JEE Main 2025 Apr 4 Shift 1, Physics Q3: Properties of Fluids

JEE Main2025Apr 4, Shift 1Physics

Two liquids A and B have $θ_{A}$ and $θ_{B}$ as contact angles in a capillary tube. If $K = cos θ_{A} / cos θ_{B}$ , then identify the correct statement:

A
$K$ is negative, then liquid A and liquid B have convex meniscus.
B
$K$ is negative, then liquid A and liquid B have concave meniscus.
C
$K$ is negative, then liquid A has concave meniscus and liquid B has convex meniscus
D
$K$ is zero, then liquid A has convex meniscus and liquid B has concave meniscus.

Solution

A liquid wets the tube (concave meniscus) when its contact angle is acute, so $cos θ > 0$ . It does not wet the tube (convex meniscus) when its contact angle is obtuse, so $cos θ < 0$ .

$K = \frac{cos θ _{A}}{cos θ _{B}} < 0$ requires $cos θ_{A}$ and $cos θ_{B}$ to have opposite signs. Hence one liquid has a concave meniscus and the other a convex meniscus.

Only option (3) describes this combination correctly: liquid A concave (wetting) and liquid B convex (non-wetting).

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JEE Main 2025 Apr 4 Shift 1, Physics Q3: Properties of Fluids

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