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TẬP ĐOÀN DẦU KHÍ VIỆT NAM

TRƯỜNG ĐẠI HỌC DẦU KHÍ VIỆT NAM

Lecturer : Assoc. Prof. Pham Hong Quang

Email : [email protected]

Fundamental of Physics



PGS. TS. Phạm Hồng Quang Fundamentals of Physics 2

Chapter 9: Liquids

1. Properties of Liquids and the Kinetic-Molecular Theory

2. Intermolecular Forces

3. Surface Tension

4. Measuring Surface Tension

5. Wettability6. Vapor Pressure

7. Evaporation

8. Boiling

9. Viscosity



PGS. TS. Phạm Hồng Quang Fundamentals of Physics3

The Kinetic-Molecular Theory states thatparticles of a liquid have no fixed space,and move about constantly.

Fluid-is a substance that can flow andtakes the shape of its container- used forliquids and gases both

9.1 Properties of Liquids and the Kinetic-Molecular Theory




Relatively High

Density

The liquids are very dense because the particles of liquids

are extremely close together. Also, different liquids havedifferent densities.

Relative

Incompressibility

Liquids are much less compressible because they have

tightly packed particles, and also transmit pressure equally.

Ability to Diffuse The liquids diffuse with most liquids, but at a slower rate

than gases because the particles are more tightly packed,

and there are many attractive forces between the particles.

9.1 Properties of Liquids and the Kinetic-Molecular

Theory




Intermolecular forces are attractive forcesbetween molecules

Intramolecular forces hold atoms together in a

molecule

Intermolecular vs Intramolecular

• 41 kJ to vaporize 1 mole of water (inter )

• 930 kJ to break all O-H bonds in 1 mole of

water (intra)

Generally, inter molecular forces are much weaker

than intramolecular forces.

9.2 Intermolecular Forces




Dipole-Dipole Forces

Attractive forces between polar molecules



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7

Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

Ion-Dipole Interaction




8

Hydrogen

Bond The hydrogen bond is a special dipole-dipole interactionbetween their hydrogen atom in a polar N-H, O-H, or F-H

bond and an electronegative O, N, or F atom.

A-H..B or A-H..A

A & B are N, O, or F




9

Various intermolecular forcesdraw the liquid particles

together. Along the surface, the

particles are pulled toward the

rest of the liquid, as shown in

the picture to the right.Surface tension (denoted with

the Greek variable gamma) is

defined as the ratio of the

surface force F to the length d along which the force acts:

gamma = F / d

The higher the attractionforces (intermolecular

forces), the higher the

surface tension.

9.3 Surface

Tension




Surface tension has the dimension of force

per unit length, or of energy per unit area.

The two are equivalent—but when referring

to energy per unit of area, people use theterm surface energy—which is a more

general term in the sense that it applies also

to solid and not just liquids.

Unit of the Surface tension are N/m, J/ m2 ,D/cm

9.3 Surface Tension




L

Soap film

Force =2Lγ

Force =mg

9.3 Surface Tension




Surface Tensions of Pure Liquids at 293

KSubstance / (10

-3 N/m)

Acetone 23.7

Benzene 28.8

CarbonTetrachloride 27.0

Methylene Iodide 50.8

Water 72.8

Methanol 22.6

n-Hexane 18.4

9.3 Surface Tension




The meniscus is the curve in the upper surface of a liquid close

to the surface of the container or another object, caused by

surface tension It can be either convex or concave. A convex

meniscus occurs when the molecules have a stronger attraction

to each other (cohesion) than to the material of the container

(adhesion). Conversely, a concave meniscus occurs when the

molecules of the liquid attract those of the container's, causing

the surface of the liquid to cave downwards.

9.3 Surface

Tension

http://en.wikipedia.org/wiki/Surface_tension

http://en.wikipedia.org/wiki/Surface_tension




If a capillary tube of inside radius=r immersed in a liquid that wet itssurface, the liquid continues to rise inthe tube due to the surface tension,until the upward movement is just

balanced by the downward force ofgravity due to the weight of theliquid.

