What Is Energy Transfers? A Visual Guide for Edexcel IGCSE Physics Students

Energy transfers is one of the most commonly examined topics in the Edexcel IGCSE Physics specification, and understanding it properly connects almost every other area of the course

Think of energy as the currency your body, your phone and every machine on the planet runs on. The Pearson Edexcel IGCSE Physics specification (4PH1) treats energy transfers as a unifying theme that cuts across forces, electricity, waves and thermal physics. The edexcel igcse energy transfers content tests whether you can identify which energy stores are involved, how energy moves between them and how much of it ends up where you want it to go.

Key Facts

The eight energy stores

The energy transfers explained approach starts with the stores. Every physical situation involves energy sitting in one or more of these eight stores, then moving to others via the four transfer pathways.

• Chemical: energy stored in the bonds of fuels, food and batteries. Released during chemical reactions.
• Kinetic: energy an object has because it is moving. Depends on mass and speed.
• Gravitational potential: energy an object has because of its height above the ground. Depends on mass, gravitational field strength and height.
• Elastic potential: energy stored in a stretched or compressed object, like a spring or rubber band.
• Thermal: the internal energy of a substance, related to the temperature and the kinetic energy of its particles.
• Magnetic: energy stored in magnetic fields, for instance around a magnet or inside a solenoid.
• Electrostatic: energy stored in electric fields, for instance between charged plates or in a thundercloud.
• Nuclear: energy stored in the nuclei of atoms, released during fission and fusion.

Transfer pathways

The four transfer pathways describe how energy moves between stores. This is where candidates gain or lose marks in the exam. Simply naming stores without saying how the energy gets from one to another is an incomplete answer.

• Mechanically: a force does work on an object. A person pushing a box transfers energy from their chemical store to the kinetic store of the box mechanically.
• Electrically: a current flows through a component. A battery transfers energy from its chemical store to the thermal store of a heater electrically.
• By heating: energy moves from a hotter region to a cooler one. A cup of tea cools as energy transfers from its thermal store to the thermal store of the surroundings by heating.
• By radiation: energy is carried by electromagnetic waves or sound waves. The Sun transfers energy to the Earth by radiation (light).

Conservation of energy

Energy cannot be created or destroyed. The total amount of energy in a closed system stays the same. What changes is which stores the energy sits in. A ball thrown upwards transfers energy from its kinetic store to its gravitational potential store as it rises. At the top, the kinetic store is empty and the gravitational potential store is at its maximum. As the ball falls, the transfer reverses.

Efficiency

No energy transfer is perfectly efficient. Some energy always ends up in stores that are not useful for the intended purpose, typically the thermal store of the surroundings.

Efficiency = (useful energy output / total energy input) x 100%

Worked example: A motor is supplied with 500 J of energy. It does 350 J of useful work lifting a load. Calculate the efficiency.

• Efficiency = (350 / 500) x 100% = 70%

The remaining 150 J has not vanished. It has been transferred to the thermal store of the motor and its surroundings by heating. The principle of conservation of energy is not violated; the energy is just in a store you did not want it in.

Sankey diagrams

The edexcel igcse physics notes on energy transfers include Sankey diagrams. These are scaled arrow diagrams where the width of each arrow is proportional to the amount of energy it represents. The input arrow on the left splits into a useful output arrow (continuing straight) and wasted energy arrows (branching off, usually downwards).

To draw a Sankey diagram: start with the total input energy as the width of the left arrow. Split it into useful and wasted portions, keeping the total width constant (because energy is conserved). Label each arrow with its value in joules and the store or pathway it represents.

Thermal energy transfer: conduction, convection and radiation

The specification requires you to describe how thermal energy transfer takes place by three mechanisms.

Conduction: energy is transferred through a material by vibrating particles passing kinetic energy to neighbouring particles. It is most effective in solids, especially metals, because the particles are close together and metals have free electrons that transfer energy rapidly.

