Energy Transfers (Oxford AQA IGCSE Physics)
Revision Note
Energy Transfers
What is a system?
In Physics, a system is defined as
An object or group of objects
Defining the system in physics is a way of narrowing the parameters of the situation 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
An apple sitting on a table can be defined as a system
If the table is removed, the apple will fall
As the apple falls, energy is transferred
Apple-table system
Energy stores
In 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:
Table of Energy Stores
Energy store | Description |
|---|---|
Kinetic | Moving objects have energy in their kinetic store |
Gravitational potential | Objects gain energy in their gravitational potential store when they are raised through a gravitational field |
Elastic potential | Objects have energy in their elastic potential store if they are squashed, stretched or bent |
Electrostatic | Objects with charge (like protons and electrons) have energy in their electrostatic store when they interact with one another |
Magnetic | Magnetic materials interacting with one another have energy in their magnetic stores |
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 energy transfer pathways
There are 4 different transfer pathways shown in the table below:
Table of Energy Transfer Pathways
Transfer Pathway | Description |
|---|---|
Mechanical working | A force acts over a distance (e.g. pulling, pushing, stretching, squashing) |
Electrical working | A charge moves 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 is transferred by electromagnetic radiation (e.g. visible light) |
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
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, that force is weight and it acts over a distance (the height of the fall)
Therefore, this is a mechanical transfer pathway
Energy is transferred mechanically from the gravitational potential store of the ball to the kinetic store of the ball
Examiner Tip
Don't worry too much about the parameters of the system. They exist to help you keep your answers concise so you don't waste time in your exam.
If you follow any process backwards, you will identify lots of the energy transfers taking place. An electric kettle heating water, for example. 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. You could take it back further to the point of electricity generation. This is beyond the scope of the question. Defining the system gives you a starting and a stopping point for the energy transfers you need to consider.
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