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.

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).