Energy: Conservation of Energy and Power

Conservation of Energy

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed.
Energy transfers occur in all physical and chemical processes, for example when you cycle up a hill or light a match.
Not all energy transfers are useful. In most scenarios, some of the initial energy is ‘wasted’ as it is transferred into a form that is not useful for the intended purpose. For example, in a light bulb, only a small portion of electrical energy is transformed into light. The rest is wasted as heat energy.
The ‘wasted’ energy is not destroyed, as this would violate the principle of conservation of energy. Instead, it is usually dissipated into the surroundings, often as thermal energy.

Energy Efficiency

The efficiency of an energy transfer is calculated by comparing the useful energy output to the total energy input. It is always a value between 0 and 1, or a percentage between 0% and 100%.
The formula for calculating energy efficiency is: efficiency = (useful energy output / total energy input).
A machine that is 100% efficient would convert all input energy into useful output energy. However, in practice, no machine can be 100% efficient due to energy losses, usually in the form of heat due to friction.
Efforts to improve energy efficiency are key to reducing energy waste, saving resources and controlling environmental impacts.

Power

Power in physics is the rate of doing work or transferring energy. It tells us how quickly energy is being transferred or transformed.
The unit of power is the watt (W). One watt equals one joule per second.
The formula to calculate power is P = E/t, where P is power, E is the amount of energy transferred or work done, and t is the time taken.
When dealing with larger units of power, it’s often useful to convert watts to kilowatts (1 kilowatt = 1000 watts). This is particularly common in the context of electricity usage.

Sankey Diagrams

Sankey diagrams are a visual tool used to represent energy transfers.
In a Sankey diagram, the width of the arrows is proportional to the amount of energy they represent.
The input energy is usually represented by a single vertical arrow on the left, while output energies are represented by arrows leading to the right.
Useful output energies are usually represented by arrows pointing upwards, while ‘wasted’ energies are shown by downward arrows. The larger the downward arrow, the less efficient the device.