Energy Types

A body posseses energy if it is capable of doing work.

The 'work done' by a system is the 'total energy transfer' by that system and is measured in Joules. That energy transfer is never totally transferred into 'useful output' and the efficiency of a system can be calculated by calculating the ratio of useful energy output to total energy input.


efficiency = useful energy transferred รท total energy transferred

Energy Conservation

Energy is neither created nor destroyed, it merely changes from one form to another. This is known as the Law of Conservation of Energy.

Energy transfers can be useful (the energy being changed into a form we need). But whenever a transfer is made or energy is stored, some of the energy is 'dissipated'. That means it is 'lost to the surroundings' - shared out between air particles and particles of a container so that it cannot be made use of.

You need to be able to recognise the changes that take place in a physical system (see Sankey Diagram) and calculate the efficiency of the transfer.

'Wasted energy' is the difference between total energy transfer and useful energy output. This is usually produced in the system in the form of heat.

Energy Types

The Gravitational Potential Energy of an object depends on how high that object is above a surface. It depends on its position. When released the gravitational potential energy will diminish and the object will gain kinetic energy. It can be used as a store for energy (hence the word 'potential').
Kinetic Energy depends on how fast the object is moving, the faster it goes the higher the kinetic energy it has.
Elastic Potential Energy (sometimes called 'stretch or compressed' potential energy) is found in springs, elastic materials (balls and bands). It comes from the structure of the materials and their desire to retain a particular shape. It can be used as a store for energy (hence the word 'potential').
Chemical Energy is energy stored because of the chemical makeup of a substance. It is determined by the bonding within the substance and the form it takes. (Eg. ice at OOC has less energy than water at OOC). It can be used as a store for energy eg. food, fuel, batteries etc. This energy can be released by burning or other reactions. Chemical energy is used in a system when a chemical change takes place. Eg coal (carbon) + air (the oxygen in it!) = carbon dioxide + heat and light energy
Sound Energy
Sound Energy can be detected by our ears. Movement causes the air molecules to vibrate and when this vibration of the air molecules is great enough, it can be detected by our ears as sound. Most things that have moving parts convert some of their energy to sound energy.
Light Energy
Light Energy is a form of electromagnetic radiation . Light energy can be detected by our eyes. When a system converts some of its energy into light energy, heat energy often accompanies it, as this is part of the same family.
Heat Energy
Heat Energy can be detected by our skin. When it travels from the source by contact we call its method of travel conduction. In gases and liquids it also travels by convection and it travels directly (a bit like light) in the form of electromagnetic radiation (more about that later) called infra red radiation. Heat energy is almost certain to be found as an energy transfer in a system. It can often be a nuisance in an energy transfer system design.
Electrical Energy
Electrical Energy derives from the movement of electrons (the kinetic energy of electrons). It is a very convenient form of energy and we make a great deal of use of it today.
Magnetic Energy
Magnetic Energy is produced by moving charges. Some elements; (cobalt (Co), iron (Fe) and nickel (Ni)) are said to be ferromagnetic and are attracted to magnetic objects and can be made into magnetic objects themselves. When a current passes through a wire a magnetic field results (because the electrons are moving in the wire). This cannot easily be detected, but when conducting wire is wound into a coil the effect is detectable with a small compass.

Click here to go to a page discussing devices that store energy

Click here to try some questions on energy transfer

Now revise the topic from flash cards I have written for:


LOJ April 2001