surface tension definition

Surface tension and it co-efficient

The surface tension is  being defined as a force that act parallel to the surface of a liquid. That is why a surface of liquid behaves as if it is covered with elastic skin, so the surfaces appear to be under tension which is called surface Tension. The surface tension can also be defined as the ratio tangential force f, in the surface to the length d, along which the force acts.  Which is give mathematical as

T = F/d, where T = surface tension, F= force and d = length.

The unit of surface tension is Newton per meter (Nm-1)

Co-efficient of surface tension is defined as the force  per unit  length acting on the line drawn on the surface

Application of surface tension

  1. A steel needle if place gently on the surface of water will float, though the density may be greater than that of water.
  2. Insect can work on water because of surface tension

How to reduce the surface tension of water

  1. Surcface tension can be reduced by increasing the temperature of the water
  2. It can also be reduced by adding detergent or soap to the liquid

Adhesion and cohesion force of molecule.

Cohesion force: it is the force of attraction between molecules of the same element or substance. Example force of attraction between water molecules or any other elements

Adhesion force: This is the force of attraction between the molecules of difference element of substance. Example, the force of attraction between water molecule and molecule of glass etc.

Molecule explanation of surface tension

The existence of surface tension can be explained by the molecular attraction between the molecules of the liquid.

Surface tension

From above picture, Right inside the liquid we see, a molecule A in Equilibrium as it is attracted equally in all directions, so there is zero net force on A  within the liquid, but for B molecule which is near the surface , path of it is attracted by the air and path by water molecule . B molecule is more attracted toward the liquid than outward; the resultant force on B is toward the liquid, consequently the surface of the liquid pulled inward straining the surface molecule so that they appear to be in state of tension


Capillarity is the tendency for a liquid to rise or fall in narrow capillary tube when inserted into a bow of liquid. Whether a liquid will rise or fall in narrow capillary tube depend on the type of liquid. For example water will rise up in narrow capillary tube and form a concave to the air, but when the same tube is inserted into mercury, instead of rising like water it will fall, and form a convex to the air.

The rise or fall of a liquid is caused by either the force of Cohesion or Adhesion. Water and some liquid which wet glass, rise in a capillary tube because the for adhesion of the liquid to glass is greater than it cohesion force to each other.

In the case of mercury, the force of cohesion of mercury molecules is greater that its force of adhesion to glass, and therefore it does not wet glass

The common examples of capillary action are:

  1. Water rising up the stem of a plant
  2. Blood spreading through the fine capillary channel in body.
  3. Ink held on the nip of a pen
  4. Liquid candle wax rising up to the wick of a candle.


Viscosity is defined as the internal friction in liquids. When water or any other liquid flows through a pipe, the velocity is highest at center but lowest at the surface in contact with the pipe. The resistance to flow is smaller in large pipe than a narrow one. The thicker the liquid the more sluggish the flow. The liquid that flow and pour slowly is called Viscous liquid, example- engine oil.

Terminal Velocity (speed)

A ball-bearing falling through a viscous liquid such as glycerine  is acted upon by three forces, the weight mg, the viscous force V, and upthrust U, due to liquid displacement. The equation is give as

Mg-V-U =ma, where a = acceleration.

The ball-bearing falling through a viscous liquid at a stage stop to accelerate but falls with uniform speed, which is called the terminal speed. The terminal speed is reached, because the viscous force , V, is proportional to the terminal speed, v that is

V= kv, where k is constant

When the ball-bearing accelerate , the  viscous force increase also, until it reach where viscous force is equal to downward force , at this point acceleration equal to zero. So

Mg – V –U = 0 or V = mg – U

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