Energy Stores & Transfers (AQA GCSE Physics)
Revision Note
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
A system can be open, closed or isolated
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Energy Stores & Transfers
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
Energy is transferred from the hot coffee to the mug to the cold hands
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 Tips and Tricks
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.
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