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  • Phases of matter


    Temperature Scales

    Thermal expansion


    Lecture 10

  • Phases of matter


    does not retain shape


    move anywhere

     little interaction


     does not retain shape.


     freer to move

    remain close to each other





    retains shape


    linked by spring-like forces

    average positions fixed

  • Molecules are in constant disordered motion

    Velocities distributed over a large range

    Average kinetic energy directly related

    to temperature

    Greater their average kinetic energy

    •Higher the temperature


    Energy exchange between two objects at

    different temperatures

    Temperature is a characteristic of an object

    related to the average kinetic energy of

    atoms and molecules of the object.

    Temperature and Heat

  • Temperature is a measure of the average

    kinetic energy of atoms and molecules.

    Brownian motion

    A suspended small particle is constantly

    and randomly bombarded from all sides

    by molecules of the liquid.


    Robert Brown, botanist noticed in 1828

    that tiny particles (pollen grains) exhibited

    an incessant, irregular motion in a liquid.

    Remained largely unexplained until

    Einstein paper in 1905

    “On the motion of small particles suspended

    in a stationary liquid demanded by the

    molecular-kinetic theory of heat”

    indirect confirmation of existence of

    molecules and atoms

  • Temperature and Heat

    polystyrene particles, 1.9 mm in diameter, in


    T = 25 C

    Brownian motion is a clear demonstration of the

    existence of molecules in continuous motion

    in any short period of time

    •random number of impacts

    •random strength

    •random directions

    Brownian motion

  • There are 3 temperature scales:

    Anders Celsius (1701-1744) - Celsius (C)

    Gabriel Fahrenheit (1686 -1736) - Fahrenheit (F)

    Lord Kelvin (1824-1907) - Kelvin (K)

    Temperature scales

    Differ by (a) the basic unit size or degree ()

    (b) lowest & highest temperature

    Celsius and Fahrenheit are defined by the

    freezing point and the boiling point of water

    (at standard atmospheric pressure):

    Range- freezing to boiling point of water

    Celsius, 100 degrees. Fahrenheit, 180 degrees

    Freezing point of water: 0C or 32F

    Boiling point of water: 100C or 212F

  • CelsiusoC Fahrenheit oF Kelvin, K







    0 Freezing



    0 -459.69 -273.15 Absolute


    SI unit of temperature is the Kelvin

    Temperature scales

    Room temperature 20o Celsius

    68o Fahrenheit

    293 Kelvin

    Absolute zero:

    Temperature at which all thermal motion ceases

  • Gas pressure depends on temperature


    Tyres have higher pressure when hot

    compared with cold.

    Temperature scales

    Most gases at atmospheric pressure and

    room temperature behave approximately

    as ideal gases

    Ideal gas:

    Is a collection of atoms or molecules

    • move randomly

    •considered to be point-like

    •exert no long-range forces on each other.

    •occupy negligible volume.

  • T oC 200 -200



    T oC 200 -200



    Kelvin Temperature Scale

    Ideal gas

    Linear relationship

    exists between pressure

    and temperature at

    constant volume

    Linear relationship

    exists between volume

    and temperature at

    constant pressure

    All plotted lines extrapolate to a temperature

    intercept of -273.15 oC regardless of initial

    low pressure (or volume) or type of gas

    Unique temperature called absolute zero

    Fundamental importance

    Constant volume Constant pressure

  • Kelvin (K) scale

    ●same basic unit size as Celsius

        15.273 CTKT Example:

    Freezing point of water : 273.15 K

    Boiling point of water: 373.15 K

    Unique temperature of -273.15oC is called

    absolute zero,

    below which further cooling will not occur

    Fundamental importance and the basis of the

    Kelvin temperature scale

    Kelvin Temperature Scale

    Kelvin Scale defined by 2 points.

     absolute zero -273.15oC

     Triple point of water- temperature at which

    3 phases, solid, liquid, and gas are in equilibrium

    0.01 oC

  • Celsius and Fahrenheit scales allow for negative


    Fahrenheit to Celsius :

    Celsius to Fahrenheit :

    Converting Temperatures


    •Alcohol in glass

    •Mercury in glass

    Depends on thermal expansion

        32 5

    9  CTFT

        32 9

    5  FTCT

  • Example.

    Body temperature can increase from 98.60F to

    1070F during extreme physical exercise or during

    viral infections. Convert these temperatures to

    Celsius and Kelvin and calculate the

    difference in each case.

         CFCT o7.4132107 9

    5 

         CFCT o37326.98 9

    5 

        KCKT 15.31015.27337 

        KCKT 85.31415.2737.41 

        32 9

    5  FTCT

    Difference DT(0C)= [41.7-37]0C = 4.70C

    Difference DT(K) = [314.85-310.15]K = 4.7K

        15.273 CTKT

  • Dental pulp is sensitive, may be damaged if

    its temperature increases >5oC)

    Temperature and Heat

    Dental drilling

    Rise in temperature of pulp during drilling

    should be less than 5 oC


    Oral environment

    temperature is not constant;

    Hot and cold food and drink

    Dental materials: Important characteristics

    transfer of heat

    Dimensional changes: expansion and contraction

  • Origin: When the average kinetic energy (or

    ‘speed’) of atoms is increased, they

    experience stronger collisions, increasing the

    separation between atoms.

    Most materials

    •expand when temperature is increased

    •contract when temperature is decreased

    Low Temperature High Temperature

    Thermal expansion

    this is called thermal expansion and contraction

  • Thermal expansion

    Thermal expansion depends on:



    •Temperature change.

    Assume no change in phase

    Linear Thermal Expansion

    Important, for example, for metals in buildings,

    bridges and dental filling materials etc.

  • a = Fractional change in length

    Change in temperature


    DT 

    Coefficient of linear expansion a for the material

    is defined as:

    Bar of initial length L changes by an amount DL

    when its temperature changes by an

    amount DT.

    Thermal expansion



    ( )T T D

    L L D




  • m m C or K

    Thermal expansion

    ( )( )( )L L TaD  D

    DL = change in length

    L = original length

    DT = change in temperature (C or K)

    a coefficient of linear expansion

    units (°C-1 or K- 1)*

    a depends on the type of material.

    linear expansion:

    oC-1 or K-1

    *Temperature interval is the same for Celsius and Kelvin scales


  • Thermal expansion

    Decayed dentine removed and replaced

    by filling.

    Thermal expansion/contraction due to hot and

    cold foods should not cause separation

    at the tooth-filling interface

    Coefficient of thermal expansion of the

    restorative material should be similar

    to that of the tooth

    Important in dental restorations

    Large mismatch in expansion coefficients:

    •Fluids leakage between filling and surrounding


  • Thermal expansion

    Coefficient of Thermal linear expansion

    Enamel Dentine

    Amalgam Composite




    11.4* x

    10-6 K-1 8.3 X

    10-6 K-1 25 x

    10-6 K-1

    ≈ 30 x

    10-6 K-1

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