Welcome back to Beyond’s Science Revision Blog! This exciting entry explores the idea of energy. We discuss how it is used commonly in everyday language and how students may have come across its use in familiar contexts before they study it at secondary school. For example:
- They may have been asked to turn off the lights to ‘save energy’.
- They may have been told that someone has ‘run out of energy’.
- They may be familiar with ‘energy saving’ light bulbs.
- They may know about ‘energy bills’ that charge for the ‘energy used’ over the period.
Rather than being helpful, these experiences can make energy challenging to teach. They present energy like a substance that can be saved or used up. This is in contrast to the scientific concept that we will teach which tells them that energy is conserved.
Energy is not an invisible substance; it is an abstract mathematical concept and that can make it difficult for students to grasp. Feynman said, “It is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate that number again, it is the same.”1 Abstract concepts like this are difficult for students to understand and that adds to the challenge for teachers.
It is likely that when you studied energy at school, or even when you trained as a teacher, it was taught as being one of nine types, and that it could be converted from one type of energy to another. Again, this implies that energy is a substance and one that can be changed from one thing to another, which is not the case. As a result, there have been significant changes in the most recent national curriculum which have implications for the way that we teach energy.
- Discuss energy as a quantitative tool rather than as a substance. The focus of an energy analysis will be on calculations.
- Compare the conditions at the start with the conditions after a change to the system, instead of introducing chains of energy transfers.
- Explain the intermediate steps that bring about changes in energy stores using physical processes and mechanisms rather than energy.
|energy store||A way that energy is stored in or by objects due to their motion, position, shape or processes.|
|energy transfer||The movement of energy from one store to another.|
|pathway||A process that causes a change in the way energy is stored in a system.|
|system||An object or group of objects.|
Ensure that your students are clear about the definitions of these and other key words with our Energy Glossary.
A typical exam question:
Describe the energy changes as a person cycles to the top of a hill.
In the past, we would have taught students to draw an energy diagram that might have looked something like this:
This is problematic for a few reasons:
- It presents the energy as different types, which can be converted to others, implying it is some kind of substance.
- It includes sound and heat as ways that energy can be stored when these are in fact methods of energy transfer.
- It creates a chain of energy transfers that replaces the physical processes that are happening as an intermediate energy step.
In this scenario, we have included ‘kinetic energy’ as an intermediate step in the process. This is a distraction from the underlying physics that is actually bringing about the change. The cyclist exerts a force on the pedals of the bike; this means the system does work mechanically to move the bike against gravity and friction. Including ‘kinetic energy’ in this chain does not help us to understand what is happening, nor is it helpful in calculating the energy before or after the change in the system.
Instead, when we analyse the energy in the system, we need to define the start and end points. From here, it is clearer where energy is stored both before and after the system changes.
Start point: The cyclist is stationary at the bottom of the hill.
End point: The cyclist is stationary at the top of the hill.
Now we can compare the conditions before and after the system changed:
- The energy associated with the chemicals in the system (food and oxygen) has decreased.
- The energy associated with the cyclist being in the Earth’s gravitational field has increased.
- The thermal stores associated with temperature rises of the cyclist, the bike and the surroundings have also increased.
To simplify these descriptions, we can instead refer to energy stores. The chemical energy store has decreased, the gravitational potential energy store has increased and the thermal energy stores have increased.
The total energy stored at the end is the same as the total energy stored at the start. Energy is conserved.
This Energy Analysis Graph Skills Worksheet can be used to analyse data on the ways energy is stored before and after a system changes, including drawing a bar chart.
Energy stores are not physical entities; they represent a value that contributes to the system’s total energy. For example, the position of the cyclist in the Earth’s gravitational field contributes to the total energy of the system.
For each energy store, there is a formula for calculating its value. This means we can calculate how much the position of the cyclist in the Earth’s gravitational field contributes to the total energy of the system.
|chemical||The energy stored in the bonds of a substance or group of substances.|
|elastic potential||The energy stored when an object has been stretched or compressed.|
|electrostatic||The energy stored when repelling charges have been pushed closer together or when attracting charges have been pulled further apart.|
|gravitational potential||The energy stored in an object at height.|
|internal (thermal)||The total kinetic and potential energy stores of all the particles (atoms and molecules) that make up a system. Sometimes referred to as the thermal energy store. In most cases, this is the vibrations – also known as the kinetic energy store – of particles. In hotter objects, the particles have a larger internal energy store and vibrate faster.|
|kinetic||The energy stored in a moving object.|
|magnetic||The energy stored when repelling poles have been pushed closer together or when attracting poles have been pulled further apart.|
|nuclear||The energy stored in the nucleus of an atom.|
Students will be introduced to the internal energy store at KS4. The internal energy store is the total kinetic and potential energy of the particles in an object. In hotter objects, the particles have more internal energy and vibrate faster. When energy is transferred to an object by heating, the object’s internal energy store will increase. At KS3, this store is usually referred to as the thermal energy store.
