Edexcel GCSE Physics (1PH0) — Conservation of energy
Edexcel requires a strict distinction between an energy store (where energy is kept) and a transfer pathway (the process that moves the energy). A qualitative flow diagram must follow the sequence: [Initial Store] [Transfer Pathway] [Final Store].
Example: An Electric Motor (powered by a battery) In this system, energy begins in a chemical store and is transferred to fulfill its purpose (useful) or lost to the environment (wasted).
While block diagrams show the sequence of transfers, a Sankey diagram shows exactly how much energy is moving.
An electric winch uses 600 J of electrical energy to lift a heavy crate. During this process, 150 J of energy is transferred usefully to the crate's gravitational potential store, while the rest is wasted as thermal energy due to friction. Sketch a Sankey diagram for the winch, using a scale of 1 grid square = 50 J.
Step 1: Calculate the wasted energy.
Step 2: Apply the scale () to find the required arrow widths.
Step 3: Describe the sketch.
EDEXCEL RULE: Always use the sequence 'Store -> Pathway -> Store'. For example, never say energy transfers from an 'electrical store'—electricity is a pathway. Input energy must always originate from a store (like Chemical or Magnetic).
Students often refer to 'light energy' or 'sound energy' as stores. Edexcel mark schemes penalise this—always identify light and sound as 'radiation pathways' that transfer energy to a thermal store.
When sketching energy flow diagrams, always include the 'wasted' branch. A linear diagram that only shows useful energy is incomplete and will lose marks.
In a 'Sketch a Sankey diagram' question, ensure the useful arrow points straight right, the wasted arrow points straight down, and their combined widths exactly equal the input arrow width.
Principle of Conservation of Energy
The fundamental law stating that energy cannot be created or destroyed, only transferred from one store to another.
dissipated
Energy that has spread out into the surroundings (usually as thermal energy) and become less useful for doing work.
wasted energy
Energy transferred to non-useful stores, typically the thermal store of the surroundings.
energy store
The specific way or place that energy is kept within a system (e.g., kinetic, chemical, thermal, GPE).
transfer pathway
The process or mechanism that moves energy from one store to another (mechanical, electrical, heating, or radiation).
Sankey diagram
A scale flow diagram where the vertical width of the arrows is proportional to the magnitude of energy or power.
useful energy
Energy transferred to the intended store to fulfill the specific purpose of a device.
efficiency
A measure of how much input energy is transferred usefully, visually represented by the ratio of the 'useful' arrow width to the input arrow width.
Put your knowledge into practice — try past paper questions for Physics
Principle of Conservation of Energy
The fundamental law stating that energy cannot be created or destroyed, only transferred from one store to another.
dissipated
Energy that has spread out into the surroundings (usually as thermal energy) and become less useful for doing work.
wasted energy
Energy transferred to non-useful stores, typically the thermal store of the surroundings.
energy store
The specific way or place that energy is kept within a system (e.g., kinetic, chemical, thermal, GPE).
transfer pathway
The process or mechanism that moves energy from one store to another (mechanical, electrical, heating, or radiation).
Sankey diagram
A scale flow diagram where the vertical width of the arrows is proportional to the magnitude of energy or power.
useful energy
Energy transferred to the intended store to fulfill the specific purpose of a device.
efficiency
A measure of how much input energy is transferred usefully, visually represented by the ratio of the 'useful' arrow width to the input arrow width.
