PHYSICS [3 Mark Question Answer]

Fluids expand on heating. On expansion, the density of the heated portion of the fluid decreases, and it becomes light. Warm fluid, being less dense and lighter, rises upwards. The cooler portion of the fluid, being comparatively denser and heavier, moves downwards and occupies the space created by the upward movement of the hot fluid. If the heat is supplied continuously, the entire cycle of hot fluid expanding and rising upwards, followed by cold fluid moving downwards to take its place continues. This constant upward and downward circulation of fluid forms currents in the medium called convection currents. Convection currents distribute the heat throughout the mass of the fluid.

A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely. It usually contains a paper scale inside the stem, to read the relative density directly.

Cooling unit of a refrigerator is at the top. This is because hot air rises and cold air sinks. The air near the cooling unit loses heat, so it sinks to the bottom. The less denser hotter air at the bottom moves up and gets cooled by the cooling unit. This then sinks to the bottom and the cycle repeats. So, the convection currents cool the entire refrigerator uniformly. If the cooling unit is placed at the bottom, then there will be no convection currents and the cooling of refrigerator will not be effective.