20
Question Question 20
Find the energy in joules consumed in 10 hours by four devices of power 500 W each.
Given,
Power rating (P) = 500 W
Converting W into kW
1000 W = 1 kW
500 W = = 0.5 kW
Time (T) = 10 h
We know,
Power =
Or
Energy consumed by each device = Power x Time taken
Substituting we get,
Energy consumed by each device = 0.5 x 10 = 5 kWh
Energy consumed by four devices = 4 x 5 = 20 kWh
Converting kWh to J
1 kWh = 3.6 x 106 J
So, 20 kWh = 3.6 x 106 x 20 = 7.2 x 107 J
Hence, the energy consumed = 7.2 x 107 J
BRIGHT TUTORIALS
BRIGHT TUTORIALS
CBSE Class IX | Academic Year 2026-2027
9403781999
Excellence in Education
Science | Chapter 10: Work and EnergyWeb Content — Quick Reference
Chapter 10: Work and Energy — Quick Reference
Quick Revision Points
- Work W = Fs cosθ (joule); W = 0 when F ⊥ s or displacement = 0
- KE = ½mv²; PE = mgh; Work-energy theorem: W = ΔKE
- Conservation: energy transforms but total remains constant; PE + KE = constant in free fall
- Power P = W/t (watt = J/s); 1 HP = 746 W
- 1 kWh = 3.6 × 10⁶ J = 1 unit of electricity
Exam Tips for Chapter 10
- Read the detailed chapter notes for complete coverage of all NCERT topics.
- Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
- Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
- For numericals (if applicable), practice at least 20 problems of varying difficulty.
- Refer to the practice question bank (200+ questions) for thorough preparation.
Related Resources
- Detailed Notes: ch10-work-and-energy.html
- Practice Questions: 100+ questions with answers in 05-practice-questions/
- Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
- Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html