Temperature  
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T
emperature is the degree of coldness or hotness of a body  
The SI unit of temperature is Kelvin, K. Other unit is Celsius, (0C) or  
Fahrenheit,( 0F)  
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Kelvin scale is also known as absolute or thermodynamic scale.  
The following formula may be useful when converting temperature from one  
unit to another:-  
푲 = ퟐퟕퟑ + 휽°푪  
푭 = ퟗ × 휽°푪 + ퟑퟐ°푪  
ퟓ
Whereby: (휽°푪) 풊풔 풕풉풆 풕풆풎풑풆풓풂풕풖풓풆풊풏 풄풆풍풔풊풖풔  
Difference between heat and temperature  
Heat  
Temperature  
Is a form of energy possessed by a body  
due to its temperature change  
Its SI unit is Joule (J)  
It cannot be measured directly  
It can be transferred from one body to another  
I
s the degree of hotness or coldness of a  
body  
Its SI unit is Kelvin (K)  
It can be measured by a thermometer  
It cannot be transferred  
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S
imilarities between Heat and temperature  
→
Heat and temperature all relate to the energy of a body  
Measurement of Temperature  
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Temperature is measured by using a thermometer  
The measurement starts with the establishment of any of the following  
temperature scale.  
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Fahrenheit scale (0F)  
Celsius scale  
Kelvin scale  
(0C)  
(K)  
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The units of both the Celsius scale and Fahrenheit scale are called degree  
The SI units of Kelvin scale is Kelvin  
0
The freezing point of water in Celsius scale is 0 C while its boiling point is 1000 C  
Absolute zero temperature: Is the theoretical value of temperature where it is  
assumed that a substance has zero volume  
OR  
Is the temperature at which the molecules of a  
material have zero kinetic energy  
Example:  
0
0
1. Convert 600 F into the C scale  
Solution:  
0
ퟓ
푭풓풐풎: 푻풆풎풑풆풓풂풕풖풓풆 풊풏 C =  
풙 ( 푭 − ퟑퟐ)  
ퟗ
ퟓ
ퟓ
휽 0 C =  
ퟓ
풙 ퟓퟔퟖ = ퟑퟏퟓ. ퟔ 0C  
(
)
ퟔퟎퟎ − ퟑퟐ =  
풙
(
)
풙 푭 − ퟑퟐ =  
ퟗ
ퟗ
ퟗ
2. Convert 398 K to degree Celsius.  
Solution:  
푭풓풐풎: 푻풆풎풑풆풓풂풕풖풓풆 풊풏 0 C = Temperature in K – 273  
∴ 푻풆풎풑풆풓풂풕풖풓풆 풊풏 0 C = 398 – 273 = 1250 C  
Individual task – 1  
1. After being mixed with impurities, the melting of ice was found to be-  
22 0 C. What is this temperature in Kelvin?  
(
ANS: 휽 0 C = 251 K)  
2. The temperature of the liquid nitrogen is 77 K. What is this temperature on  
the Celsius and Fahrenheit scale?  
(
ANS: -196 0C and -320 0F)  
A temperature scale is built from  
:
(a) At least two fixed points which are:-  
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Upper fixed point  
Lower fixed point  
(b) Fundamental interval  
NB:  
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Upper fixed point is the maximum temperature a reference object can attain  
without change of the desired state  
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Lower fixed point is the minimum temperature a reference object can attain  
without change of the desired state  
Fundamental interval refers to the span of numbers between the upper fixed  
point and the lower fixed point  
OR  
Fundamental interval Is the difference between the upper fixed point and  
the lower fixed point on a temperature scale.  
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The Upper fixed point (steam point): Is the temperature of the pure boiling  
water at normal atmospheric pressure  
The lower fixed point (ice point): Is the temperature of a pure melting ice at  
normal atmospheric pressure  
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To determine the temperature for a given height of the liquid above the  
lower fixed point ,the formula below is used:  
풉풆풊품풉풕 풐풇 풍풊풒풖풊풅 풂풃풐풗풆 풕풉풆 푳.푭.푷  
× 푼. 푭. 푷 풊풏 (0C)  
푻풆풎풑풆풓풂풕풖풓풆 =  
푭풖풏풅풂풎풆풏풕풂풍 풊풏풕풆풓풗풂풍  
Whereby L.F.P = Lower Fixed Point,  
U.F.P = Upper Fixed Point  
Example  
1. The upper fixed point of a thermometer is 23 cm above the lower fixed point .If  
the upper fixed point is 90 0C while the lower fixed point is 25 0C ,What is the  
temperature when the mercury thread is 14 cm above the lower fixed point?  
