Energy Stores & Transfers

Systems in Physics

In physics, a system is defined as:

An object or group of objects

An apple sitting on a table can be defined as a system
Defining the system in physics is a way of narrowing the parameters to focus only on what is relevant to the situation being observed

When a system is in equilibrium, nothing changes and so nothing happens
When there is a change in a system, things happen, and when things happen energy is transferred

If the table is removed, the apple will fall
As the apple falls, energy is transferred

Energy is measured in units of joules (J)

A thermodynamic system can be isolated, closed or open
- An open system allows the exchange of energy and matter to or from its surroundings
- A closed system can exchange energy but not matter to or from its surroundings
- An isolated system does not allow the transfer of matter or energy to or from its surroundings

Types of systems, downloadable IB Chemistry revision notes

A system can be open, closed or isolated

Energy Stores

Energy is stored in objects
When a change happens within a system, energy is transferred between objects or between stores
The principle of conservation of energy states that:

Energy cannot be created or destroyed, it can only be transferred from one store to another

This means that for a closed system, the total amount of energy is constant
There are many different energy stores that objects can have, these are shown in the table below:

Energy Stores Table

Energy Store	Description
Kinetic	Moving objects have energy in their kinetic store
Gravitational	Objects gain energy in their gravitational potential store when they are lifted through a gravitational field
Elastic	Objects have energy in their elastic potential store if they are stretched, squashed or bent
Magnetic	Magnetic materials interacting with each other have energy in their magnetic store
Electrostatic	Objects with charge (like electrons and protons) interacting with one another have energy in their electrostatic store
Chemical	Chemical reactions transfer energy into or away from a substance's chemical store
Nuclear	Atomic nuclei release energy from their nuclear store during nuclear reactions
Thermal	All objects have energy in their thermal store, the hotter the object, the more energy it has in this store

Energy Transfer Pathways

Energy is transferred between stores via transfer pathways
Examples of these are:
- Mechanically
- Electrically
- By heating
- By radiation
These are described in the table below:

Energy Transfer Pathway Table

Transfer Pathway	Description
Mechanical working	When a force acts on an object (e.g. pulling, pushing, stretching, squashing)
Electrical working	A charge moving through a potential difference (e.g. current)
Heating (by particles)	Energy is transferred from a hotter object to a colder one (e.g. conduction)
(Heating by) radiation	Energy transferred by electromagnetic waves (e.g. visible light)

An example of an energy transfer is a hot coffee heating up cold hands

1--thermal-energy-transfer--new

Energy is transferred from the hot coffee to the mug to the cold hands

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Worked example

Describe the energy transfers in the following scenarios:

a) A battery powering a torch

b) A ball falling

Answer:

Part a)

Step 1: Determine the store that energy is being transferred away from, within the parameters of the defined system

- - For a battery powering a torch
  - The system is defined as the battery and the torch
  - So the energy transfer to focus on is from the battery to the bulb
  - Therefore, the energy begins in the chemical store of the cells of the battery

Step 2: Determine the store that energy is transferred to

- - When the circuit is closed, the bulb lights up
  - Therefore, energy is transferred to the thermal store of the bulb
  - Energy is then transferred from the bulb to the surroundings, but this is not described in the parameters of the system

Step 3: Determine the transfer pathway

- - Energy is transferred by the flow of charge around the circuit
  - Therefore, the transfer pathway is electrical

Energy is transferred electrically from the chemical store of the battery to the thermal store of the bulb

Part b)

Step 1: Determine the store that energy is being transferred away from, within the parameters of the defined system

- - For a ball falling, the system is defined as the ball
  - In order to fall, the ball must have been raised to a height
  - Therefore, it began with energy in its gravitational potential store

Step 2: Determine the store that energy is transferred to

- - As the ball falls, it is moving
  - Therefore, energy is being transferred to its kinetic store

Step 3: Determine the transfer pathway

- - For an object to fall, a resultant force must be acting on it, and that force is weight and it acts over a distance (the height of the fall)
  - Therefore, the transfer pathway is mechanical

Energy is transferred mechanically from the gravitational potential store of the ball to the kinetic store of the ball v

Examiner Tip

Don't worry too much about the parameters of the system. They are there to help you keep your answers concise so you don't end up wasting time in your exam.

If you follow any process back far enough, you would get many energy transfers taking place. For example, an electric kettle heating water. The relevant energy transfer is from the thermal store of the kettle to the thermal store of the water, with some energy dissipated to the surroundings. But you could take it all the way back to how the electricity was generated in the first place. This is beyond the scope of the question. Defining the system gives you a starting point and a stopping point for the energy transfers you need to consider.

Energy Stores & Transfers (AQA GCSE Physics: Combined Science)

Revision Note

Systems in Physics