PHYSICS (THERMOMETER)

SECOND TERM: WEEK ONE

TOPIC: -                THERMOMETER

INSTRUCTIONAL MATERIALS: -                  Clinical  and laboratory thermometer

REFERENCES BOOK: -                                       

1.       FARINDE O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.

2.       M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY SCHOOLS, Africana first publishers.  

3.       INTERNET

PREVIOUS KNOWLEDGE: - The Students have been familiar with thermometer.

INSTRUCTIONAL OBJECTIVES: -      At the end of the lessons students should be able to: -

i.                     State what thermometer is used for.

ii.                   List the qualities of good thermometer

iii.                  Mention the thermometric substance of some thermometer.

iv.                 Convert from one temperature scale to another.

CONTENT: -

THERMOMETER

Thermometer is an instrument used in measuring temperature. A thermometer must possess the following qualities:

i.                     CONSTRICTION: A thermometer must possess certain properties before it is used. This properties change as a result of increase in temperature i.e. it must contain a thermometric substance like mercury or alcohol.

ii.                   CALIBRATION: A thermometer must be well calibrated starting with two reference points. The calibration must be made with shiny ink that can be viewed.

iii.                  SENSITIVITY: A thermometer is said to be sensitive if :

I.                    It shows the temperature of a body in a short time.

II.                  It can detect even small change in temperature.

III.                Does not absorb large amount of heat for its own heating or cooling from the body whose temperature is being measured.

TYPES OF THERMOMETER AND THEIR THERMOMETRIC SUBSTANCES

There are many types of thermometers, but some common thermometers are laboratory thermometer, clinical thermometer, thermocouple, Bimetallic strip thermometer, Pyrometers, etc.

Some substances have the property that changes with temperature. Substances that show a change in temperature can be used as a thermometric material. For example, some substances expand on heating, some change their colors, some change their electric resistance, etc. Nearly all the substances expand on heating. Liquids also expand on heating and are suitable as thermometric materials. Common thermometers are generally made using some suitable liquid as thermometric material. Which properties should present in thermometric liquid?

A thermometric  liquid should have the following properties:

1.  It should be visible.          2. It should have uniform thermal expansion.     3. It should have a low freezing point.

4. It should have a high boiling point.          5. It should not wet glass.               6. It should be a good conductor of heat.

7. It should have small specific heat capacity.

CLINICAL THERMOMETER

 Clinical thermometer is used in homes. It has a glass tube with a bulb at one end and the other end closed. This bulb and some part of the tube is filled with thermometric liquid, such as mercury or alcohol. The rest of the volume of the tube has vacuum in it. There is a kink near the end of the glass tube which prevents the falling back of the thermometric liquid once it is removed from the hot body under observation.
It is basically used to measure the temperature of humans. A clinical thermometer has a temperature range of only 35 ºC to 42 °C. 

LIQUID- IN- GLASS THERMOMETER

(a) A Bulb: Contains the thermometric liquid. It is thin walled to increase sensitivity of the thermometer (i.e. allow quick transfer of heat).

(b) Capillary Bore: It allows the liquid in the bulb to rise and fall when temperature changes. It has a small diameter to increase its accuracy so that a small change in temperature can easily be measured.

(c) Glass Stem: It is made up of thick glass to strengthen the thermometer and therefore protect the liquid inside. Thick glass also acts as a magnifying glass to magnify the liquid thread.

THERMOMETER

THERMOMETRIC SUBSTANCE

1.  Liquid in glass thermometer

Mercury or alcohol

2. Constant volume gas

Gas

3. Resistance thermometer

Resistance wire

4.  Thermocouple

Copper and Constantant  wire

SCALE OF TEMPERATURE

Thermometers are usually in Celcius scale of two references. There are upper and the lower fixed points. The upper fixed point is the boiling point (100oC) in water and lower fixed point is the melting point i.e. 0oC in water.

TEMPERATURE SCALE RANGES

Fundamental interval is the interval between the upper fixed point and lower fixed point. In celcius scale is of 100 equal divisions.

Scale Factor

oC

oRe

oF

K

oRa

Boiling point of water at 1 atm.

100

80

212

373

672

Freezing point of water at 1 atm.

0

0

32

273

492

Where oC – degree celcius (centigrade); oRe – Reaumur ; oF – degree Fahrenheit;  K- Kelvin; oRa – Rankine.

The celcius and the Kelvin scale are both express as T= 273 + oC. The  Kelvin scale has no negative value but celcius scale has.

Formular for converting from one temperature scale to another

I.                    Fahrenheit to celcius scale

 =        or        =

II.                  Rankine  to celcius scale

 =   or      =

Advantages of Mercury as a Thermometric Liquid

I.                    It does not wet glass.

II.                  It response quickly to slight change in temperature.

III.                The liquid does not vapourize easily.

IV.                Mercury is opaque, it can be seen easily.

V.                  It has regular expansion.

Disadvantages of Mercury as a Thermometric Liquid

I.                    It is expensive.

II.                  It cannot be used to measure very low temperature.

Advantages of Alcohol as a Thermometric Liquid

I.                    It is less expensive than mercury.

II.                  It has a large expansion on heating than mercury

III.                It can be used to measure a very low temperature.

Disadvantages of Alcohol as a Thermometric Liquid

I.                    It is not opaque and so must be coloured

II.                  It is vapourize easily

III.                It wet glass

IV.                It has an irregular expansion.

V.                  It has low boiling point of 78oC

DETERMINATION OF UPPER AND LOWER FIXED POINT

UPPER FIXED POINT

This can be determined by passing the thermometer into a steam jacket which is obtained from the boiling water. The container is made with a soluble copper wall vessel called hypsometer which is used to prevent loss of heat. The thermometer should be placed so that the mercury thread is visible. The manometer is fixed with the hypsometer so as to equalize pressure at 760mmHg when the thermometer thread is steady and the pressure of the room is 760mmHg, the meter read 100oC.

LOWER FIEX POINT

The thermometer is inserted into a funnel containing a melting ice and pure water mixture, when the temperature is steady in the thermometer, the reading can be taken which will show that it is 0oC.

Example

1.       Convert (a)  60oC to oF         (b) 65oF to Kelvin

Solution

(a)     =60oC,            =   = ;   =12; 108 = F – 32; F= 108+32; F=140o.

(b)   = 65oF,        =  ;    = ; 33X5 = 9  ;  = =18.2; K= 273 + oC = 273+18.2 = 291.2K.

2.       If the difference between upper and lower fixed point of a celcius thermometer is 85cm, and the liquid value during usage is 30cm. Calculate the temperature at that point.

Solution     

                              

                    

PRESENTATION

STEP I: The teacher gives the students a welcome test.

STEPII: The teacher explains thermometer and their properties.

STEP III: The teacher leads the students to identify scales of temperature and how to find the fundamental interval

STEP IV: The teacher states advantages and disadvantages of both Mercury and Alcohol as a thermometric liquid.

STEP V: The teacher leads the students to convert temperature from one scale to another.

EVALUATION

The teacher evaluates the lessons by asking these questions.

i.                     State what thermometer is used for.

ii.                   List the qualities of good thermometer

iii.                  Mention the thermometric substance of some thermometer.

ASSIGNMENT

1. Differentiate between clinical and laboratory thermometer.

Clinical thermometer

Laboratory thermometer

This thermometer is used in homes. It is basically used to measure the temperature of human body. 

This thermometer is used to measure the temperature of all things, except the human body.

A clinical thermometer has a temperature range of only 35 ºC to 42°C.

A laboratory thermometer has a temperature range of -10 ºC to 110 ºC.

It has kink.

It does not have kink.


  

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