How does an 'energy store' differ from an 'energy pathway'?

An energy store is the form in which energy is held within a system (e.g., kinetic or chemical), whereas a pathway is the mechanism by which energy moves between those stores (e.g., electrical or heating). You can think of stores as containers and pathways as the pipes connecting them.

When should you analyze a system using the 'Mechanical' pathway versus the 'Heating' pathway?

The mechanical pathway is used when a force acts over a distance, such as a weight being lifted. The heating pathway is used when energy is transferred due to a temperature difference between objects, such as a hot drink cooling down in a room.

What is the difference between thermal conduction and thermal convection?

Conduction involves energy transfer through direct particle-to-particle contact and vibrations, mostly in solids. Convection involves the actual bulk movement of heated particles within a liquid or gas, where density changes create rising and falling currents.

What is the common mistake in assuming energy is 'lost' in a machine?

Energy is never lost; it is simply dissipated into non-useful stores, usually the thermal store of the surroundings. Using the term 'lost' implies a violation of the Law of Conservation of Energy, which states total energy must remain constant.

Why is it incorrect to say that 'heat rises'?

Heat itself is a transfer of energy and doesn't have a direction; rather, it is the heated fluid (liquid or gas) that becomes less dense and rises. This distinction is important because energy transfer via radiation can travel in any direction, including downwards.

What error occurs if you calculate efficiency and get a value of 1.2 or 120%?

This error indicates that you have put the total energy input in the numerator instead of the denominator. By definition, a machine cannot output more useful energy than it receives, so efficiency must always be less than or equal to 100%.

What is the definition of the Gravitational Potential Energy store?

It is the energy stored in an object due to its position in a gravitational field. When an object is lifted higher, energy is transferred into this store, and when it falls, energy is transferred out of it.

State the principle of Conservation of Energy.

The principle states that energy cannot be created or destroyed, only transferred between different stores. Consequently, the total energy in a closed system remains constant throughout any process.

What is the formula for Efficiency in terms of energy?

Efficiency = (Useful Energy Output / Total Energy Input) × 100%. This calculation determines what percentage of the energy supplied to a system is actually used for its intended purpose.

Why are metals better thermal conductors than non-metals?

Metals contain delocalised electrons that can move freely through the metal lattice. These electrons collide with ions and other electrons, transferring thermal energy much faster than the simple vibration of atoms found in non-metals.

Energy Stores & Transfers | Pearson Edexcel IGCSE Physics

1. Energy Stores & Systems

A system in physics is defined as an object or a group of objects chosen for study, allowing scientists to ignore external complexities and focus on specific interactions. When a system undergoes change, energy is shifted between different stores, such as kinetic, gravitational potential, chemical, or thermal stores.

Kinetic energy is associated with the motion of an object, while potential energy (gravitational, elastic, or chemical) represents energy stored due to position, deformation, or molecular structure. All objects possess a thermal store, which increases as their temperature rises due to the increased internal kinetic energy of their particles.

A Sankey diagram illustrating energy flow from input to useful and wasted output.

2. Energy Transfer Pathways

Energy does not move between stores spontaneously; it requires a transfer pathway to act as a mechanism for movement. The four primary pathways are mechanical (force acting over a distance), electrical (charge moving through a potential difference), heating (temperature gradients), and radiation (waves like light or sound).

It is critical to distinguish between a 'store' (where energy is kept) and a 'pathway' (how it moves). For example, a battery holds energy in a chemical store, but it transfers that energy to a lightbulb via an electrical pathway.

3. The Principle of Conservation

The Law of Conservation of Energy dictates that energy cannot be created or destroyed, only transformed from one form to another. This means the total energy in a closed system remains constant, and any energy that seems to 'disappear' has actually been dissipated to the surroundings, usually as thermal energy.

In a typical machine, energy is transferred usefully to a desired store, but some is always shifted to non-useful stores due to friction or air resistance. We describe this non-useful energy as wasted, though it still exists within the total energy count of the universe.

4. Thermal Energy Mechanisms

Conduction occurs primarily in solids where vibrating particles transfer kinetic energy to their neighbors through collisions. Metals are superior conductors because their delocalised electrons can move freely through the lattice, rapidly spreading thermal vibrations throughout the material.

Convection is the transfer of heat through fluids (liquids and gases) via the movement of the fluid itself. When a fluid is heated, it expands and becomes less dense, causing it to rise while cooler, denser fluid sinks to take its place, creating a continuous convection current.

Radiation is the only method of heat transfer that does not require particles, as it travels through a vacuum via infrared electromagnetic waves. Surfaces that are dark and matte are excellent absorbers and emitters of infrared, whereas light, shiny surfaces reflect radiation and are poor emitters.

5. Efficiency Calculations

Efficiency is a numerical measure of how much input energy is converted into useful output, expressed as a ratio or a percentage. It is calculated by dividing the useful energy output by the total energy input, ensuring that the result is never greater than 100% due to unavoidable dissipation.

Formula: $\text{Efficiency} = \frac{\text{Useful Energy Output}}{\text{Total Energy Input}} \times 100\%$

To improve efficiency, engineers focus on reducing wasted transfers, such as using lubrication to reduce friction (mechanical waste) or insulation to reduce thermal dissipation.

6. Key Distinctions & Comparisons

Understanding the difference between temperature and thermal energy is vital: temperature is the average kinetic energy of particles, while thermal energy is the total energy of all particles in a substance. A large iceberg has more thermal energy than a cup of boiling water because it contains vastly more molecules, even though its temperature is lower.

Feature	Conduction	Convection	Radiation
Medium	Solids (mostly)	Fluids (Liquids/Gases)	Vacuum/Transparent media
Mechanism	Particle collisions	Bulk fluid movement	Electromagnetic waves
Speed	Slow to Moderate	Moderate	Speed of Light

7. Exam Strategy & Tips

When describing energy transfers, always use the format: 'Energy is transferred from the [Store A] to the [Store B] via the [Pathway]'. This structure ensures you identify all three components required by examiners to earn full marks.

Check Units: Efficiency has no units (it is a ratio), but energy must always be in Joules (J) before performing calculations.
Verify Logic: If your calculated efficiency is over 100%, you have likely swapped the numerator and denominator; remember that output can never exceed input.
Identify Waste: In almost every mechanical scenario, friction results in energy being dissipated to the thermal store of the surroundings.

Energy Stores & Transfers

Summary

1. Energy Stores & Systems

2. Energy Transfer Pathways

3. The Principle of Conservation

4. Thermal Energy Mechanisms

5. Efficiency Calculations

6. Key Distinctions & Comparisons

7. Exam Strategy & Tips

Energy Stores & Transfers

Summary

1. Energy Stores & Systems

2. Energy Transfer Pathways

3. The Principle of Conservation

4. Thermal Energy Mechanisms

5. Efficiency Calculations

6. Key Distinctions & Comparisons

7. Exam Strategy & Tips