Every card in Energy, Year 9
The whole deck, in order — so you can read it through before your child ever sees it.
- A stone at the top of a cliff has 200 J in its gravitational potential store. It falls, and just before it lands that store is empty. Ignoring air resistance, how many joules are in its kinetic store at that moment? (number only)
200
HintNothing has been added and nothing has leaked away on the way down.
WhyThe total energy has the same value before and after a change. With no air resistance there is only one place for the energy to go, so everything that left the gravitational store is now in the kinetic store.
- Energy cannot be created or ____; it can only be transferred from one store to another.
destroyed
HintThe total can never go up, and there is one other thing it can never do.
WhyThis is the conservation of energy. However a change happens, add up the energy in every store afterwards and you get exactly the total you started with. A battery "running out" has emptied one store and filled others.
- A student writes that an electric motor is supplied with 50 J and transfers 30 J to movement and 30 J to heating. What is wrong with this energy account?
More comes out than goes in
HintAdd up the two amounts on the way out and set that beside the amount supplied.
WhyThe outputs add to 60 J, which is 10 J more than the motor was given. Energy cannot be created, so the totals before and after must match exactly. One of the three figures has to be a mistake.
- As a pendulum swings down to its lowest point, energy moves from its ____ store to its ____ store, while the total amount of energy stays the ____.
gravitational potential; kinetic; same
HintOne store depends on height and the other on speed. Ask which is shrinking on the way down.
WhyAt the top of its swing the bob is high and momentarily still; at the bottom it is low and moving fastest. Energy has shifted from the store that depends on height to the store that depends on speed, and then shifts back as the bob climbs the other side.
- Why does a phone get warm while it is charging?
Some of the energy supplied heats the phone instead of being stored
HintNot every joule arriving from the charger ends up in the battery.
WhyCharging is meant to fill the chemical store in the battery, but the current also heats the wires and the battery itself. That share warms the phone and then the room, and it cannot be used to run the phone later.
- The energy a kettle transfers to the kitchen air, rather than to the water, is described as "wasted". What does "wasted" mean here?
It still exists but is not doing the job we wanted
HintThink about what the kettle is for, and whether warming the air helps with that.
WhyWasted energy has not vanished. It has been transferred somewhere that is no use to us, usually by heating the surroundings a little. Whether energy counts as useful or wasted depends on the purpose of the device, not on the energy itself.
- In almost every energy transfer, some energy is wasted by heating the ____.
surroundings
HintThink of the air and the objects close to the device.
WhyMotors, lamps, engines and even muscles all warm the air and objects around them as they work. That is why a games console needs a fan and why you feel hot after running.
- The warmth that escapes from a heated house ends up in the air outside. The energy still exists, so why can it not be collected and used again?
It has spread out too thinly
HintPicture one drop of ink stirred into a whole swimming pool.
WhyOnce energy has spread through a huge amount of air, each bit of air is only very slightly warmer than before. Nothing has been destroyed, but energy scattered that widely is no longer concentrated enough to do anything useful with.
- Oiling a bicycle chain means less of the rider's energy is wasted heating the chain. Which force does the oil reduce?
Friction
HintIt acts wherever two surfaces rub against each other.
WhyWherever moving parts rub, some energy is transferred to heating them instead of to the job in hand. A lubricant lets the surfaces slide more easily, so less energy is wasted this way and more reaches the back wheel.
- A torch is supplied with 50 J. On its Sankey diagram the arrow for light is labelled 15 J. How much energy does the wasted arrow carry?
35 J
HintThe arrows leaving must account for everything that entered.
WhyA Sankey diagram is conservation of energy drawn as a picture: the input arrow splits into output arrows, and nothing appears or disappears at the split. 50 J goes in and 15 J leaves as light, so the remaining 35 J must leave by heating.
- You are drawing a Sankey diagram on squared paper using a scale of 1 square of width for every 5 J. How many squares wide should the arrow for 30 J be? (number only)
6
HintAsk how many lots of five joules the arrow has to stand for.
WhyEvery arrow on the diagram uses the same scale, which is what lets you compare them by eye. 30 J at 5 J per square needs 30 ÷ 5 squares. An arrow drawn to no scale is just a sketch, and its width tells you nothing.
- On the Sankey diagram for an old filament lamp, the arrow for light is thin and the arrow for heating is several times wider. What does this tell you about the lamp?
Most of its energy is wasted
HintDecide which of the two arrows is the job a lamp is bought to do.
WhyThe lamp exists to give light, so the thin arrow is the useful share and the wide one is energy that only warms the room. An LED lamp giving the same light has a much narrower input arrow, because far less is lost to heating.
- On any Sankey diagram, the widths of all the arrows leaving add up to the width of the ____ arrow.
input
HintIt is the single arrow on the left, before any branch splits away.
WhyThis is the check that a Sankey diagram has been drawn correctly. Energy is conserved, so the useful and wasted branches together must be exactly as wide as the arrow that came in.
- You push a box 4 m across a floor with a steady force of 50 N. How much work do you do on the box?
200 J
HintMultiply how hard you push by how far the box travels.
WhyWork done = force × distance moved in the direction of the force, so 50 × 4. A newton multiplied by a metre is a joule, which is why work has the same unit as energy: doing work is one way of transferring energy.
- A shopper stands still at a bus stop holding a heavy bag. Her arm aches, yet in the physics sense she is doing no work on the bag. Why not?
The bag is not moving
HintWork needs two ingredients, and only one of them is present here.
WhyWork is done only when a force moves something through a distance. She is certainly pulling upwards on the bag, but the distance moved is zero, so force × distance is zero. Her arm aches because her muscles are transferring energy inside her body, not to the bag.
- The energy transferred when a force moves an object
Work done
HintIn everyday speech the first of the two words means a job.
WhyLifting, pushing, pulling and stretching are all ways of transferring energy with a force. The amount transferred depends on the size of the force and on how far the object moves in the direction of that force.
- A ramp lets you raise a heavy crate onto a lorry with a smaller force than lifting it straight up. What do you have to accept in return?
Pushing it a longer distance
HintCompare the length of the slope with the height of the lorry floor.
WhyA simple machine cannot give you energy for nothing. The work needed to raise the crate is the same either way, so if the force is smaller the distance must be bigger to keep force × distance unchanged. Levers and pulleys make the same trade.
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Keep what you learn
Here, nothing is saved. In your child’s own sky every card is scheduled — it comes back just before they’d forget it — and the professor who wrote it is one tap away.