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Physics · Grades 8–11

Useful output energy

Convert the efficiency percentage to a fraction; wasted energy remains part of the energy account.

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Rule and assumptions

Convert the efficiency percentage to a fraction; wasted energy remains part of the energy account.

Euseful = ηEin

Worked examples

A device receives 1370 J and has useful-energy efficiency 10%. Find the useful output energy. Round to 3 decimal places if needed. Answer in J.

Euseful = ηEin 1370 × 10/100 = 137

137 J

A device receives 490 J and has useful-energy efficiency 70%. Find the useful output energy. Round to 3 decimal places if needed. Answer in J.

Euseful = ηEin 490 × 70/100 = 343

343 J

Try two questions

  1. A device receives 1080 J and has useful-energy efficiency 90%. Find the useful output energy. Round to 3 decimal places if needed. Answer in J.

  2. A device receives 1220 J and has useful-energy efficiency 5%. Find the useful output energy. Round to 3 decimal places if needed. Answer in J.

Answers and reasoning

Answers and marking notes
  1. 972 J

    Euseful = ηEin 1080 × 90/100 = 972

  2. 61 J

    Euseful = ηEin 1220 × 5/100 = 61

Further reading and review references

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Learning preview · independent educational and translation review required. Suggested grades are not official curriculum alignment.