A metal rod is heated at one end, and the temperature at the other end rises slowly over time. At the same time, a sealed glass f…

Question

A metal rod is heated at one end, and the temperature at the other end rises slowly over time. At the same time, a sealed glass flask filled with water is placed over a flame, and the water temperature rises quickly.

Which of the following statements is correct regarding modes of heat transfer in each scenario (Rod and Flask)?

1. A. Rod: Convection; Flask: Conduction and Radiation
2. B. Rod: Conduction; Flask: Convection and Radiation (Correct answer)
3. C. Rod: Conduction; Flask: Convection
4. D. Rod: Radiation; Flask: Convection

Correct Answer

Option B — Rod: Conduction; Flask: Convection and Radiation

Detailed Solution & Explanation

The correct answer is Rod: Conduction; Flask: Convection and Radiation, and both option B and option C were accepted for this question.

Key Points

• The metal rod — conduction. Heat travels along the rod from the hot end to the cold end without any bulk movement of the metal itself. Energy passes from particle to particle by molecular vibration and by free electrons. The slow, gradual rise in temperature at the far end is characteristic of conduction.
• The water in the flask — convection. Water near the flame becomes less dense and rises, while cooler, denser water sinks to take its place. This sets up convection currents that carry heat rapidly through the whole volume, which is why the water heats quickly.
• Radiation is also present, since the flame emits infrared radiation that reaches the flask directly without needing a medium.
• Because radiation from the flame is genuinely present but not the dominant mechanism inside the water, both B (Convection and Radiation) and C (Convection alone) are defensible, and the Board accepted both.

Additional Information

• The three modes of heat transfer:

• Why metals conduct best: they possess free (delocalised) electrons that carry kinetic energy through the lattice far faster than vibration alone. Silver is the best conductor, followed by copper and aluminium.
• Everyday convection: land and sea breezes, monsoon circulation, the working of a chimney, and the ventilators placed high on the walls of a room.
• Radiation obeys the Stefan–Boltzmann law, $E = \sigma T^4$, so the energy radiated rises very steeply with temperature.