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KS4 Energy transfers in a system

If you are at this section of the AQA KS4 Physics syllabus then you should already have a good grasp of Energy Stores and Energy Transfers, and you should know about the variety of Energy Pathways. If these terms are meaningless to you then you should go back to 4.1.1 Energy changes in a system.

In this section we are going to continue to explore Energy Transfer in a little more detail and we will encounter the Law of Conservation of Energy and the concepts of Useful and Wasted energy transfers.

Energy Can Be Stored

We have discussed this in quite some detail, haven't we?

We named the stores. Let's repeat them -
A Gravitational Potential store
A Kinetic store
An Elastic Potential store
A Thermal (or Internal) store
A Magnetic store
A Nuclear store
A Chemical store
An Electrostatic store

Energy Can Transfer

We know these 2 facts about energy transfer:

1. Energy can move from store to store and when it does it doesn't stop in between stores.

2. Energy can only transfer along one of a few energy pathways.
These are -
Radiation pathway
Electrical work pathway
Mechanical work pathway
Heating pathway

All of the above is revision of what has already been stated in earlier sections, particularly in stores and systems.

But now we have some new points to add concerning Energy Transfers in a system:

Energy can be transferred usefully and it can be wasted

For example, when energy transfers from the Chemical store of a car's fuel tank into a Kinetic store of a car, moving it along, doing useful work, it is a Useful energy transfer.

But whilst this useful energy transfer is taking place, some of the energy from the Chemical store also transfers to a Thermal store causing the car engine to get very hot; this energy eventually dissipates (spreads out) into the surrounding air, increasing the Internal energy of the air particles, but not doing any useful work. So this energy is eventually wasted!

So, you can see, in this one example, some energy from the Chemical store is transferred Usefully, but some is transferred and Wasted.

This, sadly, is almost always the case in real world examples.

We say that "no transfer is perfectly efficient". Or, "no transfer is 100% efficient".
(More on "efficiency" in the next section.)

Using Diagrams To Describe Useful and Wasteful Energy Transfers

If you look back to section Energy stores and systems you will find many examples of everyday energy transfers with diagrams showing the various energy stores and energy pathways involved.
So, what are we going to do here that is different?
We are going to simplify our diagrams; we are going to focus on the energy transfer from the original Store to other Stores, ignoring the energy pathways.

Consider the following energy flow diagram for a "car system".

This diagram shows very clearly how the energy flows in the "car system".

Notice how the original energy from the Chemical store ends up in either the Kinetic store, which is a Useful energy transfer, or in the Internal store of the surrounding air, where it will dissipate; this is a Wasteful energy transfer. The down arrow tells you it is a Wasteful energy transfer.

Two more points to notice:
1. Look at the widths of the 2 arrows; they represent the relative amounts of energy, and what you can see is that the Wasted amount is greater than the Useful amount! Car engines are only about 25% efficient; the diagram shows this.
2. If we repeat the diagram, but this time put some numbers in for amounts of energy, eg.

What you notice is that the total amount of energy going into the system (100 J) is equal to the total amount of energy within the system (25 J plus 75 J).
In other words there is no net change to the total energy in a closed system such as this one (as long as we consider the whole surrounding air as part of "the system").

Let's look at another energy flow diagram; this time for an "electric kettle system".

The kettle derives its energy from a power station so the initial energy store is most likely to be a Chemical store.

The end result with a kettle is "hot water", so a Thermal store.

Notice how almost all of the energy from the Chemical store is transferred Usefully to the Thermal store.
Only a small amount is Wasted, transferring once again to an Internal store in the surrounding air.
So the kettle is very efficient (compared to the car) as long as the hot water is used quickly and not allowed to cool needlessly or to need re-heating.

Let's repeat this diagram with some values added:

Once again you notice that there is no net change to the total energy in a closed system.

It seems that you can't actually "get rid" of energy; what you put into a system, you will always get out, though not always in the ways you might want. You can't destroy it!

Also, you cant get energy from "nothing"; with the kettle, you can't get energy in a Thermal store without first having it in a Chemical store, in a fuel!
In other words, you can't create energy.

The two example that we have considered are enough to enable us to make a final conclusion about Energy Transfer.

Energy can be stored, transferred usefully or dissipated but it cannot be created or destroyed.

The above statement is very important and you must learn it; it summarises all that we have been saying and it does so very neatly, so it is well worth learning.
It is often called "The Law of Conservation Of Energy".

Minimising Wasted Energy Transfers

We have already stated that it is impossible to have perfect (100 % efficient) energy transfers, so whenever a system changes, in any way, there will be some wasted energy.

