Wood is often collected and burnt to give us heat and light. On a cold evening, it is wonderful to sit around a fire to tell stories and warm yourself with friends.

Wood comes from plants, specifically trees. Plants use light energy from the Sun, as well as carbon dioxide and water to grow. Plants take the energy and store it in their leaves, roots and all parts of the plant. Wood also contains this energy stored by plants. Burning wood allows us to change this stored energy into light and heat which is useful to us.

Coal is a type of fossil fuel that is also burnt to provide us with heat that we can use. The heat from coal can be used to cook our food and warm our houses.

Fossil fuels like coal were made from prehistoric plants. The plants took up energy from the Sun and stored it in their bodies. Millions of years ago, the Earth was mostly covered by water. The plants that died sank to the bottom of the water. Over millions of years, the layers of plants were covered by layers of sand and compressed by the weight of the sand. The plant material was buried deeper and deeper under the ground where it is much hotter than on the surface of the Earth. Over millions of years, the plant remains changed into fossil fuels.

These fuels get their name, "fossil fuels", because they are made from plants and animals that lived a very long time ago.

Natural gas and oil are also examples of fossil fuels. Scientists have realised that tiny sea organisms also died, sank to the the bottom of the ocean, and were buried under the sand. Over millions of years, many layers of dead sea animals got buried like this. Over millions of years, the dead sea animals changed into oil and natural gas.

Wax in a candle is burnt to provide light. By burning the candle, the stored energy in the wax is released as light and heat energy.

Paraffin is also a fuel that contains stored energy. Paraffin is burnt in paraffin lamps and paraffin stoves to provide us with useful energy in the form of light and heat.

Gas is another fuel that can be burnt to release stored energy in the form of heat and light. We can use gas heaters to keep warm, and gas stoves to cook food and boil water. Natural gas is odourless and colourless, and it is also known as 'clean gas' because, unlike other fossil fuels, it doesn't produce harmful byproducts when it burns.

Humans and animals need energy to live. We get our energy from the food that we eat. Do you remember learning about food chains in the beginning of the year in Life and Living?

Food contains stored energy that our bodies can change into useful energy that we need when we run, jump, breathe, learn and do everything else that we do.

The energy value of food is often shown on the packaging of foods that we buy. The energy of food is measured either in calories (Cal) or in joules (J). A snack such as a packet of chips gives you thousands of joules of energy. Therefore, we rather talk about kilojoules (kJ) of energy when talking about the energy in food.

Have a look at the photo of the side of a mealie meal packet below. The side of the packet contains a lot of information about what the mealie meal contains. The very top line tells us that 100 g of mealie meal will supply your body with 1368 kJ of energy.

The energy value of a food tells us how much energy that food is worth to our bodies as fuel. An average adult man needs about 2500 kcal or 10 000 kJ per day. Children and adults that are not very active need less energy. People that are very active need more energy. These numbers are just to give us an idea of the amount of energy your body needs as fuel everyday.

It is important to eat a balanced diet. In the next activity we are going to look at how much energy different food gives us. In Gr. 6 we will learn a lot more about nutrition and what you should eat to be healthy!

Fuels can also be used to give us other forms of useful energy.

Petrol or diesel is used in cars and trucks to make them go. The stored energy in the fuel is changed into movement energy of the car or truck.

Petrol and diesel are made from fossil fuels. Can you see that even energy for cars and generating electricity comes from the Sun?

Coal is not only burnt in our homes for cooking and keeping us warm. It can also be used to make electricity. A power station is a large factory where the coal is burnt in large amounts to produce electricity.

We can also carry out an investigation to find out how much energy is stored in fuels.

In order to light the fuel, you had to put in a small amount of energy. The fuel however gave out a lot more energy than what was put in. The difference between the energy you put in and the energy the fuel gave out is how much energy was stored in the fuel.

The OUTPUT ENERGY obtained from a fuel is GREATER THAN the INPUT ENERGY needed to make the fuel burn.