What Is Energy Transfers, Conservation And Dissipation Of Energy? A Visual Guide for OxfordAQA IGCSE Physics Students

oxfordaqa igcse energy transfers, conservation and dissipation of energy is the study of how energy moves between stores as a system changes, why some of that transfer is always dissipated rather than usefully delivered, and how efficiency quantifies the difference between the two. Students who have studied physics under other European syllabuses will recognise the same underlying principle here, energy conservation, though the way OxfordAQA structures and examines it has its own particular emphases, notably the use of Sankey diagrams and efficiency calculations expressed as percentages.

This is a definition-first guide answering what is energy transfers, conservation and dissipation of energy igcse exams expect, with energy transfers, conservation and dissipation of energy explained the specific way it appears on the OxfordAQA specification, alongside the diagrams and worked calculations that recur across past papers. Consider this your oxfordaqa igcse physics definition-first reference, to return to whenever the wording of a question feels unfamiliar.

Key Facts

A system is an object, or a group of objects; when a system changes, energy is transferred.
Energy can be transferred usefully, stored, or dissipated, but it can never be created or destroyed.
Friction and air resistance dissipate energy by heating the surroundings.
Efficiency compares useful energy output with total energy input, expressed as a decimal or as a percentage.
A Sankey diagram represents how the total energy in a system is redistributed, without any net change to the overall total.

Defining the Core Idea

Start with the definition itself, because it's the one most frequently misapplied. When a system, an object or a group of objects, changes, energy is transferred. Candidates should be able to identify when and where energy has been transferred, using the concept of the energy store involved: kinetic energy in a moving object, gravitational potential energy in an object raised against gravity, or elastic potential energy in a stretched or compressed spring, among others.

A useful, concrete illustration of this idea is a simple pendulum, which oscillates by continuously transferring energy back and forth between kinetic energy and gravitational potential energy. At the highest point of its swing, the pendulum bob is momentarily at rest, and its energy is entirely gravitational potential; at the lowest point, its height above the resting position is smallest, and its energy is largely kinetic. Between these two extremes, the energy is shared between both stores, changing continuously as the bob swings.

Notice, too, that in an idealised pendulum with no resistive forces acting on it, the total mechanical energy, kinetic plus gravitational potential, would stay exactly constant throughout the swing; the pendulum would swing forever at the same height. In practice, air resistance and friction at the pivot dissipate a small amount of energy on every swing, which is why a real pendulum gradually loses height and eventually comes to rest. That gradual loss is a clean, visible example of dissipation in action, and it's a scenario examiners return to often because it makes an abstract idea genuinely observable.

Conservation: Energy Is Never Lost, Only Redistributed

The principle of conservation of energy states that energy can be transferred usefully, stored, or dissipated, but the total amount is always conserved, it is never created and never destroyed. This is worth stating precisely in an exam answer, because loosely describing dissipated energy as "lost" is one of the most common ways candidates undermine an otherwise correct explanation.

When energy is transferred, only part of it may be transferred usefully; the remainder is dissipated, becoming stored in a less useful way, commonly as thermal energy spread through the surroundings. Friction and air resistance are the two forces most frequently responsible for this dissipation, converting kinetic energy into thermal energy through the heating effect of surfaces moving against each other.

It's worth being precise about the mechanism here, since "friction causes heating" is only half an explanation. As two surfaces move against each other, microscopic irregularities on each surface collide and catch, and the work done overcoming these collisions is converted into the random kinetic energy of the particles in both surfaces, which is what we observe macroscopically as a rise in temperature. The same underlying mechanism applies to air resistance, where a moving object collides with air particles ahead of it, transferring some of its kinetic energy to those particles and to the surrounding air as thermal energy.

Efficiency: Comparing Useful Output With Total Input

Efficiency quantifies exactly how much of the energy supplied to a device ends up transferred usefully, rather than dissipated. The specification gives two equivalent forms of the relationship:

Form	Equation
As a decimal	efficiency = useful energy out ÷ total energy in
As a percentage	efficiency = (useful energy out ÷ total energy in) × 100%

The same relationship applies equally to power, since power is simply the rate at which energy is transferred: efficiency = useful power out ÷ total power in, again expressible as a decimal or a percentage. Candidates should be comfortable moving between both forms, since a question may specify which one it wants.