Solution  
:
The length of mercury thread above the lower fixed point = 14 cm  
Fundamental interval  
Upper fixed point  
Lower fixed point  
= 23 cm  
= 90 0C  
= 25 0C  
풉풆풊품풉풕풐풇풍풊풒풖풊풅 풂풃풐풗풆 풕풉풆 푳.푭.푷  
× 푼. 푭. 푷 풊풏 (0C)  
From: 푻풆풎풑풆풓풂풕풖풓풆 =  
푭풖풏풅풂풎풆풏풕풂풍 풊풏풕풆풓풗풂풍  
ퟏퟒ  
∴ 풕풆풎풑풆풓풂풕풖풓풆 =  
풙 ퟗퟎퟎ = ퟓퟒ. ퟕퟖퟎ푪  
ퟐퟑ  
Thermometers  
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A thermometer is a device that is used to measure the temperature of a  
system in a quantitative way  
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The liquid which is enclosed in these types of thermometers is called  
“thermometric liquid”  
Characteristics of Thermometric Liquid  
(a) The liquid must be a good thermal conductor  
(b) The liquid must be easily visible  
(c) The liquid must have a wide range of temperature  
(d) The liquid should not stick on to the walls of the container  
(e) The liquid should be sensitive to minute temperature changes  
(f) It should have a small specific heat capacity  
NB:  
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T
he most common thermometric liquids are mercury and alcohol  
Alcohol thermometer  
Is a type of thermometer that uses a bulb filled with alcohol as the temperature sensor  
Advantages of alcohol as a thermometric liquid:  
0
a) Has a low freezing point of -115 C  
b) Its expansion is uniform (It expands more than mercury)  
c) It is cheap and easily available  
d) It is clear visible through glass  
e) It is less toxic and less hazardous  
Disadvantages of alcohol as a thermometric liquid  
a) It cannot measure a high temperature, because its boiling point is low  
b) Less durable (alcohol evaporates)  
c) It makes wet the wall of the glass  
d) The liquid should be dyed before filling the bulb  
Mercury thermometer  
Is a type of thermometer that uses a bulb filled with mercury as the temperature sensor  
Advantages of mercury as a thermometric liquid:  
a) It has a high boiling point (357°C) and low melting point (-39°C).  
b) It is visible, because its color silvery shining  
c) It does not stick to the wall of its place  
d) It is a good thermal conductor  
e) Its expansion is regular  
f) It is very sensitive to temperature changes.  
Disadvantages of Mercury as Thermometric Liquid  
a) It is relatively expensive  
b) It is toxic  
c) Its freezing point is – 39 0C, hence is not suitable in reading very low  
temperature  
Why water is not used as a thermometric liquid?  
a) Its expansion is not linear  
b) It has high freezing point and low boiling point  
c) It has high specific heat capacity  
d) It is transparent (colorless)  
e) It is poor conductor of heat  
f) It wets glass and sticks to the sides of the glass.  
Types of thermometer  
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Mercury in glass thermometer  
Alcohol in glass thermometer  
Bimetallic thermometer  
Thermocouple thermometer (works on Emf)  
Thermistor thermometer (Resistance thermometer)  
NB:  
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Thermometric property  
Is a physical property of matter on which a thermometer is based on.  
OR  
Is the property of a material that varies with the temperature of it  
OR  
Is the property of a substance which changes uniformly with the uniform  
change in temperature  
Liquid in Glass Thermometer  
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Liquid – in glass thermometer are the most often used for temperature  
measurement  
A typical liquid in glass thermometer consists of a glass cylinder with a bulb at  
one end , a capillary tube inside the glass cylinder  
Examples of liquid in glass thermometers are clinical thermometer, six’s  
thermometer, laboratory thermometer  
Clinical thermometer (medical/doctors –thermometer  
)
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Is typically a mercury in – glass thermometer used to measure human body  
temperature  
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It consists of  
(a) A thin walled bulb containing mercury which warms quickly and makes the  
thermometer quick – acting  
(b)  
A
narrow capillary bore for providing sensitivity of thermometer  
(c) Narrow constriction just above the bulb for maintaining the reading (Makes  
the reading not to change).It prevents the mercury in the bore from  
returning to the bulb.  
Mode of Action  
Consider the diagram below  
The thermometric liquid expands with increase of temperature and tends to raise the  
height of the thermometric liquid through the bore and vice versa  
NB:  
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Constriction  
:
is a bend on the capillary tube or a very thin bore found at the  
neck of a clinical thermometer  
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Short stem: is the glass cover of a liquid – in – glass thermometer where the  
scale is calibrated  
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Fine bore: is the space inside a capillary tube  
It is not advisable to sterilize a clinical thermometer in boiling water at normal  
atmospheric pressure because the glass will crack (burst) due to excessive  
pressure created by expansion of mercury  
Limitation of Clinical Thermometer  
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They do not reflect the core temperature of the body  
May spread infection if not properly sterilized  
They are delicate and can break easily  
Precautions taken when using Clinical Thermometer  
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Don’t drop or subject it to heavy shock  
Do not bend it or bite the bulb  
Do not use damaged or broken thermometer as it can cause injury  
Keep it away from unsupervised children  
Sterilize it after use to avoid contamination  
Maximum and Minimum Thermometer (six’s thermometer)  
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Is a thermometer that is used to measure the maximum and minimum  
temperature of a place during a day  
It consists of a U – shaped capillary tube with two separate temperature  
readings ,and a small piece of steel called an index  
(
marker  
)
A
maximum thermometer uses mercury which pushes an index when the  
temperature rises and the mercury expands  
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A
minimum thermometer uses alcohol which contracts when the temperature falls  
.
Maximum thermometer records highest temperature reached while minimum  
thermometer records lowest temperature  
Laboratory thermometer  
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Is a kind of thermometer designed mainly for scientific experiments which  
involve large temperature changes  
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It consists of a bulb filled with mercury , a capillary tube attached to the bulb  
,a glass cover (stem)  
Difference between clinical thermometer and laboratory thermometer  
Clinical Thermometer  
Laboratory thermometer  
Is the thermometer designed to  
measure the body temperature of  
humans and animals  
Is the thermometer designed to  
measure the temperature of liquid and  
gas substance  
Used at homes, hospitals and clinics  
Used in laboratories  
Its range of temperature is 35 0 C to  
43 0C  
Its range of temperature is -10 0C to  
110 0C  
May be used by almost anybody  
It is small in size  
Mainly used in the scientific filed  
It is large in size  
Temperature can be read after  
removing the thermometer from body  
(armpit or mouth)  
Temperature is read while keeping the  
thermometer in the substance