Energy Stores & TransfersEdexcel IGCSE Physics: Revision notes

Energy can be held in different energy stores: chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic and nuclear.

Energy is moved between stores by different transfer pathways: mechanically (by a force doing work), electrically (by an electric current), by heating, or by radiation (light and sound).

Describing an energy change means identifying the starting store, the transfer pathway, and the ending store(s).

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another. The total energy in a closed system is always the same before and after any change.

This principle underlies all energy calculations: whatever energy leaves one store must appear in another store (though some is often transferred to the surroundings as wasted thermal energy).

Not all the energy supplied to a device ends up doing something useful — some is wasted, usually as heat to the surroundings. Efficiency measures how much of the total input energy becomes useful output energy:

efficiency = (useful energy output / total energy output) × 100%

Sankey diagrams are used to represent these transfers visually: the width of each arrow is proportional to the amount of energy, showing the useful output, wasted output, and total input at a glance.

Thermal energy transfers by three processes:

• Conduction — energy transfer through a material (especially solids) by vibrating particles passing energy to neighbouring particles, without the particles themselves moving position
• Convection — energy transfer in fluids (liquids and gases) as warmer, less dense fluid rises and cooler, denser fluid sinks, creating a circulating current
• Radiation — energy transfer by electromagnetic waves (infrared), which does not require a medium and can travel through a vacuum

Convection in everyday life: convection currents drive weather patterns, central heating systems (warm air/water rising and circulating), and cooling in refrigerators.

Emission and absorption of radiation: dark, matt surfaces are good emitters and absorbers of thermal radiation; light, shiny surfaces are poor emitters and absorbers (and good reflectors). Hotter objects emit more radiation than cooler ones.

Unwanted energy transfers (usually wasted heat) can be reduced using methods such as:

• Insulation — e.g. loft/cavity wall insulation reduces heat loss by conduction and convection from buildings
• Lubrication — reduces friction between moving parts, reducing unwanted heating and energy loss to the surroundings
• Streamlining — reduces air resistance, reducing unwanted energy transfer to the surroundings as a vehicle moves

Reducing unwanted transfers increases the efficiency of a device or system, because more of the input energy reaches the useful output store.