Test: Surface Tension - SSC CGL MCQ

When a small amount of water is poured onto a clean glass plate and it spreads out thinly, this behavior is a demonstration of surface tension. Surface tension is the property of liquids that arises from the cohesive forces between the liquid molecules. It causes liquids to minimize their surface area, leading to behaviors such as water spreading thinly on a clean glass plate.

Besides gravity, another force that affects how liquids take shape based on the surfaces they touch is surface tension. Surface tension is responsible for the behavior of liquids like water forming small drops on a greasy glass plate and mercury forming round drops on a glass plate. It is the result of the cohesive forces between the liquid molecules.

Surface tension is defined as the force per unit length in the plane of the liquid surface, acting at right angles on either side of an imaginary line drawn in that surface. This force per unit length is a measure of the cohesive forces between liquid molecules at the surface.

Surface energy refers to the extra energy per unit area of the surface that arises when the surface area of a liquid is expanded. This concept is crucial in understanding how liquids behave at their interfaces and plays a significant role in various natural phenomena, such as capillary action and surface tension.

Cooling occurs when the surface area of a liquid is increased because heat from the surroundings is absorbed to maintain a constant temperature. This phenomenon is linked to the concept of surface energy, where the additional energy required to expand the surface area results in a cooling effect as heat is drawn from the surroundings to compensate for the energy change.

Increasing the surface area of a liquid requires work against the force of attraction among molecules. This process results in the storage of potential energy in the new surface, contributing to the concept of surface energy. By expanding the surface area, the force of attraction among molecules is effectively weakened, leading to the accumulation of additional energy at the interface.

The downward force F required to keep the wire PQ in equilibrium is directly proportional to the length l of the liquid film in contact with PQ. This relationship can be expressed as F = T/2l, where T represents the surface tension of the liquid. The force is inversely proportional to twice the length of the film in contact with PQ. This relationship helps us understand the equilibrium condition and the role of surface tension in maintaining it.

The work done by the force F when wire PQ is moved downward by a small distance Δx is given by the formula W = F * Δx = T * 2l * Δx. As 2l * Δx represents the total increase in area (ΔA), the work done can be simplified to W = T * ΔA. This equation illustrates how the surface tension T corresponds to the work needed to increase the liquid film's surface area by unity at constant temperature, denoted in joules per square meter.

In the scenario where the liquid film's surface area is increased by unity at constant temperature, the surface tension T symbolizes the work needed to accomplish this task. Specifically, T denotes the energy required per unit area to expand the liquid film. Understanding the concept of surface tension provides insight into the relationship between work, force, and the behavior of liquids under different conditions.

The difference in pressure just inside and outside the surface of a liquid drop is given by the equation P₂ - P₁ = 2T/R. This excess pressure inside a drop is a result of the surface tension acting over the surface area of the drop. This difference in pressure is essential for understanding various phenomena in fluid mechanics, such as the stability of liquid drops and the behavior of bubbles.

For a soap bubble, the relationship between the pressures inside and outside the bubble is articulated by the equation P₂ - P₁ = 4T/R. This excess pressure inside a soap bubble is twice that of the excess pressure inside a liquid drop due to the presence of two surfaces. Understanding this pressure differential is crucial for comprehending the stability and characteristics of soap bubbles, which are fascinating examples of surface tension in action.

The pressure difference, as described, plays a critical role in influencing the stability of both liquid drops and soap bubbles. This pressure differential, characterized by equations like P2 - P1 = 2T/R for drops and P2 - P1 = 4T/R for bubbles, is a key factor in understanding the equilibrium and behavior of these structures. The excess pressure inside these entities is a direct consequence of surface tension, impacting their shape, size, and overall stability in various environments.

The correct answer is Capillarity. This phenomenon occurs due to the surface tension of liquids, causing liquids to move up or down in a capillary tube based on their interaction with the tube material. Capillarity is a crucial concept in understanding the behavior of liquids in narrow tubes.

In the given equation, 'h' represents the height to which the liquid rises in a capillary tube due to capillary action. This height is influenced by factors such as the surface tension of the liquid, the radius of the capillary tube, the density of the liquid, and the acceleration due to gravity.

Liquid does not emerge in the form of a fountain from the upper end of a short capillary tube due to the surface tension of the liquid. The surface tension forces dominate in such scenarios, causing the liquid to rise to a certain height and then spread out, rather than flowing out continuously in a fountain-like manner.