1.1.1 Energy Stores and Systems

In this lesson, you will learn what systems and energy are, explore the different types of energy, and understand concepts such as total energy, net energy, and closed systems. You will also examine how changes in systems occur through energy transfers. Finally, you will apply these ideas by exploring how energy is transferred in familiar, everyday situations.

Systems and Energy Stores

A system is just an object or a group of objects (e.g. a ball), while energy is what makes things happen or makes things change (e.g. a ball moves when kicked by a person because of the energy given to the ball by the person). All objects are able to hold energy in their energy stores. Energy never disappears or gets “used up”, instead it moves from one energy store to another.

There are eight different types of energy stores that you need to know, each of which holds a different type of energy in an object, these are shown in Table 1 below.

Energy Store	Description
🔥 Thermal Energy	The energy stored in an object from the movement of the particles that the object is made of. Hot objects have greater thermal energy stores because their particles move faster than the particles in cold objects.
🚴‍♀️ Kinetic Energy	The energy stored in an object because it is moving. As an object speeds up, its kinetic energy stores increase, as the same object slows down, its kinetic energy stores decrease.
🌍 Gravitational Potential Energy	The energy stored in an object from the gravitational force (or gravity) pulling the object towards the centre of our planet. The higher up an object is or the further away from Earth it is, the more its gravitational potential energy store increases.
🪀 Elastic Potential Energy	The energy stored in an object when it is stretched out or compressed, like a spring or rubber band.
⛽ Chemical Energy	The energy stored in an object’s chemical bonds. Food, batteries and fuel are sources of chemical energy. Whenever a person or animal performs an action they are using energy from their chemical energy stores.
🧲 Magnetic Energy	The energy stored when two magnets, or a magnet and a magnetic material, are attracted or repelled.
⚡ Electrostatic Energy	The energy stored when charged objects (e.g. electrons), are attracted or repelled.
⚛️ Nuclear Energy	The energy stored inside the nucleus of an atom. This energy is released during nuclear reactions.

Total Energy, Net Energy and Closed Systems

The total energy of a system is the sum of all the energy in all the energy stores inside the system. You can see an example of how you can workout the total energy of a system in Figure 1 below.

How much the total energy of a system has increased or decreased after the system has changed is called the Net energy. Net energy is calculated as the energy transferred into the system, minus the energy transferred out of the system. You can see an example of a calculation like this in Equation 1 below. The units we use to measure how much energy there is are joules (J).

\(\mathrm{Energy\ }\mathit{\color{green}{Added}}\ \mathrm{by\ the\ Change} = \color{green}{10\,\mathrm{J}}\)

\(\mathrm{Energy\ }\mathit{\color{red}{Removed}}\ \mathrm{by\ the\ Change} = \color{red}{4\,\mathrm{J}}\)

\(\mathrm{Net\ Energy} = \mathrm{Energy\ }\mathit{\color{green}{Added}}\ \mathrm{by\ the\ Change} – \mathrm{Energy\ }\mathit{\color{red}{Removed}}\ \mathrm{by\ the\ Change}\)

\(\mathrm{Net\ Energy} = \color{green}{10\,\mathrm{J}} – \color{red}{4\,\mathrm{J}}\)

\(\mathrm{Net\ Energy} = \color{blue}{6\,\mathrm{J}}\)

Closed systems are systems where no energy or objects can enter or leave the system, which is why we call them “closed”. This means the total energy of a closed system is always the same, since no new energy can be added or removed from a closed system. This also means the net energy of a closed system is always zero. However, energy can still move between the objects and the energy stores already inside a closed system.

System Changes and Energy Transfers

For a change in a system to happen (e.g. for a ball that was sitting still on a table to be picked up and thrown), energy must move from one place to another. We call these movements of energy “energy transfers”. In all energy transfers, energy moves from one energy store to another energy store. This can be between energy stores of different types within the same object, or to an energy store of the same or different type in another object.

Energy can be transferred in two ways, by doing work or by heating. Doing work or work done is just another way of saying energy transferred. There are two ways in which work can be done:

1. By a force moving an object. For example, a person pushing a box exerts a force on the box, the force does work causing an energy transfer which moves the box.
2. By an electric current flowing.

Heating is the name we use specifically for an energy transfer from the thermal energy store in one object, to the thermal energy store of another object. For energy transfers of all other types we say that work was done, not heating.

Energy Changes in Common Situations

Now we will look at some common situations where a change in a system is taking place and identify why the change is happening and what is the energy transfer.

---

When a Ball is Thrown Up or Falls Down

When a person throws a ball up, the force exerted by the person on the ball does work. Since work done is the same as energy transferred, this is the same as saying that the force exerted caused energy to be transferred. In this case energy is transferred from the person’s chemical energy stores to the ball’s kinetic energy stores, causing the ball to move upwards. This is shown in Figure 3 below.

In Figure 3 above, you can also see what happens when the ball falls back down. Since the ball is above the ground it has stored gravitational potential energy. The gravitational force does work on the ball. Energy is transferred from the ball’s gravitational potential energy stores to its kinetic energy stores, causing the ball to move downwards.

---

An Electric Kettle Bringing Water to a Boil

When an electric kettle brings water to a boil, heating takes place. Remember that heating is the energy transfer from one object’s thermal energy store to another object’s thermal energy store. In this case the energy is being transferred from the kettle’s heater’s thermal energy store, to the water’s thermal energy store, causing the temperature of the water to increase.

---

When Car Brakes Are Pressed

When a car’s brakes are pressed, friction between the car’s brakes and the wheels of the car exerts a force that does work in the opposite direction in which the car is moving. Energy is transferred from the wheels’ kinetic energy stores to the thermal energy stores of the surroundings (e.g. to the ground, air or wheels).

---

When a Cyclist Hits a Fence

When a cyclist hits a fence the normal contact force between the cyclist and the fence does work causing the cyclist to slow down and stop. Energy is transferred from the cyclist’s kinetic energy stores to other types of energy stores in the fence and surroundings (e.g. energy will be transferred to the thermal energy stores in the fence).