Distinguish between useful and waste energy conversions. Interpret models for representing energy transfers. Calculate the efficiency of different devices.
Energy Transfer Diagrams
We know that energy is always conserved. However, not all energy transfers are useful.
An energy transfer diagram shows the useful energy transfers taking place in a process. For example, the energy transfer diagram in Fig. 16.2.1 shows the conversion of chemical energy in a battery to light when the battery is used to power a filament bulb.
Energy conversion process
Transferred as electrical energy
Transferred as light energy
Battery (store of chemical energy) z Fig. 16.2.1
Energy transfer diagram for a filament bulb
Filament bulb
Figure 16.2.1 shows only the useful energy transfers that take place. It is important to note that the bulb also gets hot. Energy transfers that are not useful are known as waste. In this case, the thermal (heat) energy is waste.
Sankey Diagrams
A more accurate way of showing all of the energy transfers going on in the bulb is to draw a Sankey diagram.
Electrical energy 100 J
Light energy 10 J
In a Sankey diagram, the thicker the arrow is, the more energy there is involved in the process. A typical Sankey diagram for a filament bulb is shown in Fig. 16.2.2.
It is clear from Fig. 16.2.2 that most of the energy transfer in this bulb is in the form of heat rather than light.
z Fig. 16.2.2
Sankey diagram for a filament bulb
276 276
Heat energy 90 J
Unwanted heat energy can be transferred through different devices and processes. In this case, the energy is transferred to the area around the bulb and heats the room. When this happens, we say that the energy is dissipated in the form of heat.
