Power and Commercial Unit of Energy - UNSOLVED PRACTICE SET

Q7. Define power. Write its formula, SI unit, and give one real-life example that clearly shows the concept of power as a rate.

Q8. What is the commercial unit of energy? Define 1 kWh and show how it relates to Joules (1 kWh = 3.6 × 10⁶ J). Why is Joule not used for household electricity billing?

Q9. A 60 W fan runs for 8 hours every day. Calculate the electrical energy consumed in one month (30 days) in kWh. What would this cost at ₹8 per unit?

Q10. Distinguish between power and energy. Can two machines do the same amount of work but have different powers? Explain with an example from a school setting.

Q11. A horse is said to have a power of 'one horsepower'. One horsepower ≈ 746 W. If a horse works for 4 hours, how much energy does it expend in Joules and in kWh?

Q12. Explain the difference between a 100 W bulb and a 40 W bulb. If both are left on for the same time, which consumes more energy and why? Which one is more 'powerful' in the Physics sense?

Q13. Define power and derive its relationship with force and velocity (P = Fv). Your answer must cover:
(a) definition of power as P = W/t,
(b) substituting W = F × s and simplifying to P = F × (s/t) = F × v,
(c) the unit of power and the definition of 1 Watt,
(d) expressing power in horsepower (1 hp = 746 W) with one Indian example of a machine rated in hp, and
(e) a real-life example showing that two workers doing the same work can have different powers.

Q14. An Indian household uses the following appliances daily: A 1500 W air conditioner for 6 hours, a 200 W television for 4 hours, two 10 W LED bulbs for 10 hours each, and a 1000 W washing machine for 1 hour.
(a) Calculate the energy consumed by each appliance in kWh per day.
(b) Calculate the total daily consumption in kWh.
(c) Calculate the monthly electricity bill (30 days) at ₹7.50 per kWh.
(d) Suggest two ways to reduce the monthly bill without changing lifestyles significantly.

Q15. Compare the power outputs in the following situations and answer all parts:
(a) Sana climbs a 9 m staircase (mass 45 kg) in 15 seconds. Calculate her power (g = 10 m/s²).
(b) A lift motor raises a 600 kg load to 20 m in 30 seconds. Calculate the power of the motor.
(c) A 250 W water pump fills a 2000 litre tank (1 litre = 1 kg of water) at a height of 10 m. How long does it take? (g = 10 m/s²)
(d) Which is more powerful — Sana or the lift motor? Express the ratio.
(e) What is the efficiency of the water pump if it consumes 300 W from the mains but delivers 250 W of useful power?

Q16. A motor lifts 200 kg of water from a well 15 m deep in 1 minute. (g = 10 m/s²)
(a) Calculate the work done by the motor.
(b) Calculate the power of the motor in Watts.
(c) If the motor runs for 5 hours a day, find the energy consumed in kWh.
(d) Find the cost at ₹6 per unit.

Q17. Two workers, Ramesh and Suresh, are loading trucks at a warehouse in Surat. Ramesh loads 50 boxes (each 20 kg) onto a truck 1.5 m high in 10 minutes. Suresh loads the same 50 boxes in 25 minutes. (g = 10 m/s²)
(a) Calculate the total work done by each worker.
(b) Calculate the power output of each.
(c) Who is more powerful?
(d) Do they both do the same total work?

Q18. An electricity meter in a home records the following usage in one month: Air conditioner (2000 W) — 90 hours. Refrigerator (150 W) — 720 hours (runs all month). Geyser (2000 W) — 30 hours. Lights and fans (combined 200 W) — 300 hours.
(a) Calculate the energy consumed by each appliance in kWh.
(b) Which appliance consumed the most energy?
(c) Calculate the total bill at ₹8 per kWh.
(d) If the family switches the AC off when not needed and reduces AC use to 45 hours, how much money do they save?