Energy Stores & Transfers

Energy stores and transfer pathways are a model for describing energy transfers in a system

Systems in physics

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 to focus only on what is relevant to the situation being observed
A system could be large or small, incorporating just one object, or a whole group of objects and their surroundings

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

If an apple sits on a table, and that table is suddenly removed, the apple will fall
As the apple falls, energy is transferred

apple-table-system

Energy stores

Energy is stored in objects in different energy stores

Table of energy stores

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 transfers

Energy is transferred between stores by different energy transfer pathways
The energy transfer pathways are:
- Mechanical
- Electrical
- Heating
- Radiation
These are described in the table below:

Table of energy transfer pathways

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 by heating is a cup of hot coffee heating up cold hands

1--thermal-energy-transfer--new

Energy is transferred by heating 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 falling object

Answer:

Part a)

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

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

Step 2: Determine the store that energy is transferred to, within the parameters described by the defined system

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 described by the defined system

For a falling object
In order to fall, the object 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, within the parameters described by the defined system

As the object 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 from the gravitational store to the kinetic store of the object via a mechanical transfer pathway

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 (Edexcel IGCSE Physics: Double Science)

Revision Note