Section 13'2 Forces Within Liquids

1
Section 13.2 Forces Within Liquids

• Objectives
• Explain how cohesive forces cause surface
  tension.
• Explain how adhesive forces cause capillary
  action.
• Discuss evaporative cooling and the role of
  condensation in cloud formation.

2
INTRODUCTION

• Cohesive Forces the electromagnetic forces of
  attraction that like particles exert on one
  another. They are responsible for surface
  tension and viscosity.

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COHESIVE FORCES

• Surface Tension the tendency of the surface of
  a liquid to contract to the smallest possible
  area. It is the result of the cohesive forces
  among the particles of a liquid. Example, the
  water drops the bead up on a freshly washed and
  waxed car.
• Why does surface tension produce spherical drops?
• The force pulling the surface particles into a
  liquid causes the surface to become as small as
  possible, and the shape that has the least
  surface for a given volume is a sphere.

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COHESIVE FORCES

• The higher the surface tension of the liquid, the
  more resistant the liquid is to having its
  surface broken.
• Viscosity the measure of internal friction
  between the cohesive forces and collisions
  between fluid molecules that slows the fluid flow
  and dissipates mechanical energy.
• Water is not very viscous but motor oil is.
• Motor oil flows slowly over the parts of an
  engine to coat the metal and reduce rubbing.

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ADHESIVE FORCES

• Adhesive Forces the electromagnetic forces of
  attraction that particles of different substances
  exert on one another. They are responsible for
  capillary action.
• Capillary Action - a manifestation of surface
  tension by which the portion of the surface of a
  liquid coming in contact with a solid is elevated
  or depressed, depending on the adhesive or
  cohesive properties of the liquid.
• Thus, water is lifted higher in a narrow tube
  than in a wider one.

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ADHESIVE FORCES

• Capillary action causes molten wax to rise in a
  candles wick and water to move up through the
  soil and into the roots of plants.
• When a glass tube is placed in a beaker of water,
  the surface of the water climbs the outside of
  the tube.
• The adhesive forces between the glass molecules
  and water molecules are greater than the cohesive
  forces between the water molecules.
• In contrast, the cohesive forces between mercury
  molecules are greater than the adhesive forces
  between the mercury and glass molecules, so the
  liquid does not climb the tube.
• These forces also cause the center of the
  mercurys surface to depress, as shown in the
  figure 13-9b.

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EVAPORATION AND CONDENSATION

• Evaporation - The process by which any substance
  is converted from a liquid state into, and
  carried off in, vapor for instance, the
  evaporation of water.
• Evaporation has a cooling effect.
• From chapter 12 a decrease in average kinetic
  energy is a decrease in temperature.
• Volatile Liquid a liquid that evaporates
  quickly
• Condensation - change from the gas or vapor state
  to a liquid.
• Fog often forms when moist air is chilled by the
  cold ground.
• Skip 13.2 Section Review p. 351