Convection: energy is transferred through fluids (liquids and gases) by the movement of the fluid itself. Warm fluid becomes less dense and rises; cooler, denser fluid sinks to replace it, creating a convection current. This is why radiators are placed near the floor and why hot air rises in a room.

Radiation: energy is transferred by infrared electromagnetic waves. No medium is needed, which is why the Sun's energy reaches Earth through the vacuum of space. Dark, matte surfaces absorb and emit radiation more effectively than light, shiny surfaces.

Reducing unwanted energy transfers

Insulation reduces the rate at which energy leaves a system. The edexcel igcse physics explained approach to this topic covers several practical examples.

• Cavity wall insulation: foam or mineral wool between the inner and outer walls traps air, reducing convection and conduction through the wall.
• Loft insulation: fibreglass laid across the loft floor reduces conduction and traps air to limit convection.
• Double glazing: two panes of glass with a gap (often filled with argon) reduce conduction and convection through windows.
• Draught excluders: strips around doors and windows reduce convection currents carrying warm air out of the building.

Common mistakes

• Saying energy is "used up." Energy is never used up; it is transferred to another store. Always specify which store it moves to.
• Listing stores without pathways. "Chemical to kinetic" is incomplete. "Chemical to kinetic, mechanically" is what the mark scheme expects.
• Confusing efficiency with energy. Efficiency is a ratio, expressed as a percentage. It is not an amount of energy in joules.
• Drawing Sankey diagrams with arrows that do not add up. The total width of all output arrows must equal the width of the input arrow. If it does not, you have violated conservation of energy in your diagram.

Energy resources and electricity generation

The specification also requires you to describe the energy transfers involved in generating electricity from various sources. This connects the energy transfers topic directly to real-world power generation and is tested in both papers of the Edexcel IGCSE Physics exam.

Fossil fuels (coal, oil, gas): Chemical store of the fuel is transferred to thermal store of steam by heating, then to kinetic store of the turbine mechanically, then to electrical energy via the generator electrically.

Wind: Kinetic store of moving air is transferred to kinetic store of the turbine blades mechanically, then to electrical energy via the generator.

Solar cells: Energy is transferred from the nuclear store of the Sun by radiation (light), then converted directly to electrical energy by the photovoltaic cell.

Hydroelectric: Gravitational potential store of water at height is transferred to kinetic store of falling water, then to kinetic store of the turbine mechanically, then to electrical energy via the generator.

Each source has advantages and disadvantages. Renewable sources (wind, solar, hydroelectric, geothermal) do not run out and produce no carbon dioxide during operation, but they depend on weather conditions or geography and may have high initial costs. Non-renewable sources (fossil fuels, nuclear) provide reliable base-load power but produce waste (carbon dioxide and particulates from fossil fuels, radioactive waste from nuclear). The exam expects you to compare these advantages and disadvantages for specific scenarios, not to recite a generic list.

Nuclear power is a special case. The energy comes from the nuclear store of uranium-235 atoms. Fission releases kinetic energy of the fission products, which heats water to produce steam, driving turbines connected to generators. The transfer chain is: nuclear store to thermal store (by heating), to kinetic store of turbine (mechanically), to electrical energy (electrically). The efficiency of the overall process, from fuel to delivered electricity, is typically around 30-40%, with the remainder transferred to the thermal store of the cooling water and surroundings.

Self-check questions

1. A light bulb is supplied with 100 J of energy and produces 15 J of useful light energy. Calculate its efficiency.
2. Describe the energy transfers that occur when a ball is thrown upwards and falls back down, naming both stores and pathways.
3. Explain why metals are good conductors of thermal energy.
4. Describe how a convection current forms above a heater in a room.
5. A kettle transfers 300,000 J of energy to the water and wastes 45,000 J to the surroundings. Draw a Sankey diagram to represent this.

Practise energy transfers questions on the Green Bridge CBT platform using Edexcel IGCSE Physics past questions filtered by topic to build speed and accuracy before your exam.