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KS4 Energy stores and systems

You should already have read Energy - an introduction.

Now, on this page, we will begin the AQA Energy Chapter properly by first gaining a good grasp of ideas met in the Introduction, the ideas of energy systems and energy stores.

What is an Energy System?

An Energy System consists of an object or a group of objects that are themselves Energy Stores.

So, if we take the Heat Engine example from our previous page, the whole Heat Engine is the Energy System, and it consists of the 3 Energy Stores, numbered 1,2 and 3.

Let's repeat the Heat Engine diagram here:

Strictly speaking, the Heat Engine energy system also includes the "surroundings", mostly the air that surrounds all the parts.
So, some energy will flow to the surroundings which means that we should take note that the Cooler source, Energy store 2, includes the surroundings.
As long as we do this, then our diagram of the Heat Engine energy system is complete.

OK, I think we have a fair understanding of "Energy System". What about Energy Store?

What is an Energy Store?

In our Heat Engine example, the Hot source, the Cooler source and the Spinning wheel are Energy Stores.

Quite simply, Energy Stores are ways in which energy can be stored, occasionally for a long time, but often for very little time.

Sometimes the Energy store is an actual object:
For example

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

A Kinetic store - energy is stored in a moving object, for as long as the object moves.

An Elastic Potential store - energy is stored in a stretched elastic band or spring (objects), for as long as these objects are stretched (or compressed).

Thermal (or Internal) store - energy is stored in 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 Magnetic store - energy is stored when two magnets (objects) are attracting or repelling, for as long as their magnetic poles remain apart. NB. Can also be a magnet and a magnetic material such as a piece of steel.

Sometimes the Energy store is not an identifiable object:
For example
A Nuclear store - energy is stored within the nuclei of atoms which can be released in nuclear fission, fusion or radioactive decay. Depending on the atom, the energy could be stored for seconds or for many thousands of years.

A Chemical store - energy is stored within the bonds between 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.

An Electrostatic store - energy is stored between separated positive or negative charges, that are attracting or repelling. The energy can be stored for as long as the separated charges exist, which is usually minutes or hours.

System Changes and Energy Transfers

When an Energy System changes, energy may flow or transfer from one energy store to another.

For example, in our Heat Engine, if both Sources are at the same temperature (both "cool") then no energy will flow between them. This is the initial state of the Heat Engine.

But now if we change the system by raising the temperature of Energy Store 1, then energy will begin to flow between the energy stores and we will have a working Heat Engine.

Another example of a system change would be - giving a ball a nudge so that it begins to roll down a hill.

Another would be - pressing the switch on an electric hedge trimmer so that it draws an electric current (all the way from a power station) and its motor begins to make its blades move back and forward, plus it gets hotter.

I'm sure you could think of lots of similar changes in systems that bring about energy transfers.

OK, let's say more about Energy Transfers

How Energy Travels

There are 2 principles to grasp:

1. Energy will only travel between energy stores; it will not "stop" part way between the stores and it won't travel to something that isn't an energy store.

2. Energy will only travel on certain energy "pathways".

The first rule is pretty obvious, I hope, so no more needs to be said.

The second rule, about energy pathways, needs more detail.

Let's look at a few examples of energy pathways:

Energy changes in common situations

The AQA syllabus requires that you be able to describe all the changes involved in the way energy is stored when a system changes in five particular situations.
So that is what we are going to do next:

1(a). An object projected upwards.

OK, let's assume that the object is a mass such as a tennis ball and it is already moving upwards, vertically or at an angle (it doesn't matter for now).

So, the moving ball is a Kinetic energy store and we'll call it energy store 1.

As it moves upwards, it slows down so the Kinetic energy store, store 1, decreases, but the ball gets higher and higher, so a Gravitational potential energy store, energy store 2, increases.

The energy pathway, labelled a, is a Mechanical Work-energy pathway because it is the force of gravity which acts constantly on the ball, slowing it down as it moves higher and higher; this force does Work on the ball.

Let's make the example a little more interesting by moving the starting point to where the object, the tennis ball, is stationary but ready to be fired from a catapult!

1(b). An object projected upwards from a catapult.

Now, energy store 1 is an Elastic potential energy store.

So, the catapult is released and in a very short time all of the energy from energy store 1 is transferred to the Kinetic energy store of the ball, which is energy store 2.

Then, as the ball goes higher and higher, like before, the Kinetic energy store decreases but the Gravitational potential energy store, energy store 3, increases.

Both of the energy pathways, a and b, are Mechanical Work-energy pathways since the energy change in pathway a is brought about by the force of the catapult elastic acting on the ball, and b is as described above.

NB in both examples, when the ball reaches its highest point, all of the original energy from the Kinetic store has been transferred into the Gravitational potential energy store.
Also note, when the ball falls back towards the ground, the reverse transfer happens - just before it hits the ground, all of the energy that was in the Gravitational potential energy store has fully transferred back into the kinetic energy store.