Worked example. An electric motor is supplied with 500 J of energy and usefully transfers 350 J as kinetic energy, with the rest dissipated as heat and sound. Calculate the efficiency of the motor as a percentage.
Efficiency = (useful energy out ÷ total energy in) × 100% = (350 ÷ 500) × 100% = 70%.
This means 70% of the input energy is usefully transferred, and the remaining 30%, that is 150 J, is dissipated rather than delivered as useful kinetic energy.

Sankey Diagrams

A Sankey diagram represents energy flow visually, using arrows whose width is proportional to the amount of energy each represents. A single arrow enters, representing the total energy input, and it splits into two or more arrows leaving the diagram, representing the useful output and the dissipated portion, or portions, of the energy. Because energy is conserved, the total width of the arrows leaving the diagram always equals the width of the arrow entering it; no energy simply vanishes from the diagram.

Candidates should be able to both draw and interpret Sankey diagrams, including reading off approximate proportions from the relative widths of the arrows, and constructing a diagram to scale when given numerical values for the useful and dissipated energy.

How to interpret one correctly. If a Sankey diagram for a lightbulb shows a single wide arrow entering, splitting into a narrow arrow labelled "light" and a much wider arrow labelled "heat," this tells you immediately, without needing any numbers, that the bulb is not very efficient: most of the input energy is dissipated as heat rather than converted into the useful output, light.

How This Compares Internationally

This is oxfordaqa igcse physics explained with an eye to how the same underlying physics is presented across different systems. The underlying physics here, conservation of energy and the distinction between useful and dissipated transfer, is universal, and students moving between different national curricula will find the same conceptual core wherever they've studied it. What differs, often, is emphasis and notation: some systems place heavier weight on formal energy-store bookkeeping using named stores, while OxfordAQA's approach leans more on Sankey diagrams and percentage-based efficiency as the primary tools of assessment. If your prior study used a different visual convention for energy diagrams, it's worth specifically practising OxfordAQA-style Sankey diagrams before the exam, since the format itself, arrow width representing energy quantity, carries assessment weight independent of the physics you already understand.

A Second Worked Example: Working Backwards From Efficiency

Worked example. A device is 80% efficient and usefully transfers 240 J. Calculate the total energy supplied to the device, and the energy dissipated.
Rearranging efficiency = useful energy out ÷ total energy in gives total energy in = useful energy out ÷ efficiency.
As a decimal, 80% = 0.8, so total energy in = 240 ÷ 0.8 = 300 J.
Energy dissipated = total energy in - useful energy out = 300 - 240 = 60 J.

This kind of "working backwards" question is common precisely because it tests whether you understand the relationship as a genuine algebraic relationship you can rearrange, rather than a fixed sequence of steps memorised in only one direction. Practise rearranging the efficiency equation both ways, so that whichever quantity a question asks for, forward or backward, feels equally routine.

Common Mistakes

Describing dissipated energy as "lost," implying it has disappeared rather than been transferred to a less useful store.
Forgetting that a Sankey diagram's arrow widths must be proportional to the energy values they represent, drawing a rough sketch instead when a scaled diagram is specifically requested.
Calculating efficiency as total energy in ÷ useful energy out, inverting the ratio and producing a value greater than 100%, which should immediately signal an error.
Treating 100% efficiency as achievable in practice, when in reality some dissipation, however small, is essentially unavoidable in any real energy transfer.

Self-Check Questions

A device is supplied with 200 J and usefully transfers 120 J. Calculate its efficiency as a percentage.
Explain, using the idea of energy stores, what happens to a pendulum's energy as it swings from its highest point to its lowest point.
Explain why "energy is lost" is not an acceptable way to describe what happens to dissipated energy.
Sketch a labelled Sankey diagram for a device that is 40% efficient, showing the relative widths of the useful and dissipated arrows.
Explain, in terms of particle motion, why friction dissipates energy as heat.

Once conservation, dissipation and efficiency are held together as one connected idea rather than three separate facts to memorise, this topic becomes considerably more manageable. Use these worked examples and the Sankey diagram guidance above as your reference oxfordaqa igcse physics notes, and revisit them whenever a past-paper question on efficiency or energy transfer catches you out. Precise notes like these, paired with regular practice questions, are what turn a topic that initially feels abstract into one you can apply confidently under exam conditions.