Question

Which of the following is an example of convection?

• A. Heat moving through a metal rod
• B. Sunlight warming the Earth
• C. Heat in a vacuum
• D. Water circulating in a pot on the stove

Hint:

Convection requires a fluid medium to allow bulk movement of matter. Conduction occurs primarily in solids, and radiation can occur without any medium.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify a practical physical scenario that exemplifies convective heat transfer.

Step 2: Key Formula or Approach:
Convection is the heat transfer mechanism that occurs in fluids (liquids and gases) via the bulk movement of fluid molecules. It is governed by Newton's Law of Cooling:
\[ q = h A (T_s - T_\infty) \]
where:
\( q \) is the heat transfer rate,
\( h \) is the convection heat transfer coefficient,
\( A \) is the surface area, and
\( T_s, T_\infty \) are the surface and fluid temperatures, respectively.

Step 3: Detailed Explanation:

• Water in a Pot: When water is heated in a pot on a stove, the water at the bottom gets hot and expands, becoming less dense.
This lighter water rises, and cooler, denser water from the top sinks to replace it, establishing a natural circulation loop known as a convection current. This is a classic example of free (natural) convection.

• Analysis of Other Options:
-

Heat moving through a metal rod (Option A) is an example of conduction, where heat is transferred via lattice vibrations and free electron motion without bulk movement of the medium.
-

Sunlight warming the Earth and

heat in a vacuum (Options B and C) are examples of radiation, which requires no material medium to transfer energy.


Step 4: Final Answer:
Therefore, water circulating in a pot on the stove is an example of convection, matching Option (D).