Energy is needed to make things happen. In physics, energy is described as the ability to do work. When work is done, motion would happen and the energy is converted into a different form. In the previous example of pushing a broken car, the chemical energy in your body is transferred to the car to become the energy of motion. We can say that work is just a kind of energy transferred:

Work done = Energy transferred

Energy therefore has the same unit of Joule (J) as work.

Different forms of Energy

Fig. 2-1 Penny's Bay Power Station

In fact, energy exists in many different forms and may change from one form to another.

Plants take up radiation energy from the sun convert it into the chemical energy of their tissue. Animals convert the chemical energy in the food into the kinetic energy of their body movement. Automobile engines also convert the chemical energy in the fuels into kinetic energy.

In power stations, chemical energy in the fossil fuel is converted into the electrical energy. Nuclear power stations, on the other hand, convert nuclear energy into electrical energy. Electrical energy is then transmitted through power cables to your home, where it is changed into various forms of energy by the electrical appliances, such as light energy and sound energy in a television, internal energy of the water in a boiler, and kinetic energy in a fan, etc.

Fig. 2-2 A China archer

Sometimes energy is stored because an object is in a particular position or state. This is known as potential energy. For example, an apple on the tree has gravitational potential energy. When it falls, it gains speed because the potential energy is being converted into kinetic energy. The elastic potential energy stored in a drawn bow is changed into the kinetic energy of the arrow as it is shot (Fig. 2-2).

No matter how energy is changed into different forms, the amount of energy always remains the same before and after a conversion. In other words, energy is conserved in the conversion. In physics, this is known as the principle of conservation of energy:

Energy cannot be created or destroyed. Energy may be changed from one form to another but the total amount of energy always remains constant.

The principle is in fact central to all science subjects. Conservation of energy governs all the mechanisms and reactions in any process in nature, and is therefore a very powerful law for making scientific predictions.

 Flash animation: Energy of a diver