# Surface Tension: Plate-Water Interface

• vcsharp2003
In summary, the conversation discusses the role of surface tension in the interaction between water and a plate. It is argued that the force of surface tension would act upwards on the plate due to the stretched membrane effect of the water's surface. The second diagram supports this idea, as the imaginary line on the water's surface would be pulled upwards by the liquid surface. The conversation also mentions the importance of external forces in holding up the plate, and the assumption that the meniscus between the plate and water surface is concave. It is acknowledged that making assumptions is a necessary part of problem solving in Physics.
vcsharp2003
Homework Statement
A rectangular plate of dimensions 6 cm by 4 cm and thickness 2 mm is placed with its largest face flat on the surface of water. Calculate the force due to surface tension on the plate. What is the downward force due to surface tension if the plate is placed vertical and its longest side just touches the water? (Surface tension of water = 7000 N/m.)
Relevant Equations
##S= \frac {F}{l}##, where ##S## is the surface tension perpendicular to an imaginary line on the liquid's surface of length ##l## on which a force of ##F## acts due to surface tension ( The force ##F## lies in the plane of the interface between liquid and another material)
I would think that a force due to surface tension would act upwards on the plate and not downward, as shown in the diagram I have come up with. The surface of water would act as a stretched membrane and therefore there would be a surface tension on water surface in contact with the bottom of the plate edges and it would act along the area of contact created at the edges.

The interface between water and plate would be as shown in second diagram, which supports the direction of surface tension in first diagram since we can consider the imaginary line on water surface coinciding with the plate edge and then the liquid surface which extends vertically up the edge face of plate would pull this imaginary line upwards.

In the second part, they mean with some external force holding up the plate such that if the force were increased the plate would lose contact with the water .

vcsharp2003
haruspex said:
In the second part, they mean with some external force holding up the plate such that if the force were increased the plate would lose contact with the water .
So the meniscus between plate and water surface is assumed to be concave i.e. angle of contact is acute and we idealize by assuming angle of contact is 0 degrees i.e. the liquid surface is vertical next to each edge else the surface tension would not point vertically downwards. ( As shown in diagram below)

Is the above scenario what we need to assume for this situation?

Last edited:
yes

vcsharp2003
haruspex said:
yes
I hope these questions were given with more clarity. But I guess, making assumptions is part of the process of problem solving in Physics.

## 1. What is surface tension?

Surface tension is a physical property of liquids that describes the force exerted on the surface of the liquid due to the cohesive forces between the liquid molecules. It is the measure of how difficult it is to break or stretch the surface of a liquid.

## 2. How is surface tension measured?

Surface tension is measured in units of force per unit length, such as newtons per meter (N/m) or dynes per centimeter (dyn/cm). It can be measured using various methods, including the drop weight method, the capillary rise method, and the maximum bubble pressure method.

## 3. What factors affect surface tension?

Surface tension is affected by factors such as temperature, intermolecular forces, and the presence of impurities or surfactants. Generally, as temperature increases, surface tension decreases, and as intermolecular forces increase, surface tension increases. Impurities and surfactants can also decrease surface tension by disrupting the cohesive forces between liquid molecules.

## 4. How does surface tension relate to the plate-water interface?

The plate-water interface is the boundary between a solid surface (such as a plate) and a liquid (such as water). Surface tension plays a crucial role in this interface, as it determines the shape and behavior of the liquid near the solid surface. For example, surface tension causes water droplets to form a spherical shape on a plate, as it minimizes the surface area of the liquid.

## 5. What are some real-world applications of surface tension at the plate-water interface?

The surface tension at the plate-water interface has many practical applications, such as in the formation of bubbles, the behavior of water droplets on surfaces, and the movement of liquids through narrow channels. It also plays a role in the functioning of biological systems, such as the movement of water and nutrients through plant cells.

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