Energy cannot be created or destroyed, therefore the total energy before and after a change or transfer remains the same. This is known as the conservation of energy.
Therefore, the total energy before a change is equal to the total energy after the change. However, energy can also be wasted.
When transfers between different energy stores take place, there can be wasted energy.
For example, when a bulb is lit, even if it is small, it releases heat energy to the surroundings. This heat energy isn’t needed for the bulb’s primary function of producing light, so it’s considered wasted energy.
Most wasted energy ends up in the thermal energy stores of the surroundings, which causes the surroundings to heat up a little.
Sankey diagrams are also used to show energy transfers. In these diagrams, the size of the arrows is important, as it shows the amount of energy.
The Sankey diagram above is for an electric lamp. As you can see, the total amount of energy given out by the lamp is 100 J. Of the total 100 J, 10 J is transferred as light energy (which is the transfer that is useful to us) and 90 J is transferred as thermal energy (which is the transfer that is unuseful to us).
The arrow representing the transfer to heat energy is larger, indicating that more energy is directed there. However, this is considered wasted energy since the primary purpose of lamps is to produce light, not heat.
In physics, we call a particular object or a group of objects a system. When a system changes, energy is transferred between objects or between different forms of energy.
An open system can exchange both energy and matter with its surroundings. The surroundings consist of all the matter outside the system. Open systems gain or lose energy as they interact with their surroundings.
A closed system can exchange energy with its surroundings, but it cannot exchange matter.
In an isolated system, while energy can be transferred internally, the system does not interact with its surroundings. In an isolated system, neither energy nor matter can enter or exit the system.
Therefore, the net energy change in an isolated system is zero.