Topic: Heat Transfer
A thin copper wire at $300^{\circ}C$ is suddenly immersed in water at $30^{\circ}C$. It cools down to $150^{\circ}C$ in 70 seconds. It is then reheated to the initial temperature of $300^{\circ}C$ and suddenly exposed to air at $30^{\circ}C$ where it cools down to $150^{\circ}C$ in 200 seconds. This difference in cooling time is due to
- A. Larger specific heat of water
- B. Larger heat transfer coefficient in water
- C. Smaller heat transfer coefficient in water
- D. None of the above
Answer: B. Larger heat transfer coefficient in water
Explanation: Heat transfer coefficient is the proportionality constant between the heat flux and the temperature difference $\Delta T$. Water has a significantly larger convective heat transfer coefficient compared to air. So the larger the heat transfer coefficient, the lesser the time needed for the body to cover the temperature difference.