Energy makes things happen. When you switch on a lamp, ride a bicycle or heat water, energy is being changed from one form into another. The Principle of Conservation of Energy states that energy cannot be created or destroyed; it can only be transformed from one form to another. The total amount of energy in a closed system always stays the same.
Forms and transformations of energy
Common forms include kinetic energy, gravitational potential energy, chemical, electrical, heat, light and sound energy. In any device, one form changes into others. For example, a battery-powered torch changes chemical energy into light and heat energy. Energy from the Sun can be stored by plants as chemical energy and later released as heat and light when wood is burned. In each of these changes the form of the energy is different, but the grand total of energy stays exactly the same before and after.
Key idea
Total energy before a change = total energy after the change. Energy is measured in joules (J).
Energy is never lost
When a ball falls, its gravitational potential energy is converted into kinetic energy. When you brake a bicycle, kinetic energy becomes heat energy in the brakes. No machine is 100% efficient because some useful energy always spreads out as heat to the surroundings. This wasted heat is not destroyed; it is still counted in the total.
Example
A swinging pendulum has maximum potential energy at its highest point and maximum kinetic energy at the lowest point. Energy keeps changing between the two forms while the total stays constant.
Remember
- Energy is conserved: total stays the same.
- Energy can only change form, not disappear.
- "Lost" energy usually becomes heat.