The reason it is wasted is that this energy finds itself mostly in a low temperature Thermal store where it can bring about no further useful changes. (Remember our "Heat engine"; it worked by having a large temperature difference in order to do some work like spinning a wheel, so if there isnt a large temperature difference then it can't do such useful work.)
Furthermore, eventually this low grade energy from the Thermal store moves to the surrounding air's Internal store where it is dissipated, spreading out across millions of particles such that it can never be put to further use.

So, the question is, how can we minimise the transfer of energy to these two Wasted energy stores, low temperature Thermal and Internal?

There are 2 approaches to this:

1. One approach to minimising the transfer of energy to Wasted stores or to reducing the "leaking" of energy from a Useful store is to try to stop whatever it is that is causing the "leak".
For example, in the car engine that we illustrated above, some energy "leaks" into the Internal store of the surrounding air because the engine's surroundings get too warm. This happens when friction (you should know about this from KS3) between all the moving parts within the engine causes heating up of its surrounding surfaces.
If we can stop (or at least reduce) this friction then we should be able to minimise the amount of energy "leaking" into and being Wasted in the Thermal and Internal stores.

So, can we do this?
Yes, we can !
We can reduce friction within the engine by using and regularly changing the engine oil. This oil acts as a lubricant, a smooth layer between engine parts enabling them to move together more feely with less friction and so producing less unwanted heating.
We can further reduce friction by ensuring that all parts are machined so they have very smooth surfaces so they move over each other with minimum resistance.

2. The second approach to minimising the transfer of energy to Wasted stores or to reducing the "leaking" of energy from a Useful store is quite simply to block its flow to a Wasted store.
For example let's consider a gas fire being used to heat a house. The source of the energy is a Chemical store (the gas). We want all the energy that is released in burning the gas to stay inside the house. But we know that some energy is going to flow outside and be wasted, increasing the energy in the Internal store of the surrounding air.
It is the walls, roof, floor and windows of the house that "leak" energy to the outside.

Can anything be done to reduce this "leakage" of energy? Can we block its flow?
The simple answer is - we insulate each of them with effective insulating materials.

A good insulator is a material which does not provide a good Heating Energy Pathway! It will reduce the flow of energy to a Thermal store or eventually to an Internal store.
Wool, polystyrene, fibre pile and fibre glass are a few examples of particularly good insulators.

So, in a house, the roof (or its loft) will be insulated using fibre glass matting. The cavity walls will have polystyrene sheets or beads inserted. The floor will be covered with woolen carpets (or a newer alternative). The single glazed windows will be replaced with double glazed windows.
Once all of these actions are taken, the house will "leak" less of its Useful energy to the Wasted stores.

Insulation and Thermal Conductivity

Since we are discussing the use of insulation to reduce the flow of energy to wasted stores, we will introduce the idea of Thermal Conductivity.

Materials that are designed to be used as insulators will have a known thermal conductivity. If you go to a DIY store such as B&Q to buy some insulating material to wrap around your hot water tank, for example, you can check its thermal conductivity value and compare it with other materials. The higher the value the greater the amount of energy will conduct across the material and be lost.

So, for your hot water tank insulation, you would look for a material with a low thermal conductivity. Simple as that.

Here are a few materials and their thermal conductivity values:

Copper 401 W/m °C
Steel 43 W/m °C
Brick 0.6 W/m °C
Wood (oak) 0.17 W/m °C
Wool 0.04 W/m °C
Fibre Glass 0.04 W/m °C

So, as you would expect, copper and steel are hugely more thermally conductive than any of the other materials listed. And wool or fibre glass are the least thermally conductive and so the best as far as insulation is concerned.

So, the rate of cooling of a building with walls made from a material of small thermal conductivity will be small. (This is good!)
But the rate of cooling of a building with walls made from a material of large thermal conductivity will be large. (This is bad!)

(NB. You can't make a wall out of fibre glass matting as shown above, so what builders do is they make a wall out of 2 layers of brick but with a cavity in between which they fill with fibre glass. So, they get the solidity of the brick with the low thermal conductivity of the fibre glass.)

You can also see from the unit (and from common sense) that thicker materials will be less conductive than thinner materials. This simple fact means that we should try to use a thick layer of insulation whenever possible eg a thick layer of fibre glass in a loft or a thick carpet on a floor. This is also why two layers of glass (double glazing) is better than one!

So, the rate of cooling will be low (slow) if the walls are thick. (This is good!)
But the rate of cooling of a building will be high (fast) if its walls are thin. (This is bad!)