A filament lamp takes in 60 J of electrical energy. 6 J leaves as light, and 54 J leaves as heat.
A diagram can show that whole budget at a glance, sizes included. How?
Here is the budget drawn a new way: one arrow per flow, and each arrow's width drawn in proportion to the energy it carries.
The 54 J heat arrow is nine times as wide as the 6 J light arrow, because it carries nine times the energy.
Reading one takes three steps. First find the input: the arrow entering the lamp, 60 J.
Next find the useful output: the arrow that carries on to do the lamp's job, the 6 J of light.
Then find the wasted output: the arrow that bends away, the 54 J of heat.
The widths must add up: 6 + 54 = 60. No energy appears or disappears in the diagram.
Last, judge the efficiency: 6 ÷ 60 = 0.10, and 0.10 × 100 = 10. The lamp is 10% efficient.
A glance now does real work: a thin useful arrow beside a wide wasted arrow means a low efficiency.
Q: A Sankey diagram for an electric motor shows 400 J entering, a 320 J arrow carrying on to the turning shaft, and an 80 J arrow bending away as heat. Find the input, the useful output, and the efficiency.
The arrow entering the motor is the input: 400 J.
The arrow carrying on to the shaft is the useful output: 320 J. The 80 J arrow bending away is the wasted output.
efficiency = 320 ÷ 400 = 0.80, and 0.80 × 100 = 80
Input 400 J; useful output 320 J; the motor is 80% efficient.
Q: A Sankey diagram for a soundbar speaker shows 200 J entering, an arrow carrying 50 J of sound toward the room, and an unlabeled arrow bending away as heat. Find the heat arrow's value and the efficiency.
The widths must add up, so the heat arrow carries the rest of the input.
200 - 50 = 150. The heat arrow carries 150 J.
efficiency = 50 ÷ 200 = 0.25, and 0.25 × 100 = 25
The heat arrow carries 150 J; the soundbar is 25% efficient.
The Sankey diagram shows a box fan. Which arrow carries the fan's useful output?
The Sankey diagram shows a water pump: 250 J enters, a 140 J arrow carries on as useful work, and a 110 J arrow bends away as heat. Enter the pump's efficiency, as a percentage.
The Sankey diagram shows a projector: 480 J enters, a 72 J arrow carries on as light to the screen, and a 408 J arrow bends away as heat. Enter the projector's efficiency, as a percentage.
In a Sankey diagram for a charger, the heat arrow is drawn three times as wide as the useful-output arrow. Which statement follows?
The Sankey diagram shows a gaming laptop: 300 J enters, a 210 J arrow carries on as useful output, and the arrow bending away as heat has no printed value. Enter the heat arrow's value, in J.
An energy-flow diagram draws each arrow's width in proportion to the energy the arrow carries. What is the diagram called?