ACARA v9 CONTENT DESCRIPTION “apply the law of conservation of energy to analyse system efficiency in terms of energy inputs, outputs, transfers and transformations”
Builds on earlier work sorting energy into kinetic and potential forms and tracing how it transfers and transforms through simple systems. Here that single idea, conservation, becomes a tool: we use it to account for every unit of energy a device takes in and to measure how efficient that device really is.
The law of conservation of energy
Energy is never created and never destroyed. It only moves from one place to another, or changes from one form into another. This is the law of conservation of energy, one of the most reliable rules in all of science. It means we can treat energy like money in an account: whatever goes into a system must come out somewhere, even if it leaves in a less useful form. If we add up every output, including the parts we did not want, the total always matches the input.
Energy in equals energy out
One bar of input energy splits into a useful part and a wasted part. The split shifts, but the two parts always add to the same total.
Almost all of the input becomes useful output here: 100 useful and only 0 wasted. The two parts add up to 100.
Inputs, outputs, transfers and transformations
To analyse a system we name its energy inputs and its energy outputs. A transfer is energy moving from one object to another while keeping the same form, like heat passing from a flame to a pan. A transformation is energy changing form, like chemical energy in fuel becoming light and heat. Most devices do both, often in a chain. At every transfer and every transformation a little energy slips away, usually as heat, so the useful output is always smaller than the input even though the grand total is unchanged.
Losing a little heat at every step
Follow energy as it changes form down a chain. At each transformation some energy escapes as heat, so less useful energy carries forward.
The chain starts with 100 units of stored chemical energy. Press Next transformation to change its form and watch heat losses appear.
Measuring efficiency
Efficiency tells us what fraction of the input energy a device turns into useful output. We work it out by dividing the useful output by the total input, then writing the answer as a percentage. A device that turns 90 of every 100 units into useful work is 90% efficient; one that manages only 5 is 5% efficient and wastes the other 95 as heat. Comparing efficiencies lets us choose better technology, such as swapping an incandescent bulb for an LED that puts far more of its electricity into light.
How efficient is the device?
Efficiency is the useful output divided by the total input. Choose a device and compare how much of its input energy becomes useful work or light.
The incandescent bulb turns 5% of its input energy into useful output and wastes 95% as heat. Efficiency is the useful output divided by the total input, written as a percentage.
Tracking where the energy goes
A flow diagram is a clear way to analyse a system. The input enters as a single stream and then splits, with each branch drawn as wide as the share of energy it carries. The useful branch leads to the job we wanted done; the wasted branch leads to heat lost to the surroundings. Because energy is conserved, the widths of all the branches leaving the device must add back up to the width of the stream that entered. Nothing escapes the accounting.
Following the flow of energy
An input stream enters on the left and splits into a useful branch and a wasted branch. The branch widths show the share of energy each one carries.
Of every 100 units flowing in, this efficient device sends 80 down the useful branch and 20 down the wasted branch. The two branch widths always add back to the full input, because energy is conserved.
Why nothing reaches 100%
No real device is perfectly efficient, because every transfer and transformation produces some waste heat through friction, electrical resistance or sound. That wasted heat is not destroyed; it counts as part of the output, just in a form we cannot easily use again. When we include it, useful plus wasted always equals the input. Conservation is never broken. The reason a device falls short of 100% is not lost energy but unavoidable waste, and good design aims to make that waste as small as possible.
Why nothing is 100% efficient
Look only at the useful output and the energy seems to go missing. Count the wasted heat as well and the books balance back to the full input.
Looking at only the useful output, the energy seems to vanish. It has not: the rest left as low-grade heat. Real devices always lose some energy this way, so none can be perfectly efficient.
Why this matters
Applying the law of conservation of energy turns a vague sense that machines waste energy into precise analysis. By tracking inputs, outputs, transfers and transformations we can say exactly how efficient a device is and where its energy goes. That skill drives real decisions, from designing low-loss power systems to choosing appliances that do more with less, and it underpins every later study of energy and engineering.
Quick self-check
1. The law of conservation of energy says that in any change, the total energy...
2. A device takes in 100 units of energy and produces 30 units of useful output. Its efficiency is...
3. In that same device, where do the other 70 units of energy go?
4. Why can no real device be 100% efficient?
5. In an energy transformation chain, what happens to the total energy as it changes form at each step?