2. A moving object hitting an obstacle.

For this example let's consider a train crashing into the buffers at the end of a line (it doesn't usually happen but the buffers are there just in case!)

Before we start to write about the energy stores and the changes let's just think about what will happen when the train hits the buffers?

There will be a loud bang;
there will be heat due to friction as the train grinds its way into the buffers;
the buffer has springs which will compress.

OK, now we can start.

We start with the moving train which is a Kinetic energy store, energy store 1.

When it hits the buffers the energy in energy store 1 is going to transfer to a Thermal energy store, 2 a, plus an Elastic potential energy store, 2 b as the springs compress.

But what about the energy that goes to make the loud bang?
Well, energy will travel from energy store 1 via a Radiation - energy pathway in the form of a sound wave; this will spread out and out into the air shifting air particles to and fro, so the whole surrounding air becomes an energy store and it would be an Internal energy store, 2 c.

The 3 energy pathways labelled a to c would be:
a would be a Heating-energy pathway
b would be a Mechanical Work-energy pathway (because the springs get squashed by a pushing force)
c would be a Radiation-energy pathway, as already explained.

Before we leave this example, what if after compressing the buffer springs, the train "bounced back" a bit?
We would have to amend our diagram to add another pathway and energy store, as shown below.

Energy store 3 is, of course, a Kinetic energy store, since the train has "bounced back";
the pathway labelled d is another Mechanical work-energy pathway since it is the action of the springs which produce a pushing force which produces this motion of the train.

3. An object accelerated by a constant force.

OK, this could be a car or a motor bike where the driver/rider is maintaining the engine on a steady driving force such that the vehicle accelerates continuously, faster and faster.

Let's take this example all the way back to the fuel in the tank.
So energy store 1 is a Chemical energy store.

Energy will transfer from this store to a Thermal energy store via a Heating energy pathway, a, when the fuel is ignited and it, quite literally, explodes inside the cylinders of the engine.
Flow of energy from this Thermal store will occur via a Mechanical work pathway (the turning of all the mechanical parts within the engine and gearbox), b, ending up in the Kinetic store and producing the movement of the vehicle.

So, the 2 main energy pathways are:
a will be a Heating-energy pathway.
b will be a Mechanical work-energy pathway

Friction due to the movement of the pistons within the cylinders will further increase the temperature of the engine, adding to the Thermal energy store of the whole engine, and much of this energy will eventually transfer by another Heating-energy pathway, c (shown by a dashed line) to the Internal store of the surrounding air where it will be lost!

4. A vehicle slowing down.

Let's imagine the car or the motorbike from the previous example has reached a high speed, so the driver/rider begins to apply the brakes to bring the vehicle to a stop.

This time we start with a Kinetic energy store, store 1.

When the brakes are applied, pads are pushed against metal discs which are attached to each wheel; these pads rub against the discs producing friction; the friction produces Heating.

So, there is an energy transfer from the Kinetic energy store, store 1, to a Thermal energy store, store 2.

The energy pathway labelled a is a Mechanical Work-energy pathway since it is a braking force that pushes the pads against the discs which reduces the speed of the vehicle.

Because the energy within the Kinetic energy store decreases, the vehicle slows down! This is how brakes work.

NB. The brake pads and discs become VERY hot, so hot that they can glow red hot on racing vehicles. Even bicycle disc brakes can become hot enough after a downhill run to burn your fingers if you touch the discs (I have done so, ouch!)

We could extend the above example to describe what happens to the energy within the Thermal energy store, store 2.

It will transfer via a Heating-energy pathway, labelled b below, to all the surrounding air which will become another Thermal energy store, store 3.

5. Bringing water to a boil in an electric kettle.

Our "story" here has to start at the power station where the electricity or the electric current is generated. So, we have to ask ourselves, "from what type of energy store is our electricity generated?"
And the answer can vary depending on the type of power station, but most likely it will be a power station that uses a Chemical energy store, for example oil or natural gas or coal. So we are going to start our answer by stating that Energy store 1 is a Chemical energy store.

Energy from store 1, the Chemical energy store, will travel via an Electrical Work-energy pathway (labelled a in the diagram) to a Thermal energy store, store 2 which is the element at the bottom of the kettle. This will cause the element to become Hot and so energy will transfer from this store via a Heating-energy pathway (labelled b) to an Internal energy store, store 3, to increase the vibration speed of the particles of water surrounding the hot element. When their vibration speed increases sufficiently, the water will have boiled.

Notice how we don't mention an Electrical Current energy store at the power station! That is because there is no such thing as an electrical current energy store. An electric current provides an Energy Pathway, a means of transferring energy, but not an Energy Store.

Now that we have successfully described how energy changes take place when systems change and we have described all the various energy stores that could be involved, it is time to turn our attention to how we CALCULATE the amount by which energy changes when a system changes!

Time for some equations!