogren nuclei overcome electrostatic repulson to fuse and form helium. 

Photosynthesis

Plants on Earth absorb the energy from the Sun and use it to combine carbon dioxide and water to create carbohydrates. In terms of physics, charged particles are being pushed together, increasing their potential energy on a nano scale. Macroscopically, we call this chemical energy.


When an animal eats a plant, this chemical energy is converted into heat and work by a process known as respiration, which takes place in the animal's cells. This chemical energy can be transfered along a food chain.

Plant material such as wood can be burned directly to produce heat. Wood has a specific energy of about 20 MJkg-1 and an energy density of 3 MJ l-1. More on these terms here: Power stations.

Dead plants compressed for millions of years turn into coal with a specific energy of 32 MJkg-1. This doesn't appear to be much higher than wood, but the compression increases the density of the material, resulting in an energy density of 72 MJ l-1. Dead animals form crude oil when compressed.

Domestic heating 

Wood can be burned in the house to provide heating. This is a good point to recap efficiency from the Work and energy topic.

A wood burning stove is about 80% efficient which means that 80% of the energy goes to heat the house and 20% goes up the chimney.

E f f i c i e n c y space equals space fraction numerator U s e f u l space e n e r g y space o u t over denominator T o t a l space e n e r g y space i n end fraction 

Sankey diagrams

Sankey diagrams display the flow of enery through a system:

  • The left-most inward section shows the energy input (normally labelled with the type of energy)
  • The arrow continued the right shows the useful energy output
  • Downward arrows (of which there is often more than one) shows the wasted energy

The widths of the arrows are proportional to the amount of energy (length is irrelevant).

 In an exam, if squared paper is provided in the question paper, then your diargam must be perfectly to scale. If the answer space is blank, a sketch with approximate proportions is fine (but you should label all quantities). 

Essentials

Heat into work

To convert heat into work requires some sort of engine, for example, a turbine.

Fuel is used to boil water in an enclosed vessel causing the pressure of the steam to increase. The high pressure steam is directed at the vanes of a turbine causing it to turn.

More info is included in Power stations.

Motion into electricity

An electromotive force is induced if a coil is rotated in a magnetic field. This is the principle of a generator.


A power station produces electricity by burning fuel. The fuel could be coal, oil or gas but the principle is the same.

Test Yourself

Use flashcards to practise your recall.


MY PROGRESS

How much of Energy sources have you understood?