Energy transfers · Model
Energy transfers: before and after
A phone battery holds about 40,000 joules. Run it flat and the phone weighs exactly the same as it did when full. So what actually left it?
Start here
Weigh the battery. Twice.
A fully charged power bank on a balance accurate to a milligram. Use it until it is completely flat, then put it back on the same balance. The reading has not changed.
Commit to what that tells you about energy.
Energy is not stuff. Nothing was poured out of the battery and nothing was consumed. The chemicals inside were rearranged, and a number you can calculate about that arrangement got smaller — while the same number, calculated about the room, got bigger by exactly as much. There is no substance to weigh, which is why the balance has nothing to report.
Last lesson you learned where energy can sit. This lesson is about the only thing you ever actually do with that list: put a number next to each store before, put a number next to each store after, and check that the two columns add up to the same total. That check is the whole of energy physics. Everything else is arithmetic.
The before-and-after tally · put numbers on it
Two columns. One total.
0 of 4 devices tallied
Pick a transfer, then drag the slider to decide how much of the starting energy ends up doing the job you wanted. The rest is not lost — watch where the tally puts it.
Commit first. An old filament bulb takes in 60 J of energy each second and gives out about 3 J as light. Where are the other 57 J?
Before
After
In, from the store
Doing the job
Into the surroundings
A filament bulb is a heater that happens to glow. About 3 J of every 60 leaves as light and the rest warms the room — which is why they were banned, and why an LED doing the same job takes 8 J instead of 60.
A kettle is close to the best a device can be, and for a simple reason: the job you want is a thermal store, and the energy that escapes to the room is thermal too. There is almost nothing for it to be wasted as.
The 30 J in every 100 that does not lift the load warms the motor windings, the gearbox and the cable. Run a winch hard for ten minutes and you can feel every joule of it.
A human is a poor machine and a good heater. Three-quarters of the chemical store you spend sprinting ends up as a thermal store in your own body, which is precisely why you sweat.
Key fact
To describe any transfer, say which store empties, which stores fill, and by how much. The two columns must add to the same total.
The word “wasted” · say what you mean
Wasted energy has not gone anywhere strange
For each of these, decide whether the energy ending up in the surroundings is a problem or the entire point.
A filament bulb warms the room it is lighting.
Wasted — you were buying light, and you are paying for 57 J of warm room you did not ask for.You were paying for light. The warm room is real energy in a real store, but it is not the job you wanted done.An electric heater warms the room it is in.
The point. Identical physics to the bulb, opposite verdict — because this time the warm room is what you were buying.Not wasted. This is the one device where “energy ends up as a thermal store in the room” is exactly the job.A phone gets warm while charging.
Wasted. You wanted the chemical store filled, not the back of the phone warmed — and it is why fast charging is harder than it sounds.You were trying to fill the battery. Warmth in the phone case is energy that did not get there.A tumble dryer warms the clothes inside it.
The point — warming the water in the clothes is exactly how a dryer works.This one is genuinely the job. The dryer warms the water so it evaporates; the thermal store is the mechanism, not a side effect.Identical physics, opposite verdicts. A heater and a kettle and a filament bulb all end with energy spread thinly through a warm room; the only difference is whether that was what you were paying for. “Wasted” is a judgement about your intentions, not a statement about where the joules went — and a physicist who says “wasted” always means “ended up somewhere too spread out to be useful”, never “ceased to exist”.
Think again
“There is energy inside the battery, and using the phone lets it leak out until there is none left.”
Two things. First, a battery does not contain energy the way a bottle contains water — it contains chemicals in a particular arrangement, and the energy is a number you calculate about that arrangement. Nothing physically drains. Second, and worse: the leaking picture has energy vanishing at the far end. Follow it honestly and you have to ask where the leak goes, and the answer is always another store you could point at and measure.
The test is the balance from the hook. A flat battery weighs the same as a full one to a milligram. Whatever left it, it was not a substance — and the phone, the charger and the air around them are all very slightly warmer than they would otherwise have been, by an amount that adds up to exactly what the battery started with.
“The light between the torch and the wall is a store of energy.”
A pathway is a route, not a container. Light, sound, an electric current and heating are all ways energy gets from one store to another; none of them holds it. If you switch the torch off, the light in the room is gone in a few billionths of a second, because there was never anything there to empty. The chemical store in the cell is what emptied, and the thermal store of the wall is what filled.
Key fact
Describe a transfer by naming the store it started in, the store it ended in, and the pathway between the two. Before and after is the whole method — and the total is the same in both pictures.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What must be true about the total energy before and after any transfer?
Rung 2 · The one that catches people
A 60 W filament bulb takes in 60 J each second and emits 3 J of light. What is the correct thing to say about the other 57 J?
Rung 3 · Explain
An electric winch uses 500 J to lift a crate, and the crate gains 350 J in its gravitational store. Account for all 500 J, and explain why the missing amount is not a failure of conservation.
Rung 4 · Take it somewhere new
A shopkeeper replaces every filament bulb in a shop with LEDs and finds that the heating bill goes up slightly in winter. Explain why, and say whether the change was still worth making.
Key note
Name the store that empties, name every store that fills, and make the two totals match. Energy that ends up spread thinly through the surroundings is called wasted — it has not gone anywhere you cannot point at.
Going further
There is a direction to all of this that the tally does not show. You can take 60 J of chemical store and end up with 60 J spread through a warm room, easily, every time. You cannot take a warm room and get the 60 J back into a battery — not because it would break the sum, but because the energy is now shared out among so many particles, moving so randomly, that there is no way to gather it up. Every transfer in this lesson runs downhill in that sense, from concentrated to spread out, and none of them will run backwards on their own. This is the second law of thermodynamics, and it is the reason the universe has a past and a future rather than just a sequence of frames.
Before this lesson
At GCSE this becomes
- Sankey diagrams and efficiency — the same two columns, drawn to scale and turned into a percentage.
Where to next
Ask Mr Badmus AI
Still picturing energy as something that drains?
The tally is a teaching model. Each device is given a round input in joules and a single useful output, so that the before and after columns balance exactly. Real efficiencies vary with the model, its age and how it is used, and no appliance matches its figure to the joule. The useful and wasted split depends on the job you wanted done, which is why the same warm room counts as waste from a bulb and as the whole point of a heater.
Lesson content © MrBadmusAI.