Topic: Surface Tension

What is Surface Tension?

Surface tension is the property of the surface of a liquid that allows it to resist external force. It acts like a stretched elastic skin on the surface. It results from the cohesive forces between liquid molecules, especially at the surface.

✅ Definition: Surface tension is the force per unit length acting along the surface of a liquid that tends to minimize the surface area.

Given below in a table is the surface tension of various liquids:

What Causes Surface Tension?

Surface tension is caused by cohesive forces – the attraction between like molecules in a liquid.

• Molecules in the bulk of a liquid experience forces in all directions and are balanced.

• Molecules at the surface are pulled inward and sideways, leading to a net inward force.

• This inward pull reduces the surface area, creating surface tension.

What is the Unit of Surface Tension?

The SI unit of surface tension is:

Newton per meter (N/m)

This unit shows that surface tension is a force acting over a line (length).

Dimension of Surface Tension

To find the dimensional formula, recall:

Surface Tension = Force / Length

Since:

• Force = Mass × Acceleration = M × L × T⁻²

• Length = L

Then,

Dimensional Formula = [M T⁻²]

Examples of Surface Tension

• Water forms droplets instead of spreading out.

• Insects like water striders walk on water without sinking.

• A needle can float on water if placed carefully.

• Soaps and detergents reduce surface tension, helping water spread.

How to Calculate Surface Tension?

Surface tension is calculated using the formula:

                              T=F/L​

Where:

• T = surface tension (N/m)

• F = force acting along the surface (N)

• L = length over which the force acts (m)

Methods of Measurement

Several laboratory methods are used to measure surface tension:

a. Capillary Rise Method

• When a thin tube is inserted into a liquid, the liquid rises or falls due to surface tension.

• Surface tension is calculated based on the height of the liquid column.

b. Drop Weight or Drop Count Method

• By measuring the weight or number of drops falling from a known tube.

c. Ring Method (Du Noüy Tensiometer)

• A ring is pulled from the liquid surface, and the force needed is used to calculate surface tension.

2025 50 Likely Physics Questions And Answers For the JAMB

Frequently Asked Questions – FAQs

Q1: Does surface tension exist in all liquids?
✅ Yes, but it varies depending on the type of liquid and temperature.

Q2: What factors affect surface tension?

• Temperature (increases reduce surface tension)

• Impurities (may increase or decrease)

• Use of detergents (usually decreases surface tension)

Q3: Why can insects walk on water?
Their small weight is supported by the surface tension of water, and their feet do not break the surface.

Q4: What is the effect of soap on surface tension?
Soap reduces surface tension, making it easier for water to spread and clean surfaces.