Practical Uses - Electric Bulb and Fuse - UNSOLVED PRACTICE SET

Q7. Explain the working principle of an electric bulb. Why does the filament glow while the connecting wires do not?

Q8. What is a fuse? What is it made of? Explain using Joule's Law (H = I²Rt) how a fuse protects a circuit from excessive current.

Q9. Why is tungsten used for the filament of a bulb? State four properties of tungsten that make it ideal.

Q10. Why is the bulb filled with an inert gas? What would happen if the bulb were in a vacuum?

Q11. Compare a fuse and an MCB as circuit protection devices. Give two advantages of an MCB over a traditional fuse.

Q12. A 100 W, 220 V bulb and a 60 W, 220 V bulb:
(a) Which has higher resistance?
(b) Calculate the resistance of each using R = V²/P.
(c) Which draws more current?

Q13. Explain the construction and working of an electric bulb in detail. Your answer must cover:
(a) the material and shape of the filament and why tungsten is chosen,
(b) why the filament is coiled,
(c) the role of inert gas filling in prolonging filament life, (d) how electrical energy is converted to heat and light (Joule's effect), and
(e) why incandescent bulbs are being replaced by CFLs and LEDs in India — compare efficiency.

Q14. A short circuit occurs in a mixer grinder in a Kolkata home causing excessive current.
(a) Define short circuit and explain what causes it.
(b) Using H = I²Rt, explain why a short circuit is dangerous. 

(c) How does a properly rated fuse protect the appliance and wiring?
(d) What current rating fuse should be used in a circuit where the total load is 800 W at 230 V? (Calculate: I = P/V, then choose appropriate fuse rating.)
(e) What must you do after replacing a fuse wire to safely restore power? 

Q15. Compare incandescent bulbs, CFLs (Compact Fluorescent Lamps), and LEDs from an energy efficiency perspective.
(a) How does each produce light?
(b) What percentage of input energy is converted to light in each?
(c) A household replaces 10 × 60 W incandescent bulbs with 10 × 9 W LEDs. Calculate the energy saving per day if used for 8 hours.
(d) Calculate the annual cost saving at ₹7 per kWh.
(e) The Government of India's LED programme (UJALA scheme) distributed millions of LEDs — estimate the national saving if 30 crore bulbs were replaced.

Q16. A tungsten filament bulb is rated 100 W, 220 V.
(a) Calculate the current drawn. 

(b) Calculate the resistance of the filament at operating temperature using R = V²/P.
(c) The filament is only 0.5 m long — calculate the resistivity of tungsten at operating temperature if the cross-sectional area is 3.5 × 10⁻⁹ m².
(d) The cold resistance of tungsten is about 40 Ω — how many times greater is the hot resistance? What does this tell you about temperature's effect on resistance?

Q17. A circuit carries a maximum safe current of 5 A and has a 1000 W water heater at 220 V. 

(a) Calculate the current drawn by the heater.
 (b) Is a 5 A fuse appropriate for this circuit? Why or why not?
(c) What is the minimum fuse rating needed?
(d) If a 2 A fuse is accidentally installed, what would happen when the heater is switched on? 

(e) Calculate the heat produced in the fuse wire (R_fuse = 0.01 Ω) per second just before it melts when 4.5 A flows.

Q18. An Indian household is upgrading from 10 incandescent bulbs (60 W each) to LEDs (8 W each). 

(a) Calculate daily energy used by incandescent bulbs (8 hours/day).
(b) Calculate daily energy used by LED bulbs.
(c) Calculate the daily energy saving in kWh.
(d) Calculate the annual cost saving at ₹8 per unit.
(e) The LED bulbs cost ₹120 each — in how many months does the investment pay for itself?