Advanced Level Examination
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PHYSICS
Unit 04
Part I
THERMAL PHYSICS
Prepared By : Samitha Rathnayake
THERMOMETRY
Thermal equilibrium
A > B
A
B
A = B
A
B
When two objects A and B of different temperatures are made to touch and when observed after awhile, it could be
observed that the temperature of A has reduced while the temperature of B has increased. We say this happens as heat
flowed from A to B.
If the observation was done after a considerable time since A and B was made touch, it could be seen that the
temperatures of A and B are equal. At this instance , there is no net heat transfer between A and B , while the heat
transfer is in dynamic equilibrium.
•
If there is no net heat transfer between several objects (for this , they should not necessarily be touching) they
are said to be in thermal equilibrium.
•
The temperatures of objects in thermal equilibrium are equal.
Temperature
When we touch an object, we get the sensation whether it is hot or cold. The quantity which generates this sensation
could simply be named as temperature.
Since several objects which are in thermal equilibrium have equal temperatures , we could define temperature in a
much better way using this situation. According to this , temperature is scientifically defined as " the physical
property which determines whether a set of objects is in thermal equilibrium or not"
Heat
Heat is not temperature . Temperature generates through heat. Although heat is often identified as a source of
energy , it is more accurate to introduce heat as a transition state of energy or as a method of energy transfer between
two systems.
Accordingly , heat is defined as " the energy transitioned from one system to another due to the temperature
difference between the two systems."
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Zeroth law of thermodynamics
A
B
C
"If two objects A and B are seperately in thermal equilibrium with another object C , A and B are also in thermal
equilibrium with each other.''
•
Since this was presented after first and second laws of thermodynamics this is known as the zeroth law of
thermodynamics.
•
The zeroth law is studied under thermometry as we use the zeroth law to draw many conclusions in
temperature measurements.
Thermometers
The method we most oftenly use to assess the temperature of an object is touching it. This is not only dangerous but
also unreliable as the temperature we feel is relative to the previous temperature that our hand was at. In our day to
day life as well as in many scientific tasks, we are required to quantitatively measure temperature. The instrument
used to quantitatively measure the temperature is called a thermometer .
Thermometric properties
What aides the function of any thermometer is a certain physical property of matter which changes with the
temperature. Such physical properties which change with temperature are called thermometric properties.
Thermometric substances
A thermometric property cannot exist in isolation. It is contained in some substance. Such substances which contain
thermometric properties are called thermometric substances.
Thermometer
Thermometric property
Glass - Liquid
Volume of a specific liquid mass
Mercury, Alcohol
Constant
volume - gas
Pressure of a fixed mass of gas with constant
volume
Helium , Hydrogen , Nitrogen ,
Air
Constant
pressure - gas
Volume of a fixed mass of gas with constant
pressure
Helium , Hydrogen , Nitrogen ,
Air
Resistors
Electric resistance
Platinum
Thermocouple
Electromotive force
Platinum and Platinum - Rhodium
alloy, Copper and Constantan,
Copper and Iron
Radiation pyrometer
Intensity of radiation
Heat radiation
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Thermometric substance
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Essentials of a thermometric property
• Should continuously vary with temperature.
• Should only have one value for a certain temperature at an instance. (single valued)
• Should linearly vary with temperature as far as possible.
Most practical thermometric properties do not satisfy all of the above
Ideal thermometric property
characteristics at once. A thermometric property which satisfies all of the
above characteristics at once is called an ideal thermometric property.
Temperature
Essentials of a thermometric substance
• Should have a high heat conductivity (for a faster response rate)
• For a small change in temperature , there should be a large change in the thermometric property.
(to increase the sensitivity)
• Should be active in a large temperature range without a change of state ( to have a high range)
• Should have a law specific heat capacity .
(to reduce the effect on the temperature which is being measured)
• Should have a law density ( to be light)
Essentials of a thermometer
• Should be light weight for the ease of usage. Should be small.
• The temperature sensitive part (probe) should be small such that even the temperature of a point could be
measured.
• The heat conductivity of the probe should be high for faster activation.
