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Physics · Grades 7–10

Energy from constant power

Power is an energy-transfer rate; watts times seconds gives joules.

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Rule and assumptions

Power is an energy-transfer rate; watts times seconds gives joules.

E = Pt

Worked examples

A model device transfers energy at constant power 725 W for 52 s. Find the transferred energy. Round to 3 decimal places if needed. Answer in J.

E = Pt 725 × 52 = 37700

37700 J

A model device transfers energy at constant power 400 W for 43 s. Find the transferred energy. Round to 3 decimal places if needed. Answer in J.

E = Pt 400 × 43 = 17200

17200 J

Try two questions

  1. A model device transfers energy at constant power 780 W for 41 s. Find the transferred energy. Round to 3 decimal places if needed. Answer in J.

  2. A model device transfers energy at constant power 715 W for 12 s. Find the transferred energy. Round to 3 decimal places if needed. Answer in J.

Answers and reasoning

Answers and marking notes
  1. 31980 J

    E = Pt 780 × 41 = 31980

  2. 8580 J

    E = Pt 715 × 12 = 8580

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

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