Question

If we switch on a fan heater, it blow hot air through a grille into the room, then
(A) Cold air rises up
(B) Hot air rises up
(C) Neither A nor B
(D) None

Answer 1

Hint When a fan blows air, it actually gives hot air instead of cold. When this air heats up the other air and it rises up, cold air rushes to fill the void. This movement of air is the real reason why we feel good when sitting under a fan.

Complete step by step answer:
Temperature measures the heat while density measures how the molecules in a substance or an object are packed or ratio of mass of the object to volume.
We know that when the heater is switched on the temperature of the air inside the room increases which affect the density of the air.
We know that the mathematical relationship between the density and temperature for a gas is given by
$P = \rho RT$
Where $\rho$ represents the density of gas and T is the temperature of the gas
From this relation we know that,
$\rho \propto \dfrac{1}{T}$
Density is inversely proportional to temperature. As temperature increases the density decreases and vice versa.
Since, the air in the room is now hotter it becomes less dense and hence rises upwards

Therefore, hot air rises upwards and the correct option is B.

Note: Temperature affects a lot of physical quantities like pressure, volume, energy, speed of gas and more. This temperature is responsible for heating the gas and making it less dense. This heated or less dense air has a tendency to rise up which actually creates a void below.