Energy, Year 9

200

HintNothing has been added and nothing has leaked away on the way down.

The 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.

destroyed

HintThe total can never go up, and there is one other thing it can never do.

The 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.

More comes out than goes in

HintAdd up the two amounts on the way out and set that beside the amount supplied.

The 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.

gravitational potential; kinetic; same

HintOne store depends on height and the other on speed. Ask which is shrinking on the way down.

The 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.

Some of the energy supplied heats the phone instead of being stored

HintNot every joule arriving from the charger ends up in the battery.

The 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.

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.

The 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.

surroundings

HintThink of the air and the objects close to the device.

The 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.

It has spread out too thinly

HintPicture one drop of ink stirred into a whole swimming pool.

The 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.

Friction

HintIt acts wherever two surfaces rub against each other.

The 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.

35 J

HintThe arrows leaving must account for everything that entered.

The 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.

6

HintAsk how many lots of five joules the arrow has to stand for.

The 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.

Most of its energy is wasted

HintDecide which of the two arrows is the job a lamp is bought to do.

The 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.

input

HintIt is the single arrow on the left, before any branch splits away.

The 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.

200 J

HintMultiply how hard you push by how far the box travels.

The 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.

The bag is not moving

HintWork needs two ingredients, and only one of them is present here.

The 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.

Work done

HintIn everyday speech the first of the two words means a job.

The 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.

Pushing it a longer distance

HintCompare the length of the slope with the height of the lorry floor.

The 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.