Surface Tension – Class 11 | Chapter – 10 | Physics Short Notes Series PDF for NEET & JEE
Surface Tension: Surface tension is a phenomenon that occurs at the interface between two different phases of matter, such as between a liquid and a gas or between two immiscible liquids. It is the force that causes the surface of a liquid to contract and minimize its surface area. Surface tension is caused by the attraction between the molecules in the liquid, which are strongly attracted to one another, and the molecules at the surface, which have no molecules above them to attract them in all directions.
The surface tension of a liquid is dependent on its intermolecular forces, temperature, and the presence of solutes or impurities. Liquids with strong intermolecular forces, such as water, have high surface tension, while liquids with weak intermolecular forces, such as oils, have low surface tension. Surface tension has many practical applications, including the formation of droplets, the capillary action, and the formation of menisci in tubes.
Causes of Surface Tension
Surface tension is caused by the cohesive forces between the molecules in a liquid. Cohesive forces are the intermolecular forces that hold the molecules of a substance together. In a liquid, each molecule is attracted to its neighboring molecules by cohesive forces, which create an inward force that pulls the molecules towards each other.
At the surface of a liquid, however, the molecules are not surrounded by other molecules on all sides. As a result, the cohesive forces between the surface molecules are stronger than the adhesive forces between the surface molecules and the air or another material. This creates an imbalance in the forces acting on the surface molecules, causing them to be pulled towards the interior of the liquid, resulting in surface tension.
The strength of surface tension depends on the strength of the cohesive forces between the molecules of the liquid. Strong cohesive forces result in high surface tension, while weak cohesive forces result in low surface tension. Other factors that can affect surface tension include temperature, pressure, and the presence of impurities or dissolved solutes in the liquid.
Formula of Surface Tension
The surface tension of a liquid can be calculated using the following formula:
γ = F ÷ L
where γ is the surface tension of the liquid in Newtons per meter (N/m), F is the force acting on the surface of the liquid perpendicular to the line of contact, and L is the length of the line of contact between the liquid and the surface.
Alternatively, the surface tension can be expressed in units of dynes per centimeter (dyn/cm), in which case the formula becomes:
γ = F ÷ L × 10
where the factor of 10 is included to convert from N/m to dyn/cm.
It is important to note that the surface tension of a liquid is affected by factors such as temperature, pressure, and the presence of impurities or dissolved solutes in the liquid. Therefore, the surface tension of a liquid can vary under different conditions.
Units of Surface Tension
The unit of surface tension depends on the system of units being used. In the SI (International System of Units) system, the unit of surface tension is Newtons per meter (N/m). In the CGS (centimeter-gram-second) system, the unit of surface tension is dynes per centimeter (dyn/cm).
Other common units of surface tension include pound-force per inch (lbf/in) and poundal per foot (lb/ft). However, these units are less commonly used than the SI and CGS units. It’s important to use the correct units of surface tension when performing calculations or reporting experimental results to ensure consistency and accuracy.
Examples of Surface Tension
Surface tension is a phenomenon that can be observed in many everyday situations. Here are a few examples:
- Water droplets: When water is dropped onto a surface, it forms spherical droplets due to the surface tension. The cohesive forces between the water molecules cause the water to pull together, forming a spherical shape that minimizes the surface area of the droplet.
- Floating objects: Small objects, such as paper clips or insects, can float on the surface of water due to surface tension. The surface tension holds the surface of the water together, creating a kind of “skin” that supports the weight of the object.
- Capillary action: Capillary action is the ability of a liquid to flow through narrow spaces, such as the pores in a sponge or the spaces between fibers in a towel. This is caused by the combination of adhesion (the attraction between the liquid and the solid surface) and surface tension.
- Meniscus formation: When a liquid is in contact with a solid surface, the surface tension causes the liquid to form a curved meniscus at the point of contact. For example, water in a glass tube forms a concave meniscus due to the adhesive forces between the water and the glass.
- Soap bubbles: Soap bubbles are formed when air is trapped inside a layer of soap solution. The surface tension of the soap solution creates a thin film that encloses the air, forming a spherical bubble. The surface tension also allows the bubbles to resist breaking and remain intact for a period of time.
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