sonia13
02-01-2009, 04:45 PM
slt :
Translate into French language the following text and try to understand why viscosity increases with temperature for gases and decreases for liquids
Viscosity in Gasses
The molecules of gasses are only weakly kept in position by molecular cohesion (as they are so far apart). As adjacent layers move by each other there is a continuous exchange of molecules. Molecules of a slower layer move to faster layers causing a drag, while molecules moving the other way exert an acceleration force. Mathematical considerations of this momentum exchange can lead to Newton law of viscosity.
If temperature of a gas increases the momentum exchange between layers will increase thus increasing viscosity.
Viscosity will also change with pressure - but under normal conditions this change is negligible in gasses.
Viscosity in Liquids
There is some molecular interchange between adjacent layers in liquids - but as the molecules are so much closer than in gasses the cohesive forces hold the molecules in place much more rigidly. This cohesion plays an important roll in the viscosity of liquids.
Increasing the temperature of a fluid reduces the cohesive forces and increases the molecular interchange. Reducing cohesive forces reduces shear stress, while increasing molecular interchange increases shear stress.
Translate into French language the following text and try to understand why viscosity increases with temperature for gases and decreases for liquids
Viscosity in Gasses
The molecules of gasses are only weakly kept in position by molecular cohesion (as they are so far apart). As adjacent layers move by each other there is a continuous exchange of molecules. Molecules of a slower layer move to faster layers causing a drag, while molecules moving the other way exert an acceleration force. Mathematical considerations of this momentum exchange can lead to Newton law of viscosity.
If temperature of a gas increases the momentum exchange between layers will increase thus increasing viscosity.
Viscosity will also change with pressure - but under normal conditions this change is negligible in gasses.
Viscosity in Liquids
There is some molecular interchange between adjacent layers in liquids - but as the molecules are so much closer than in gasses the cohesive forces hold the molecules in place much more rigidly. This cohesion plays an important roll in the viscosity of liquids.
Increasing the temperature of a fluid reduces the cohesive forces and increases the molecular interchange. Reducing cohesive forces reduces shear stress, while increasing molecular interchange increases shear stress.