Sankey diagrams
Sankey diagrams summarise all the energy transfers taking place in a process. Sankey diagrams are drawn to scale - the thicker the line or arrow, the greater the amount of energy involved. You may be required to draw a Sankey diagram to scale in an exam.
This Sankey diagram for a filament lamp shows that most of the electrical energy is transferred as heat rather than light.
Energy can be transferred usefully, stored or Spread out into the surroundings.. It cannot be created or destroyed. This is called conservation of energy.
In this Sankey diagram, note that 100 J of electrical energy is supplied to the lamp. Of this, 10 J is transferred to the surroundings as light energy. The remainder, 90 J (100 J – 10 J) is transferred to the surroundings as heat energy.
The energy transfer to light energy is the useful transfer. The rest is wasted. It is eventually transferred to the surroundings, making them warmer. This wasted energy eventually becomes so spread out that it becomes very difficult to do anything useful with it.
Modern energy-saving lamps and LEDs (light-emitting diodes) work in a different way. They transfer a greater proportion of electrical energy as light energy.
This is the Sankey diagram for a typical energy-saving lamp.
From the diagram, you can see that much less electrical energy is transferred, or wasted, as heat energy from the energy-saving lamp. It’s more efficient than the filament lamp.
Efficiency
Calculating efficiency
The efficiency of a device, such as a lamp, can be calculated using this equation.
\(\text{\% efficiency}=\frac{\text{energy [or power] usefully transferred}}{\text{total energy [or power] supplied}}\times{\text{100}}\)
The efficiency of the filament lamp
\(\text{\% efficiency}=\frac{\text{10}}{\text{100}}\times{\text{100}}={\text{10\%}}\)
This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy).
The efficiency of the energy-saving lamp
\(\text{\% efficiency}=\frac{\text{75}}{\text{100}}\times{\text{100}}={\text{75\%}}\)
This means that 75% of the electrical energy supplied is transferred as light energy (25% is transferred as heat energy).