Question 21

The Correct Answer is C **Why option C is correct:** Heat transfer (flux) is fundamentally driven by a temperature gradient. For the most common modes of heat transfer (conduction, convection, and radiation), the rate of heat transfer ($Q$) is directly proportional to the temperature difference ($\Delta T$) between the hot body (source) and the cold body (sink). This relationship is central to Fourier's Law (conduction), Newton's Law of Cooling (convection), and the Stefan-Boltzmann Law (radiation, which uses the difference in the fourth power of absolute temperatures). Therefore, the magnitude of the temperature difference ($\Delta T$) is the most critical factor determining the rate of heat transfer. **Why the other options are incorrect:** **A) Heat is given off from a high temperature region known as a heat sink.** This statement is incorrect because a **heat sink** is defined as a region or body that absorbs heat—it is typically the low-temperature reservoir in the context of heat transfer or thermodynamics. The high-temperature region that gives off heat is called the **heat source**. **B) Heat transfer rate is affected most by the size of the heat sink involved.** While the size (specifically the surface area) of the heat sink significantly affects the heat transfer rate, it is generally secondary to the temperature difference ($\Delta T$). For example, doubling the surface area might double the rate, but doubling the temperature difference often has a more profound effect, especially in radiation (where $Q \propto T^4$) and is the primary driving force in all modes. **D) Heat transfer by radiation will occur only by mass motion of a fluid substance.** This statement is incorrect. Heat transfer by **radiation** (electromagnetic waves) does not require a medium or mass motion; it can occur perfectly well through a vacuum (like the sun heating the Earth). Heat transfer that occurs by the mass motion of a fluid substance is defined as **convection**.