You will notice that light, sound and electrical energy are missing from the list of energy stores. Because these are usually transient, you can’t calculate their value in an instant. You would also need to consider time in the calculations. This means they do not have a role in an analysis that compares the conditions before and after a system changes.
Instead, they are methods (or pathways) for transferring energy.
Methods (Pathways) For Transferring Energy
A pathway is a process that causes a change in the way energy is stored in a system. Pathways are processes that bring about change over a period of time, which means we cannot calculate the energy associated with them in any instant.
Energy is transferred from one store to another as a result of doing work or heating.
|work done electrically||Energy is transferred by a current when charges move due to a potential difference. Work is done electrically.|
|work done mechanically||Energy is transferred by a force making something move through a distance. This can be by friction to a thermal energy store. Work is done mechanically.|
|heating via particles||Energy is transferred by conduction or convection because of a temperature difference between two objects.|
|heating via radiation||Energy is transferred by waves from a source of electromagnetic radiation to an absorber, e.g. visible light and infrared are emitted from the Sun.|
This Energy Transfer Pathways Match and Draw worksheet can be used to review students’ understanding of these different energy stores.
Energy can be transferred usefully, stored or dissipated. The dissipated energy is often described as being ‘wasted’. However, it is more useful for students to understand the concept of dissipation. Energy tends to spread from a more concentrated energy store to more dispersed energy stores. This means that it is stored in less useful ways. Dissipated energy often ends up increasing the thermal energy store of the surroundings.
A Note on Sound
Sound waves transfer energy due to the vibration of particles. However, the pathway is mostly very small in comparison to other transfers so perhaps not important in most examples at the school level.
Observation: The mug falls towards the ground.
Explanation: The forces on the mug are unbalanced, the force of gravity is larger than air resistance. The mug accelerates towards the ground, moving the air particles in front of it. The temperature of the air particles and the mug increases.
Start Point: The hand has let go of the mug.
End Point: The mug is halfway between the hand and the
The energy stored gravitationally in the system has decreased and the kinetic energy store of the mug has increased. The energy stored thermally by the air and the mug has increased.
Observation: The hot-water bottle warms the bed.
Explanation: Infrared radiation is emitted by the hot-water bottle. The vibration of particles in the hot-water bottle causes the vibration of the particles in the bed to increase through the process of conduction. The vibration of air particles next to the hot-water bottle increases which cause a decrease in their density. The air particles rise and are replaced with air particles at a lower density.
Start Point: The hot-water bottle is placed on the bed.
End Point: The hot-water bottle and the bed are the same temperature.
The energy stored thermally in the hot-water bottle has decreased and the energy stored thermally in the bed and the surroundings have increased. The pathways are heating by particles and heating by radiation.
A Note on Heat
In everyday language, people may talk about heat passing from the hot-water bottle to the bed. This implies that heat is a substance, when in fact heating is a process that occurs as a result of a temperature difference between objects. This heating can occur by particles (conduction and convection) or by radiation.
You can explore the processes of energy transfer by heating in more detail with our Thermal Processes Lesson Pack.
Observation: When the switch is closed, the bulb lights up.
Explanation: When the switch is closed, charges move through a potential difference. Electrons are forced through the filament of the bulb, which has a high resistance. As a result, the temperature of the bulb increases. The hot bulb emits visible light and infrared radiation. The vibration of air particles next to the hot bulb increases which cause a decrease in their density. The air particles rise and are replaced with air particles at a lower density. The temperature of the surroundings increases.
Start Point: The switch is open, there is no current flowing.
End Point: The switch is closed, there is a current flowing.
Energy is transferred electrically from the chemical energy store associated with the cell to the thermal energy store of the bulb. The chemical energy store of the cell decreases and the thermal energy store of the bulb increases. Energy is then transferred from the thermal energy store of the bulb to the thermal energy store of the surroundings by heating. The thermal energy store of the surroundings increases.
Students can complete an energy analysis for four different systems using this Energy Transfer Diagrams Worksheet, and this Energy Circus activity provides an opportunity for students to compare the conditions before and after a system changes.
Check out our Energy Stores and Transfer Pathways Resource Pack.
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