Which of the following property of water makes the raindrops to form roughly spherical structure?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 06/06/2024 (04pm-06pm)
Detailed Explanation
Molecules in the bulk of a liquid experience equal cohesive attraction in all directions, but molecules at the free surface experience a net inward pull. This inward force creates surface tension, driving the liquid to minimize its exposed surface area.
Surface energy is proportional to surface area:
\[ E_s = \sigma \cdot A \]
For a fixed volume \(V\), a sphere possesses the minimum surface area \(A\), minimizing the potential energy of the droplet.
Why “surface tension” Is Correct
Surface tension acts like a stretched elastic skin across the outer boundary of the droplet, pulling inward until the droplet attains minimum surface area for its volume, which is a sphere.
Why the Other Options Are Wrong
- A. atmospheric pressure: Atmospheric pressure acts uniformly on the exterior and does not explain droplet rounding.
- C. Acceleration due to gravity: Gravity acts downward, distorting large falling droplets into oblate spheroids rather than creating the spherical shape.
- D. air resistance: Air resistance creates aerodynamic drag that flattens falling raindrops on the bottom.
Key Point
Surface tension minimizes surface energy and surface area, making falling drops spherical.
Key Concepts for Students
- Surface tension arises from unbalanced cohesive intermolecular forces at liquid interfaces.
- For a given volume, a sphere provides the minimum surface area.
- Larger raindrops are flattened by aerodynamic drag, forming a hamburger-bun shape before breaking up.
Practice more: this is question 4 in Properties of Fluid MCQs – Part 1. Try the whole set and check every answer instantly.