Surface tension is an interesting topic to study. It is defined as the attractive force that is found mostly in liquids which usually pulls molecules in the surface together. This results in the minimization of the area of the surface. In technical terms, the surface tension is the energy required to increase the surface area of a liquid by a unit of area. The surface tension is present due to the imbalance of attractive intermolecular forces or we say the cohesive forces that are between molecules. This phenomenon mostly occurs with water generally but it can happen with other liquids as well. Let us look at some surface tension units.
The Unit of Surface Tension
We find many of the examples of surface tension in nature and we have listed a few of them below in the article:
There are small insects such as water striders walking on water because their weight is not that enough to penetrate the surface tension.
A needle that is small when carefully placed over the surface of water can be made to float on water though the needle is several times denser than water. If the surface is agitated to break the tension ultimately, then the needle disturbs the surface tension and will sink quickly.
The detergent and the soaps lower the surface tension of water so that it can more readily soak into solid and pores areas.
The droplets of water that acquire their round shape due to surface tension. They are pulled spherical shape by the force that is cohesive of the surface layer
Surface tension is the tendency of surfaces of liquid to shrink generally into the minimum surface area possible. The phenomenon that is surface tension allows insects that are water striders to float and slide on a surface of water without becoming even partly submerged.
There are primarily two mechanisms that are in play. One is a force that is inward on the surface molecules which is causing the liquid to contract. There is a second method that is the tangential force parallel to the surface of the liquid. The net effect is the liquid that generally behaves as if its surface were covered with a stretched elastic membrane.
Unit of Surface Tension of Liquid
The force that is the cohesive one between liquid molecules is responsible for the phenomenon that is called as surface tension. The molecules which are at the surface of a glass of water do not have other water molecules on all sides of them. It is not really said to be true that a "skin" generally forms on the water surface. The force which is the cohesive one between the molecules in a liquid are shared with all neighboring molecules. Those which are on the surface have no neighboring molecules above and thus they exhibit a force which is the stronger attractive forces upon their nearest neighbors on and below the surface. Surface tension could be generally defined as the property of the surface of a liquid that allows it to resist an external force which is due to the cohesive nature of the water molecules.
The molecules of water want to cling to each other. However at the surface there are fewer molecules of water to cling to since there is air above thus we say that there are no water molecules. This usually results in a bond that is stronger between those molecules that actually do come in contact with one another and a layer of strongly bonded water as well. This layer of the surface that is held together by surface tension) creates a barrier that is considerable between the atmosphere and the other water. In fact we can say that other than mercury only water has the greatest surface tension of any liquid.
Within a human body that is made up of a liquid there is a molecule that will not experience a net force because the forces by the neighboring molecules all of then get cancelled out . However for a molecule which is on the liquid's surface, there will be a net inward force since there will be no attractive force acting from above. This inward force or the net force causes the molecules that are on the surface to contract and to resist being stretched or broken. Thus we can say that the surface is under tension which is probably where the name that is "surface tension" came from.
Now due to the phenomenon of surface tension there are small objects that will "float" on the surface of a fluid, as long as the object cannot break through and the objects cannot separate the top layer of water molecules. When an object is on the surface of any of the fluids there is the surface under tension will behave like an elastic membrane.
FAQs on Unit of Surface Tension
1. What are the SI and CGS units of surface tension as per the Class 11 Physics syllabus?
According to the CBSE syllabus for Class 11 Physics, the units for surface tension are defined in two primary systems:
- The SI unit of surface tension is Newton per meter (N/m).
- The CGS unit of surface tension is dyne per centimeter (dyne/cm).
Both units are derived from the definition of surface tension as the force acting per unit length on a liquid's surface.
2. How is the unit of surface tension derived from its basic formula?
The unit of surface tension is directly derived from its defining formula, T = F / L, where 'T' is surface tension, 'F' is the force acting on the surface, and 'L' is the length over which the force acts. In the SI system:
- Force (F) is measured in Newtons (N).
- Length (L) is measured in meters (m).
Therefore, the resulting SI unit for surface tension becomes N/m.
3. What is the dimensional formula for surface tension?
The dimensional formula for surface tension is [MT⁻²]. This is determined by dividing the dimensional formula of force by that of length:
- The dimension of Force (F) is [MLT⁻²].
- The dimension of Length (L) is [L].
Thus, the dimension of surface tension (F/L) is [MLT⁻²] / [L], which simplifies to [MT⁻²].
4. How does the unit of surface tension differ from the unit of simple linear tension?
The key difference lies in what they measure. Linear tension is simply a force and is measured in Newtons (N). In contrast, surface tension is a measure of force distributed over a length (a type of force density), so its unit is Newtons per meter (N/m). It describes the cohesive energy present at the surface of a liquid, not just a pulling force at a single point.
5. Is the unit of surface tension the same as the unit for surface energy?
While they represent different conceptual definitions, the units for surface tension and surface energy per unit area are dimensionally equivalent. Surface tension can be defined as:
- Force per unit length, with the unit N/m.
- Work done (or energy) per unit increase in surface area, with the unit Joule per square meter (J/m²).
Since 1 Joule = 1 Newton-meter (N·m), the unit J/m² is equivalent to (N·m)/m², which simplifies to N/m. This shows that both definitions lead to the same fundamental unit.
6. What is a common example of the unit of surface tension for a liquid like water?
A classic real-world example is the surface tension of water at 20°C. It is typically expressed as 0.072 N/m in the SI system. In the CGS system, which is also commonly used in laboratory contexts for its convenient numbers, this value is expressed as 72 dynes/cm.
7. Why is the CGS unit dyne/cm often used for surface tension instead of the standard SI unit?
The CGS unit dyne/cm is frequently used in practical and laboratory settings because the surface tension values for most common liquids result in more convenient, whole numbers in this system. For instance, expressing water's surface tension as 72 dyne/cm is often simpler for calculations and comparisons than using the SI value of 0.072 N/m. It is a matter of practical convenience rather than fundamental correctness.
