SSC Selection Post 2025 PYQ — Question 7

Correct Answer

Heating water on a stove

Explanation

Convection is the mode of heat transfer that occurs exclusively in fluids (liquids and gases) through the actual physical movement of the matter itself. When a fluid is heated from below, the molecules near the heat source expand, become less dense, and naturally rise upward. Concurrently, the cooler, denser fluid molecules at the top sink down to take their place. This continuous cycle sets up cyclical loop movements known as convection currents.

Heating water on a stove is a classic practical example of this phenomenon. The water at the bottom of the pot directly touches the hot base, heats up, and rises to the top. The cooler water at the surface moves downward to the bottom, where it is heated in turn.

The other options represent different modes of heat transfer:

Warming your hands by a fire is predominantly an example of radiation (infrared waves traveling through space) and partly convection if your hands are placed directly above the rising hot air plumes.

Heat transmission through a vacuum can only happen via radiation, as radiation is the unique mode of heat transfer that requires no material medium.

Heat transfer in metal occurs through conduction, where heat flows through a solid material via molecular vibrations and free electron collisions without any bulk displacement of the material itself.

Key Points

Convection cannot occur in solids because the constituent molecules are tightly locked within a rigid structural matrix and cannot flow or change positions freely.

Natural convection currents are responsible for crucial planetary and atmospheric phenomena, including the generation of daily sea breezes and land breezes along coastal belts.

In engineering applications, forced convection is utilized by employing external mechanical devices like electric fans, water pumps, or air conditioners to artificially accelerate heat dissipation.

Important Questions and Answers

Question: Why does a room heater function most efficiently when positioned close to the floor level rather than near the ceiling?

Answer: Hot air naturally expands and rises upward due to lower density, creating a natural convection loop that warms the entire room from the bottom up.

Question: Which primary mode of heat transfer is responsible for driving the colossal tectonic plate movements via mantle currents deep inside the Earth?

Answer: Thermal convection currents operating within the semi-fluid asthenosphere layer of Earth's mantle drive tectonic movements.