Electric Power and Energy - UNSOLVED PRACTICE SET

Q7. Define electric power. Write three equivalent formulae for power: P = VI, P = I²R, P = V²/R. When is each form most useful?

Q8. Define the kilowatt-hour (kWh). Show that 1 kWh = 3.6 × 10⁶ J. Why is kWh used for billing rather than the joule

Q9. A 60 W bulb and a 100 W bulb are connected to 220 V. Calculate the resistance of each. Which has higher resistance and why?

Q10. A household in Bhopal uses a 1500 W AC for 8 hours per day. Calculate the daily energy consumption in kWh and the monthly electricity cost at ₹6.50 per unit.

Q11. A heater is rated '2000 W, 220 V'. What do these ratings mean? Calculate the current drawn and the resistance of the heating element.

Q12. Why does a 40 W bulb glow less brightly than a 100 W bulb even when both are connected to the same 220 V supply? Explain using the concept of power and energy.

Q13. Explain electric power and electrical energy in detail. Your answer must cover:
(a) definition of power as P = W/t = VI,
(b) three equivalent formulae (P = VI, P = I²R, P = V²/R) derived using Ohm's Law,
(c) the definition of 1 Watt,
(d) the commercial unit kWh — define 1 kWh and show 1 kWh = 3.6 × 10⁶ J, and (e) an example calculation of monthly electricity bill for an Indian household running specific appliances. 

Q14. An Indian family's monthly electricity consumption: AC (1.5 ton = 1500 W) — 6 h/day for 30 days. Refrigerator (200 W) — 24 h/day. TV (100 W) — 5 h/day. Washing machine (800 W) — 1 h/day. Four LED lights (10 W each) — 8 h/day.
(a) Calculate monthly units (kWh) for each appliance.
(b) Find total monthly consumption.
(c) Calculate total monthly bill at Rs. 7 per kWh.
(d) If they switch to a 5-star AC (900 W instead of 1500 W), how much do they save per month?
(e) Suggest two more energy-saving steps.

Q15. Compare the power consumption of series and parallel arrangements:
(a) Three 60 W bulbs connected in series across 220 V — find effective resistance, current, and actual power consumed by each.
(b) Same three bulbs connected in parallel across 220 V — find total current and power consumed by each.
(c) In which arrangement do the bulbs glow at their rated brightness? Explain.
(d) In which arrangement does the source deliver more total power?
(e) What does this tell us about why domestic wiring uses parallel connections?

Q16. A resistor of 15 Ω carries a current of 3 A for 10 minutes.
(a) Calculate power dissipated.
(b) Calculate energy consumed in Joules.
(c) Convert energy to kWh.
(d) Calculate the cost at Rs. 5 per unit.

Q17. An electric water pump rated 750 W runs for 4 hours per day. The household also uses a 2000 W geyser for 45 minutes each morning.
(a) Calculate the daily energy consumption of the pump in kWh.
(b) Calculate the daily energy consumption of the geyser in kWh.
(c) Which appliance costs more per day at Rs. 8/unit?
(d) Calculate total monthly (30-day) cost for both.

Q18. Two appliances are connected to 220 V: Appliance A draws 5 A, Appliance B draws 2 A.
(a) Calculate the power of each.
(b) Calculate energy each consumes in 3 hours.
(c) Calculate the resistance of each using P = V²/R.
(d) Appliance A is a geyser and B is a ceiling fan — which identification makes physical sense?
(e) If both are connected in series (not usual, just a thought experiment) — what is the total resistance and current?