Tracer — Soil Survey and Soil Conservation — 2016 — Official Paper — Kerala PSC PYQ Practice with Answers

Correct answer (Option A):\nUnlike liquids, the dynamic viscosity of gases increases with a rise in temperature. In gases, viscosity is primarily governed by molecular momentum transfer rather than intermolecular cohesive forces. As the temperature rises, the thermal agitation and velocity of gas molecules increase, leading to more frequent molecular collisions and greater resistance to flow.\n\nWhy others are wrong:\nOption B is incorrect because a decrease in viscosity with temperature rise happens in liquids, where cohesive forces dominate and break down under heat. Option C is wrong because temperature has a strong, highly noticeable impact on gas viscosity.\n\nStudy tip:\nRemember the distinction: Liquid viscosity decreases with temperature due to weakened cohesive forces, while gas viscosity increases due to increased molecular collisions.

Correct answer (Option B):\nBuoyant force is the net upward force exerted on an object fully or partially immersed in a fluid. This force is the resultant force acting on the body due to the static fluid pressure surrounding it, as pressure increases with depth, causing a greater upward force on the bottom of the object than the downward force on its top.\n\nWhy others are wrong:\nOption A is incorrect because it mistakenly combines gravity with upthrust. Option C is wrong because the buoyant force is equal to the *weight* of the liquid displaced, not just the volume itself. Option D incorrectly includes dynamic thrust, which applies to flowing fluids rather than static buoyancy.\n\nStudy tip:\nArchimedes' principle states that the buoyant force equals the weight of the fluid displaced by the body: FB = ρ × g × V.

Correct answer (Option D):\nA flow is defined as turbulent when the fluid particles move in a highly irregular, chaotic, and zig-zag path. Consequently, the velocity vectors at any point continuously change in both direction and magnitude from one instant to another.\n\nWhy others are wrong:\nOption A refers to highly ordered layers sliding past each other smoothly. Option B refers to flow where velocity does not change with respect to space. Option C represents a flow where fluid properties at a point do not change over time.\n\nStudy tip:\nTurbulent flow generally occurs at high Reynolds numbers (Re > 4000 for pipe flow), where inertial forces dominate over viscous forces.

Correct answer (Option D):\nThe centre of pressure is defined as the specific point on an immersed surface where the total resultant hydrostatic pressure force acts. For inclined or vertical surfaces, this point always lies below the centre of gravity because hydrostatic pressure increases linearly with fluid depth.\n\nWhy others are wrong:\nOption A is the center of gravity of the displaced volume of fluid. Option B is the point of intersection of the buoyant force line before and after a small tilt. Option C is the point where the entire gravitational weight of the body acts.\n\nStudy tip:\nFor a vertically immersed plane surface, the depth of the centre of pressure (hc) is given by: hc = x̄ + Ic / (A × x̄), where x̄ is the depth of the centroid.

Correct answer (Option D):\nA piezometer is a simple vertical tube connected to a pipe or vessel containing liquid. It cannot be used to measure gas pressure because a gas would simply escape out into the open atmosphere from the top of the column rather than forming a visible liquid surface level.\n\nWhy others are wrong:\nOption A, B, and C describe liquid flow behaviors that could impact accuracy or response times, but they do not make measurement mechanically impossible like a gas escaping an open tube.\n\nStudy tip:\nTo measure gas pressures or high differential liquid pressures safely, U-tube manometers or mechanical pressure gauges must be used.