the temperature is í63°C, the density equals 0.092 kg/m^3. At the summit of Mount Everest, a human can breathe barely enough oxygen to

stay alive.

Conversely, in deep diamond mines, air pressure and density both increase, and the temperature rises rapidly with depth. In the deepest

mines, it would be impossible for miners to work without the introduction of refrigerated air.

13.17 - Surface tension

Surface tension: A cohesive

effect at the surface of a

liquid due to the forces

between the liquid’s atoms or

molecules.

Above, you see a dewy rose. The drops of water do

not spread and flow to cover the petals on which they

rest. Instead they contract into tiny spheroids. They

do this because the surface tension of water causes

each drop to try to minimize its own surface area. The bristles of a wet paintbrush

contract into a sleek shape for the same reason. The surface tension of the water on

the bristles causes them to pull in. You see this in Concept 1.

Surface tension also enables an insect called a water strider to walk on water. The

creature’s weight, transmitted to the water’s surface through its feet, causes tiny

depressions, but the feet do not break through. The surface tension of the deformed

liquid surface provides an elastic-like restoring force that balances the insect’s weight. A

video of this interesting phenomenon is shown in Concept 2.

What causes surface tension? In some liquids like water, the molecules that make up

the liquid attract each other. These molecules are dipoles; they have regions of positive

and negative electrostatic charge. The positive pole of each molecule is attracted to the

negative poles of neighboring molecules, and vice versa.

Molecules in the interior of the liquid experience equal attractions in all directions, and

so they experience no net intermolecular force. Molecules at the surface of the liquid,

however, are pulled on only by the molecules below, and by their neighbors in the

surface, so there is a net force pulling them into the interior of the liquid. Because of

this, the surface of the liquid tends to contract and consequently to minimize its own

area. You see the intermolecular forces illustrated in Concept 1.

In the case of liquid dewdrops, the minimum surface area is roughly spherical. In the

case of the water strider, the elastic-like upward force of surface tension on its feet

results from the water’s tendency to flatten, and so minimize the area, of its surface.

Water has a strong surface tension, but that tension can be reduced. For example,

when you heat water, you reduce its surface tension because the faster moving

molecules do not attract each other as much as they would at cooler temperatures.

Diminished surface tension allows other substances that might be in the water í say,

butterí to rise to the surface. Cooks know this (at least implicitly), and they serve soups

hot because they will be more flavorful. Adding soap to water also reduces its surface

tension í and it can cause a water strider to sink!

Dewdrops on rose petals form tiny spheres.

Surface tension

Contraction of surface of liquid

Due to mutual attraction of molecules

Water strider

Surface tension lets insect walk on

water

Restoring force balances weight

13.18 - Gotchas

Pressure increases with depth in a fluid. Yes, it does. The farther below the surface of a fluid an object is, the more fluid above it and the

greater the pressure on it.

Buoyant force increases with an object’s depth in water. Only if more of the object is getting submerged, in which case the buoyant force does

increase. But a soda can five meters below the surface, and one 500 meters below the surface, both experience the same buoyant force. This

force is equal in magnitude to the weight of the displaced water.

Two surfaces have the same pressure on them, so the pressure must exert the same force on each.No, pressure is force divided by area, so if

one surface has a greater area, it experiences a greater force.

Pressure acts in every direction. Yes, this is stated by Pascal’s principle. For instance, water pressure pushes both down on the top and up on

the bottom of a scuba diver exploring under the sea.

(^266) Copyright 2000-2007 Kinetic Books Co. Chapter 13