Revision Notes

1.7.4 Efficiency

Efficient & Inefficient Systems

  • Whenever energy is transferred from one form to another, some of that energy is usually wasted and is transferred away from the system, usually in the form of heat or waves (light and sound)


Sankey diagram, IGCSE & GCSE Physics revision notesWhenever energy is transformed, some of the original energy usually ends up in an unwanted (wasted) form


  • An efficient system is one where most of the energy going into that system ends up in the form that is wanted
    Eg. An LED light bulb is efficient because most of the electrical energy (90%) ends up as light
  • An inefficient system is one where most of the energy ends up in forms that weren’t wanted
    Eg. An old incandescent light bulb is inefficient because only a small amount of the electrical energy (5%) ends up as light


Sankey comparison, IGCSE & GCSE Physics revision notesSankey diagrams comparing modern and old light bulbs



Exam Question: Efficiency

Extended Only

Calculating Efficiency

  • The efficiency of a system is the percentage of energy transferred from the original store that ends up in the intended form
  • Efficiency can be calculated using the following equation:

Efficiency equation

  • Efficiency can also be written in terms of power) the energy per second):

Efficiency equation 2

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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