Heating Effect of Current - Joule's Law UNSOLVED PRACTICE SET

Q7. State Joule's Law of heating. Write the formula H = I²Rt and explain each symbol and its unit.

Q8. Why does the filament of an electric bulb glow while the connecting copper wires do not heat up significantly? Use Joule's Law to explain.

Q9. An electric iron has a resistance of 80 Ω and draws 2.75 A. Calculate the heat produced in 30 minutes.

Q10. Why is the heating element of a room heater made of nichrome rather than copper? Give two reasons related to Joule's Law and material properties.

Q11. Explain how Joule's heating is used in the working of an electric fuse. Why must the fuse wire have a low melting point?

Q12. A wire carrying 2 A produces 200 J of heat in 50 seconds. Calculate the resistance of the wire. Show full working using Joule's Law.

Q13. State and explain Joule's Law of heating in detail. Your answer must cover:
(a) the formula H = I²Rt,
(b) deriving an equivalent form H = VIt using Ohm's Law (V = IR),
(c) deriving a third form H = V²t/R,
(d) explaining physically why heat is produced (collisions of electrons with lattice),
(e) three practical applications of Joule's heating effect used in Indian homes.

Q14. A family in Lucknow uses an electric geyser (resistance 40 Ω, current 5.5 A) every morning for 20 minutes.
(a) Calculate the heat produced using H = I²Rt.
(b) Convert this to kilocalories (1 kcal = 4200 J).
(c) Calculate the power of the geyser using P = I²R.
(d) Calculate the cost of running the geyser every morning for 30 days at Rs. 7 per kWh.
(e) Suggest one way to reduce the electricity bill from the geyser without sacrificing comfort.

Q15. Explain the heating effect of current in the context of electrical safety.
(a) How does Joule's heating lead to fire hazards if too much current flows through household wiring?
(b) Why are thick wires used for high-current appliances like geysers and ACs in Indian homes?
(c) Explain how a fuse protects a circuit — what is the fuse made of and how does H = I²Rt apply?
(d) What is an MCB (Miniature Circuit Breaker) and how is it better than a fuse?
(e) Why do appliances feel warm after extended use — is all heat production wasteful?

Q16. An electric toaster in a Mumbai home has resistance 25 Ω and is connected to 220 V.
(a) Calculate the current drawn.
(b) Calculate heat produced in 5 minutes using H = V²t/R.
(c) Calculate power using P = V²/R.
(d) If the cost of electricity is Rs. 8 per kWh, calculate the cost of using the toaster for 30 days (10 minutes per day).

Q17. Two heating coils A and B have resistances 4 Ω and 8 Ω. They are connected:
(a) in series across 24 V — calculate heat in each in 2 minutes; which produces more heat?
(b) in parallel across 24 V — calculate heat in each in 2 minutes; which produces more heat?
(c) Compare your results: in series, higher resistance coil produces more heat; in parallel, lower resistance produces more heat — verify this pattern with your numbers.

Q18. The filament of a 100 W bulb has resistance 484 Ω at operating temperature.
(a) Verify the operating current at 220 V using P = V²/R then I = V/R.
(b) Calculate the heat produced per second.
(c) Only 5% of this heat is converted to light. How much light energy is produced per minute?
(d) An LED rated 10 W produces the same light. Calculate how much heat the LED wastes per minute.
(e) What is the energy saving per day if used for 8 hours (bulb vs LED)?