The image shows a Sankey diagram for a light bulb that is supplied with 100 joules of energy, where 80 joules of energy is wasted as heat. What is the useful energy output of the bulb?
Looking at this diagram, called a Sankey diagram, it shows us all the energy inputs for a system, as well as all the energy outputs. One rule this diagram needs to follow is that the sum of all the inputs equals the sum of all the outputs. In other words, energy is conserved. It’s this reality of energy conservation that will let us solve for the useful energy output of the light bulb.
From our diagram, we see that when it comes to input energy, we have 100 joules of energy input into the system. As far as energy outputs go, we’re told that 80 joules of energy is wasted as heat. If you’ve ever brought your hand near to an incandescent light bulb, you can feel how much heat that bulb generates. In addition to that 80 joules of energy dissipated as heat, there is some amount of energy given off as light from the bulb. It’s that energy that we consider the useful energy output.
Since energy conservation as well as our diagram shows us that the input quantity is equal to the output quantities, we can see that the useful energy output of the bulb is 20 joules. That’s how much of the overall input energy is released as light.