Page 133 - Physics

Mechanical properties of matter
Surface tension
Liquid surfaces have a tendency of
shrinking into the minimum surface
area. This tendency leads to surface
tension.
Surface tension is defi ned as the
property of the surface of a liquid
that allows it to resist an external
force due to the cohesive nature of
its molecules.
This allows the liquid surface to
behave like a fully stretched elastic
skin. Small insects such as water
striders and items like plastic, paper
clips fl oat on the surface of water,
shown in Figures 6.6 and 6.7, because
of the surface tension of water.
Figure 6.6: Water strider
Figure 6.7: Paper clip fl oating on the
surface of water
Physics Form 1 Final.indd 127
Mechanism of surface tension
Surface tension is caused by cohesive forces
between liquid molecules. Taking an example
of water in a glass, the molecules at the surface
of water do not have neighbouring water
molecules above them, as shown in Figure 6.8.
Consequently, these molecules cohere more
strongly to the water molecules directly
associated with them, that is the molecules
that are next to and below them. Because of
the stronger cohesive force between water
molecules, it is more diffi cult to move an
object through the surface than to move it
when it is completely submerged.
This resistance to an external force due to
cohesive forces between molecules is called
surface tension.
Water molecules
Surface tension
Bonds between molecules
Figure 6.8: Surface tension
Acti vity 6.3
Aim: To demonstrate the surface
tension in water.
Materials: Needle, water, trough, paper clip,
chalk powder, liquid detergent
and razor blades
127
16/10/2024 20:57