Lesson 5: Conservation of Energy

The law of conservation of energy states that energy can neither be created nor be destroyed. Although, it may be transformed from one form to another. 

All the forms of energy follow the law of conservation of energy. 

In brief, the law of conservation of energy states that: In a closed system, i.e., a system that is isolated from its surroundings, the total energy of the system is conserved.

Energy can be transferred usefully, stored or dissipated, but it cannot be created or destroyed.

In all cases, energy comes from one store and is transferred to another store. This means that all the energy in the Universe was present at the Big Bang and will still be around at the very end of time.

No system is perfect. Whenever there is a change in a system, energy is transformed and some of that energy is dissipated.

Dissipation is a term that is often used to describe ways in which energy is wasted. Any energy that is not transferred to useful energy stores is said to be wasted because it is transferred to the surroundings.

• The wheels have to work hard to overcome friction between the tyres and the road.  This causes the tyres to become hot and the thermal energy is wasted. 
• The chemical energy from the fuel is converted into mechanical energy in the engine.  There are many moving parts in the engine which must overcome the force of friction.  This creates heat energy which is wasted.
• Engines also produce sound energy, which is wasted. 

When a skydiver jumps out of a plane, he begins to lose gravitational potential energy as his height decreases and he gains kinetic energy as his speed increases.

However, not all of the gravitational potential energy is transferred into kinetic energy as some work is done pushing against the air particles. Some of the gravitational potential energy is transferred to the air particles and this is waster energy. 

Figure 1: When a pendulum swings to and fro, its energy is constantly changing from potential energy to kinetic energy and back again.  This energy is gradually transferred to heat by friction with the air. 

If the amount of energy going into a system is known you can draw a Sankey diagram.  These diagrams are particularly useful in showing how much energy is in the system, transformed and into what type(s) of energy. Sankey diagrams start off as one arrow that splits into two or more points. The width of the arrow is drawn to scale to show the amount of energy.

Figure 2: Sankey diagrams comparing the energy needed, used, and wasted in two different types of light bulbs.