MYP Physics: Efficiency & Energy Diagrams FAQ

It's assessed mainly under Criterion A (Knowing and Understanding) and Criterion C (Processing and Evaluating Data), through calculations of efficiency using energy input and useful output, plus Sankey diagrams showing energy transfers and losses to the surroundings, usually as heat or sound.

An energy diagram, usually a Sankey diagram, shows how energy transfers from an input source through a device, splitting into useful output and wasted energy. Arrow width is drawn proportional to the amount of energy, so a thin arrow means less energy than a thick one, always in joules or watts.

Efficiency equals useful energy output divided by total energy input, multiplied by 100 to give a percentage. The formula is efficiency (%) = (useful energy output ÷ total energy input) × 100. It can also be written using power instead of energy if the transfer happens over a fixed time.

No real machine reaches 100% efficiency because some input energy always dissipates as heat, sound or vibration due to friction, air resistance or electrical resistance. This wasted energy still exists — it isn't destroyed — it's just spread into the surroundings in a less useful form, which is what the second law of thermodynamics describes.

Draw one input arrow on the left, splitting into a useful-output arrow (usually staying horizontal) and one or more waste arrows angling away. Arrow widths must be proportional to energy values, every arrow needs a label with a value and unit, and the diagram should be drawn using a ruler with a clear scale.

The three I see every year: forgetting to multiply by 100 for a percentage, mixing up input and output in the formula, and stating an efficiency above 100% without questioning it. Efficiency can never exceed 100% — if your answer says otherwise, you've swapped input and output somewhere in the calculation.

Criterion C rewards how you process and evaluate data, not just correct numbers. To move up a band, show full working with units at every step, state your efficiency answer to an appropriate number of significant figures, and add a sentence evaluating the result — is it realistic for that type of device?

You need efficiency (%) = (useful energy output ÷ total energy input) × 100, and the same formula using power instead of energy when a rate is given. You should also be comfortable rearranging it to find input or output energy when efficiency and one value are already known.

Yes — energy transfer and efficiency questions appear regularly in the MYP eAssessment for Sciences, often combined with real-world contexts like renewable energy or transport. Expect a mix of calculation questions and diagram-based questions requiring you to interpret or construct a Sankey diagram from given data.

Efficiency and energy diagrams sit within the Energy strand of MYP Sciences, typically taught in MYP 4 or 5 alongside work, power and thermal energy transfer. It links directly to the MYP: Sciences guide's key concept of Energy and the related concept of Transformation.

MYP efficiency work builds the foundation for DP Physics topics on energy, power and thermal physics, where the same efficiency formula reappears alongside more complex systems like heat engines and specific heat capacity calculations. Students who master Sankey diagrams in MYP find DP energy-transfer questions far less intimidating.

MYP treats efficiency and energy diagrams conceptually, with straightforward calculations and Sankey diagrams built from given data. DP Physics uses the same core formula but applies it to more demanding contexts — heat engines, specific heat capacity, and multi-step problems requiring calculus-free but layered reasoning.

Look for resources with worked Sankey diagram examples and step-by-step efficiency calculations rather than pure theory. On RevisionPrep, the MYP Physics Revision Notes and Topical Worksheets cover this exact strand with practice questions modelled on eAssessment-style tasks, which is what most students actually need before a test.

Usually it's presentation, not understanding — missing units, arrows drawn without proportional widths, or forgetting to convert a decimal to a percentage. These are Criterion C marks lost on process, not Criterion A marks lost on knowledge, and they're the easiest ones to fix with focused practice.