Surface Tension
Force per unit length along a liquid surface.
Formula
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How it works
Surface tension measures how strongly a liquid surface resists being stretched or pulled apart. The formula = F/L works because the pulling force is distributed along the length of the line where the surface is being held or pulled, so a longer line gives the same force spread over more distance and therefore a smaller force per unit length. In fluids, this effect comes from the imbalance of molecular attractions near the surface, which makes the surface behave a bit like a stretched elastic film.
When to use it
Use this formula when you know the force acting along a liquid surface or contact line and need the surface tension, or when you know the surface tension and want the force needed to stretch or hold a film. It is common in capillary action, soap films, floating needles, and experiments involving liquid interfaces.
Worked example, step by step
A wire frame holds a soap film, and the film pulls on the movable side with a force of 0.36 N over a length of 0.12 m. Step 1: Write the formula: = F/L Step 2: Substitute the values: = 0.36 N / 0.12 m Step 3: Calculate: = 3.0 N/m Final answer: The surface tension is 3.0 N/m.
Common mistakes to avoid
- using area instead of length in the denominator
- forgetting that force must act along the surface edge or line of contact
- mixing up surface tension with pressure, which has different units
Did You Know?
Surface tension lets water striders walk on ponds and pulls small droplets into near-perfect spheres.
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