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KS3 3.3.2 Energy transfer

In this section we begin to understand that, although we encounter it in many ways, "energy" is one thing but it can be stored in many different ways which we call, not surprisingly, energy stores. We will then learn that energy can move or transfer, but only from one of these energy stores to another. Finally we will learn that energy can never be lost, it is always conserved.

Energy stores

It has been known for quite a long time that energy is "one thing" but that it is stored in many different ways and that it flows from one store to another and in doing so we can make it do something good and useful, something we call work.

But before we get ahead of ourselves, what do we mean by energy stores?

Quite simply, energy stores are ways in which energy can be stored; there are many of them:

A Gravitational Potential store - energy fills an object when it is raised up against the pull of gravity, for as long as the object is raised up.

A Kinetic store - energy fills a moving object, for as long as the object moves.

An Elastic Potential store - energy fills a stretched elastic band or spring, for as long as they are stretched (or compressed).

Thermal (or Internal) store - energy fills a hot object, for as long as the object remains hot.
NB. Eventually the air around the hot object gains all of the energy, so it becomes an energy store, though its energy would be very spread out, wouldn't it?

A Chemical store - energy fills the bonds between the chemical elements in food, fuel or batteries. The energy could be stored for as long as the food or the fuel or the battery exist.

Energy transfers

Energy will transfer or flow between energy stores.
It will only transfer between energy stores; it won't move from one energy store to something that isn't an energy store.

So, it will only transfer when there are two or more energy stores in an energy system (unless the energy stores are all identical in every way eg two thermal stores at exactly the same temperature).

Example 1

A good example of an energy transfer is that described in Question 5 above:
A battery powered toy car moves along the floor-
Energy flows from a chemical store (ie from its battery) to a kinetic store (the moving car).

An easier way to illustrate this is to draw boxes to show the energy stores, and an arrow to show the flow of energy.

Another example is:

Example 2

A ball thrown upwards-
It starts off with a large kinetic store as soon as it leaves the persons hand. As it travels upwards its gravitational potential store gradually fills BUT because it rises slower and slower its kinetic store gradually empties.
So the overall energy transfer is from the kinetic store of the ball to the gravitational potential store of the ball.

Note: 1.the kinetic store of the ball is fullest at the start just when the ball starts to move; at this time its gravitational potential store is empty.
2. But when the ball reaches its highest point, its kinetic store is empty and its gravitational potential store is at its fullest.
3. The total of the two stores should always add up to the same amount ie to the inital amount in the kinetic store or the final amount in the potential store.

Sometimes energy can transfer into two (or more) stores.

Example 3

Consider a small ball being fired vertically upwards from a catapult?
The rubber catapult is pulled backwards producing a full elastic energy store, Store 1;
then the rubber and ball is released causing the ball to begin to move rapidly, so it suddenly has a large kinetic store.
However, since it is fired upwards it gains height causing its kinetic store to decrease because it slows down but its gravitational potential store increases.
Eventually it reaches its highest point when its kinetic store is empty (because it stops moving for a moment) but its potential store is at its maximum.
At the highest point the amount of energy in its potential store should equal that in its original elastic store.

Sometimes one energy store can empty into two stores but one of them is undesirable!

Example 4

For example a car (not a toy car) starts with a chemical store because it uses a fuel such as petrol (store 1); it uses this fuel to produce movement, so it fills a kinetic store (store 2) BUT at the same time the engine gets very hot so it also fills a thermal store (store 3).
The diagram to illustrate this would look like:

In the above example we would prefer it if Store 1 emptied entirely into Store 2 producing only kinetic energy and a moving car, but it is inevitable in the real world of cars, that their engines get very hot so some of Store 1 empties into the thermal store, Store 3.

The energy in Store 3 is effectively wasted energy; it will gradually transfer its energy to the surrounding air which becomes another thermal store (Store 4); but once this happens, this energy can't be used for any further useful purposes. We describe this as dissipated energy - energy that has become so spread out that it is completely useless and wasted.

It is a problem that many energy transfers involve some transfer to an undesirable and wasteful thermal store; the energy from such a store eventually becomes dissipated.
Every home appliance that uses an electric motor will generate heat when it spins, so it transfers energy to a thermal store eg a washing machine, a dish washer, a vacuum cleaner, an electric drill, a food mixer, a computer power supply etc. The list goes on and on...
Even our bodies generate a large amount of energy that goes to a thermal store - just go for a short run and you will experience "overheating"; you become a thermal store but that energy becomes dissipated - spread out into the surrounding air and wasted.

Energy is conserved

If we look back to Example 2 above, we said that:
"the total of the two stores should always add up to the same amount ie to the initial amount in the kinetic store or the final amount in the potential store".

This is true because energy is conserved.

What this means is:
whatever energy goes into a system must come out of the system, or
total energy input must equal total energy output.

Energy is never lost, it is always conserved.

Consider the following simple system where some energy in Store 1 is transferred into two stores:

What we will find is:
Total energy input = Total energy output
because energy is conserved.

It will help to understand the statement if we put some numbers into our examples.

In Physics, energy is measured in units called joules, named after the English Physicist James Joule.
The letter "J" is used as the symbol for the unit "joule".

Let's say the total energy input to Store 1 is 100 J,
and the energy transferred to Store 2 is 45 J.
What must be the energy transferred to Store 3, if energy is conserved?

The total energy input = total energy output
100 = 45 + energy in Store 3
energy in Store 3 = 100 - 45
energy in Store 3 = 55 J

Try another example.

Another example.

Another way to illustrate energy transfers in a system is to use an energy flow diagram.
An energy flow diagram for the car in Example 4 (and used in the calculation above) would look like:

You should be able to see how it usefully shows that the Kinetic store is the "useful" one, but the Thermal store, off to the side, is the "wasted" one.

Also the width of the arrows shows which of the stores gets the largest share of the input energy; so here you can see that it is the Kinetic store that gets more of the input energy.

Also notice that the total output energy equals the total input energy - this will always be true.

Calculating the percentage of energy wasted

If you look at the energy flow diagram above you can see that the amount of energy wasted is 350 J out of a total input (or output) energy of 1000 J.

So, the percentage of energy wasted = 350/1000 x 100

Finally, the percentage of energy wasted = 35 %

We can also say that 65 % of energy is not wasted,
or 65 % of energy is usefully used,
or the system is 65% efficient.


Now that we have reached the end of this section we can focus on the keywords highlighted in the KS3 specification. You have already met each one, but it is important to learn them.