# Surface Tension

The free surface of a liquid contracts so that its exposed surface area is a minimum i.e. it behaves as if it were under tension, somewhat like a stretched elastic membrane. This property is known as surface tension.

The surface tension of a liquid varies with temperature as well as dissolved impurities, etc. When soap is mixed with water, the surface tension of water decreases

Exercise : How do detergents clean dirty clothes?

Consider a wire frame, as shown in figure, equipped with a sliding wire AB. It is dipped into a soapy water. A film of liquid is formed. A force F has to be applied to hold the wire in place.

Since, the soap film has two surfaces attached to the wire, the total length of the film in contact with the wire is 2L.

surface tension $\displaystyle T = \frac{F}{2 L}$

Surface tension of a liquid is measured by the normal force acting per unit length. On either side of an imaginary line drawn on the free surface of a liquid, the direction of this force is perpendicular to the line and tangential to the free surface of liquid.

$\displaystyle T = \frac{F}{ L}$

### Properties of surface Tension:

# Scalar quantity

# Temperature sensitive

# Impurity sensitive

# Depends only on the nature of the liquid.

# Unit of surface tension, N/m

# Dimension of surface tension, MLoT-2