• To reduce the effect on the temperature which is being measured, the heat capacity of the probe should be low.
Temperature scales
Even though a thermometer is produced using a thermometric substance containing thermometric property , in order
to calibrate it , a program which assigns a numerical value to temperature is required. Such scales which have been
produced to allocate numerical values to temperature are called temperature scales
Fixed points
In order to produce any temperature scale, permament temperatures which could be easily and accurately reproduced
under given conditions are required. These permament temperatures are called fixed points.
• Two fixed points have been used to produce Celcius and Fahrenheit temperature scales , while only one fixed
point have been used in producing the Kelvin scale.
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Lower fixed point (Ice point) :-
The temperature at which ice melts (melting point) under standard
pressure (760 mm Hg)
Upper fixed point (Steam point) :-
The temperature at which the water boils (boiling point) under standard
pressure (760 mm Hg)
Setting up a temperature scale
X
XH
L –
XL –
H –
XH –
Temperature corresponding to the ice point
The value of the thermometric property at the ice point
Temperature corresponding to the steam point
The value of the thermometric property at the steam
point
 – Temperature to be measured
X – The value of the thermometric property at the
temperature to be measured
A
X
B
XL
C
L
Gradient obtained
by A and C
=
XH – XL
H – L
 =
(

H
?
X – XL
XH – XL
Gradient obtained
by B and C
X – XL
 – L
=
) (  –  ) + 
H
L
L
H – L ; Basic difference of the scale
Celcius scale
Celcius scale has been produced by allocating the temperature corresponding to the ice point as 0 , and temperature
corresponding to the steam point as 100 , and dividing the gap between them into 100 equal parts and naming one part
as one degree celcius (10C).
C
=
X – X0
( X – X ) 100
100
0
 equation of the celcius scale
1
• One degree of celcius is defined as
of the celcius temperature gap between the ice point and the steam
100
point.
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Triple point of water
The instance where liquid water , melting ice and saturated water vapour exist in thermal equilibrium is called the
triple point of water.
• Not only water , other substances too have triple points (Ex : N2 , H2 etc)
• The only fixed point which has been used to produce the Kelvin scale is the triple point of water.
Kelvin ( absolute / thermodynamic ) scale
According to the thermodynamic scale , the temperature of the triple point of water is 273 . 16 K. Therefore 1 K
is
1
th fraction of the thermodynamic temperature of the triple point of water.
273.16
X
X tr
TK
XT
XT
Xtr
273.16
– The value of the theromometric prperty at the triple point
– Temperature to be measured
– The value of the thermometric prperty at the temperature
to be measured
T
TK
XT
TK
X tr
=
273.16

TK =
XT
X tr
×
273.16
• According to thermodynamics the lowest possible temperture to exist in the universe is 0 K and it is termed as
absolute zero.
• According to the thermodynamic scale , the temperature of the melting point of ice is 273 . 15 K. This is slightly
different to the temperature of the triple point as the pressure corresponding to the triple point (4.6 mm Hg) and
the pressure corresponding to the melting point (760 mm Hg) being different to each other and liquidized air
being removed from the liquid water considered at the triple point.
• According to the thermodynamic scale the temperature of the boiling point of water is 375 . 15 K
Celcius
Thermodynamic
100
Boiling
point
C
373.15
TK
0
Melting
point
273.15
Temperature difference of 1 degree celcius (1 0C) = Temperature difference of 1 K
TK
=
C + 273.15
 Equation for the convertion
between kelvin temperatures
and celcius temperatures
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Liquid - glass thermometer
The liquid which is mostly used in liquid - glass thermometers is either Mercury or alcohol. The melting point of
Mercury is 39 0C while the boiling point of 357 0C. The melting point of alcohol is 112 0C and the boiling point is
78 0C
Advantages of using Mercury
•
•
•
•
•
Having a good heat conductivity.
Since it is an opaque liquid, the position of Mercury mensicus could be read easily.
Since it does not wet the glass, it can easily flow inside the capillary tube.
The temperatures that need to be measured in ordinary activities fall between the melting point (–39 0C) and
boiling point (357 0C) of Mercury.
