Conservation of Energy and Wasted Energy

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.

So Total energy before = Total energy after

However, energy can also be wasted.

Wasted energy

When transfers between different energy stores take place, there can be wasted energy.

For example, when a bulb is lit, it may be small, but it releases heat energy to the surroundings. This heat energy is not required for the bulb to work, which is why it is wasted energy.

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Most wasted energy ends up in the thermal energy stores of the surroundings, which causes the surroundings to heat up a little.

Sankey diagrams

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 for the transfer to heat energy is larger because more energy is transferred here. However, it is wasted energy, as we use lamps solely to produce light, not to produce heat.

Open, Closed and Isolated Systems

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.

Open system

An open system can exchange both energy and matter with its surroundings. The surroundings are all of the matter outside the system. Open systems gain or lose energy as it interacts with their surroundings.

  • An example of an open system is an open stove top. This is because both matter (food) and energy can escape the system.

Closed System

A closed system can share energy with the surroundings but are unable to share matter.

  • An example of a closed system is a pressure cooker. This is because no matter can enter it or escape, but heat can be transferred to it.

Isolated system

Although energy can be transferred within an isolated system, it does not interact with the surroundings. In an isolated system, neither energy nor matter can enter or exit the system. So, the overall change in the total energy of an isolated system is zero.