The total upward force around the

inside circumference of the tube is

θ= the contact angle between the

surface of the liquid and the capillary

wall

cos2

r a

Capillary Rise Method

9.4 Measuring Surface

Tension




For water the angle Ө is

insignificant, i.e. the liquid

wets the capillary wall so

that cos Ө = unity

The downward force ofgravity is given by

g hr 2

At Maximum height, the

opposing forces are in

equilibrium

g rh 2

1


Tension




Wtot = Wring + 4R

The method is simple and measures the detachment

force

(the surface tension multiplied by the periphery

2*2R)

The Ring Method (du Nouy

1919)


Tension




Drop Weight and drop volume method

If the volume or weight of a

drop as it is detached from a

tip of known radius is

determined, the surface andinterfacial tension can be

calculated from

r

g V

r

mg

22

Where m = the mass of the drop V = the volume of the drop

ρ = the density of the liquid r = the radius of the tip g = the acceleration due togravity

Φ = a correction factor

Why is Φ needed

1)The drop does not completely

leave the tip.

2) The surface tension forces

are not completely vertical.

3) There is a pressure

difference across the

curved surface.


Tension




9.5 Wettability

According to the nature of the liquid and the solid,a drop of liquid placed on a solid surface will

adhere to it or no. That is the wettability between

liquids and solids.

When the forces of adhesion are greater thanthe forces of cohesion, the liquid tends to wet the

surface and vice versa.

Place a drop of a liquid on a smooth surface of a

solid. According to the wettability, the drop will

make a certain angle of contact with the solid.




A contact angle is lower than 90°, the solid is called

wettable

A contact angle is wider than 90°, the solid is named

non-wettable.

A contact angle equal to zero indicates completewettability.

9.5 Wettability

9 6




9.6 Vapor Pressure

Vapor Pressure: The pressure exerted on the

surface of a liquid by the vapor that is in equilibrium

with the liquid is called as “vapor pressure”

Once equilibrium between a liquid and vapor is

reached, the number of molecules per unit volume in

a vapor does not change with time. Hence, the vapor

pressure over the liquid remains constant at a given

temperature.Vapor Pressure is independent of the volume of the

container.

9 6V P




Vapor pressure increases

with the increase in

temperature.

9.6Vapor Pressure

9 7 E ti




9.7 Evaporation

Evaporation- is the process where particles escape

from the surface of a non boiling liquid and enters the

gas state.

~Evaporation takes place because the particles of

liquids have different kinetic energies, therefore

some of the particles with higher kinetic energy

overcome the intermolecular forces and evaporate to

go in the gas phase.

9 8




Boiling- is the change of a liquid to bubbles or vapor.

Boiling occurs when the vapor pressure becomes equals

atmospheric pressure.

A l iquid bo i ls at the temp. at wh ich i ts vapor pressu re

is equal to the pressu re above i ts surface. (usuallyatmospheric pressure)

If the pressure above the liquid’s surface is 1 atm, then

this temperature is called as its “Normal Boiling Point”

B.P. of a liquid is reduced by lowering the pressure

above it.

Why does it take longer to cook at high altitudes?

9.8

Boiling




A liquid boils at a temp. when the vapor pressure P1

becomes equal to the external pressure P2 above theliquid

9.8 Boiling

9 9 Vi it




9.9 Viscosity

Defined as “resistance to flow” of a fluid.

Viscous liquids move slower.

The greater the intermolecular forces the

more is the viscosity.

9 9 Vi it




The dynamic viscosity(η) of a fluid is a measure ofthe resistance it offers to relative shearing motion.Viscosity (η) is defined as the ratio of shear stress

(τ)to shear rate (u/h)

η= F/ [A×(u/h)]

η= τ /(u/h) N-s/m²

9.9 Viscosity

9 9 Vi it




Kinematic Viscosity :

It is defined as the ratio of absolute

viscosity to the density of fluid.ν= η/ρ m²/s ; ρ= density of

fluid

9.9 Viscosity

9 9 Vi it




The viscosity of liquids

decreases with increase

the temperature.

The viscosity of gases

increases with the

increase the

temperature.

9.9 Viscosity

9 9 Viscosit




For Newtonian fluids, shear stress linearly vary

with the shear rate as shown in Figure. Viscosity

is constant for this kind of fluid.

τ = η (u/h)

9.9 Viscosity




Non Newtonian fluid doesn’t follow the linearrelation between viscosity and shear rate.

9.9 Viscosity



PGS TS Phạm Hồng Quang F d t l f Ph i

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