The change in volume corresponding to a certain change in temperature being larger when compared to
alcohol.
Disadvantages of using mercury
•
•
Since Mercury is toxic, if the thermometer breaks, it is dangerous to the envirnment and organisms.
Not being suitable to measure very low temperatures.
Advantages of using alcohol
•
•
•
Being suitable to measure low temperatures.
Being less toxic than Mercury.
Being cheaper than Mercury thermometers.
Disadvantages of using alcohol
•
•
•
•
•
Since it is colourless, has to be coloured by mixing with another substance.
Not being suitable to measure high temperatures.
Since it is not a heat conductor, takes a long time to change the temperature to the measured temperature.
Since it wets the glass, could be difficalt to flow inside the capillary tube.
The change in volume corresponding to a certain change in temperature being smaller, when compared to
Mercury.
Construction
•
Mercury
filled funnel
Create a bulb by taking a clean capillary tube of uniform internal cross
section and holding one of its ends to a bunsen burner. Until it liqudized
and blow from the other end. Connect a funnel with Mercury to the open
end, heat and leave it to cool from time to time. This will help the
Mercury enter into tube. Completely fill the tube with Mercury, such that
no air bubbles are formed. When bulb is at a higher temperature than the
highest temperature which is expected to be measured using the
thermometer, seal the open end.
Capillary
ube
Bulb
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Calibration
The ice point and steam point are marked as follows.
Hydro
manometer
Melting Point
Boiling Point
Advantages
•
•
•
•
Construction, calibration and usage is easy.
Cheap
The readings could be directly obtained.
Since the response time is quick (when Mercury is used) suituble to measure temperatures varying with time
(especialy in liquids).
Disadvantages
•
•
•
Measurable temperature range is limited.
Not very suitable to measure the temperatures of solids and gases.
Since the bulb is not too small, cannot measure the temperature of very small places.
Errors
•
•
•
•
•
The cross section of the capillary tube might not be uniform. (Therefore the liquid stem will not rise up
uniformly).
Due to the plastic deformation of glass particles, the volume of the bulb may change with time. [Then all
readings will be erroneous].
Since the bulb is not very small, there could be a certain effect on the temperature being measured.
The pressure of the vapour which is above the free liquid meniscus change with temperature. (The liquid stem
does not rise up uniformly at this instance as well).
The temperatures of the liquid inside the bulb and inside the tube (which is outside the bulb), not being equal.
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Thermo - electric effect
When a closed circuit is made using two wires of two different metals and when its joints are kept at a temperature
difference, the phenomenon of generation of an electromotive force in the circuit is called the thermo electric effect.
Thermocouple
Action
Galvanometer
If the electromotive force generated between the joints when the cold joint is at 0 0C
and the hot joint is at  0C, is E.
E
Cold
Hot
E = a + b + c2
Here a, b, c are constants for a given couple.
•
The electromotive force generated at low temperature gaps could be considered proportional to the temperature gap.
Advantages
•
•
•
•
•
Since the joints could be made small, the temperature of even a point could be measured.
Since the heat capacity of the joints is small, the effect on the temperature which is being measured is low.
Since the heat conductivity of the joints is high, it is sensitive to temperature changes.
Having a high temperature range (–250 0C to 1500 0C)
Could be constructed easily when the necessary equipments are provided.
Disadvantages
•
•
When a very high accuracy is required, a sensitive potentiometer should be used to measure the electromotive
force instead of a voltmeter.
Readings should be calculated.
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Thermistor
A thermistor is a device whose electrical resistance varies with temperature. Therefore, it can be use as a sensor to
detect the variation in temperature. Some thermometers are made using thermistors. The variation of resistance with
temperature can be converted to either a voltage variation or a current variation. When this variation is fed to an
electronic circuit setup, the temperature value is indicated by a digital display.
There are two types of thermistors.
1. Negative Temperature Coefficient (NTC)
type thermistors
2. Positive Temperature Coefficient (PTC)
type thermistors
• Most thermistors in practical use are of the NTC type.
• The external appearance and circuit symbols of a thermistor are shown below.
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