Efficiency

This lesson explains Efficiency for Cambridge IGCSE Physics 0625. It separates the Core requirements from the additional Supplement work for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. Read the explanation once, then attempt the worked method from a blank page.

What does efficiency mean at Core level?

A device transfers input energy into useful and non-useful outputs. The non-useful energy is not destroyed. It is dissipated to the surroundings, commonly by heating or sound.

A more efficient device transfers a greater proportion of its input into the intended useful output. A less efficient device dissipates a greater proportion to the surroundings.

Core questions may ask you to:

• identify the useful and non-useful outputs
• compare two devices qualitatively
• explain that total energy is conserved
• interpret a simple energy-transfer or flow diagram

Do not write that energy is “lost” or “used up”. State where it is transferred.

Which efficiency equations are Supplement content?

The current 0625 syllabus places both percentage equations in the Supplement:

$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100\%$

$\text{efficiency} = \frac{\text{useful power output}}{\text{total power input}} \times 100\%$

Use energy divided by energy, or power divided by power. Do not mix joules and watts in the same fraction.

Quantity	Unit
Energy input or output	joule (J)
Power input or output	watt (W)
Efficiency	no unit, usually written as %

An efficiency cannot be greater than 100%. A result above 100% usually means the fraction has been inverted.

Supplement worked question

An electric motor lifts a 30 N load through 2.0 m. The motor receives 90 J of energy.

(a) Calculate the useful work done on the load. [2]

$W = Fd = 30 \times 2.0 = 60\text{ J}$

(b) Calculate the efficiency of the motor. [2]

$\text{efficiency} = \frac{60}{90} \times 100\% = 66.7\%$

A suitable final answer is 67%.

(c) State what happens to the remaining input energy. [1]

It is dissipated to the internal energy stores of the motor and surroundings, mainly by heating, with some transfer by sound.

Original marking guidance

• one mark for using $W = Fd$ with the correct values
• one mark for 60 J
• one mark for useful output divided by total input
• one mark for 66.7% or a correctly rounded value
• one mark for a correct transfer to the surroundings

How do energy-flow and Sankey diagrams fit in?

A simple flow diagram shows the input and the useful and non-useful outputs. In a Sankey diagram, arrow widths represent energy quantities. The total width of the output arrows must account for the input because energy is conserved.

When values are given, check that:

$\text{input energy} = \text{useful output} + \text{non-useful outputs}$

Extended questions may combine a Sankey diagram with an efficiency calculation. Core students should still understand the useful-versus-dissipated idea, but the percentage equations are Supplement.

Common mistakes

• Putting total input on top. Useful output belongs in the numerator.
• Mixing energy and power. Use J over J or W over W.
• Giving J or W as the efficiency unit. Efficiency has no unit; use a decimal or percentage.
• Saying energy disappears. Energy is conserved and transferred to other stores.
• Ignoring the tier split. Qualitative efficiency is Core; calculation equations are Supplement.

Exam technique

Before accepting an efficiency calculation, ask two questions:

1. Is the answer between 0% and 100%?
2. Did I divide useful output by total input using matching quantities and units?

For an explanation question, name the useful transfer, name the dissipated transfer and state that total energy is conserved.

How this is examined

Core papers can test the qualitative meaning of efficiency and energy conservation. Extended papers can also require the energy or power percentage equations, sometimes after a work or power calculation. Practical questions may ask why a measured output is smaller than the supplied input; a specific transfer such as heating caused by friction is stronger than the vague phrase “energy was lost”.