Saturday 28 December 2019


8.0 Temperature
8.1 Concept of temperature
Temperature –is the degree of coldness or hotness of a body at a particular time.
       Temperature is symbolised by T
The temperature is measured by using a thermometer.
 The SI-unit of temperature is Kelvin, K

Other SI-unit commonly used is Celsius (  and some cases Fahrenheit scale
·         The zero K  is equivalent to
·          is equivalent to 273K

   8.2 Measurement of temperature
Ø  Temperature is measured using a thermometer
Ø  Thermometer –comes from two Greek words thermo and meter
·         Thermo –means heat
·         Meter –means measure
Ø  The common example of thermometer is the liquid-in-glass thermometer
Ø  Thermometer contains a liquid, usually mercury or coloured alcohol.
Ø  In thermometer liquid expands or contracts as temperature changes by changes the volume (moves up and down) of liquid in a thin tube.

Physical properties that change with temperature
Ø  The mercury and alcohol have thermometric properties.
Ø  The decision to use either mercury or alcohol in a thermometer depends on the range of temperature to be measured compared to two properties of the two liquids.

Comparison of properties of mercury and alcohol
It does not wet glass
It wets glass
It is not volatile
It is very volatile
It boils at 360
It boils at 78
It freezes at -39
It freezes at -115
It is opaque, so it is easily seen
It is colourless and has to be coloured
It has a small heat capacity so it will not absorb heat from substances being measured
Its heat capacity is 18 times that of mercury and will absorb heat and cool substances being measured
It is a good conductor of heat
It is a poor conductor of heat
It expands steadily
Its expansion is abrupt and rapid.

Ø  A liquid-in-glass thermometer depends on the volume of the liquid.

Types of thermometer
        i.            A thermistor thermometer
      ii.            A thermocouple thermometer

        i.            A thermistor thermometer –is thermometer depends on the electrical properties of materials varying with temperature.
Ø  Has a battery which provides it with current.
Ø  When its temperature rises, a higher current flows from the battery which results into a higher reading on the digital meter.

      ii.            A thermocouple thermometer –the current flow increases with increase in temperature.

Fundamental interval of a thermometer
The fundamental interval of a thermometer–is the span of numbers between its two fixed points

Ø  The conversion between Celsius and Kelvin
The ice point on the Celsius scale is  while on the Kelvin scale it is 273K.
Others conversion scales

Fixed points of a thermometer
Fixed lower point –is a standard calibration of a thermometer
Ice point –is the fixed lower point of a thermometer with a temperature mark of
Steam point – is the fixed upper point of a thermometer with a temperature mark of

Mode of action of a mercury-in-glass thermometer
Ø  A mercury-in-glass thermometer consists of a glass cylinder with a bulb at one end, a capillary down the axis, connected to the reservoir in the bulb filled with silvery mercury.
Ø  At the temperature of the thermometer changes the mercury in the reservoir expands or contracts causing the height of the liquid in the capillary tube to rise or fall.
Ø  When the thermometer reached thermal equilibrium where the temperature to be measured.
Clinical thermometer
A clinical thermometer –is typically a mercury-in-glass thermometer used to measure human body temperature.

Ø  There is a constriction in the neck close to the bulb.
Ø  As the temperature rises, the force expansion forces the mercury up through the constriction.
Ø  When the temperature falls, the column of mercury breaks at the constriction and cannot return to the bulb, thus remain stationary in the tube.
Ø  This provides accurate measurement of the body temperature after the thermometer is removed.
Ø  To reset the thermometer it must be swung sharply.

Limitations of clinical thermometer.
        i.            They do not necessarily reflect the core temperatures of the body
      ii.            They may spread infection if not properly sterilised.
    iii.            They are delicate and can break easily.

        i.            Do not drop the thermometer or subject it to heavy shock
      ii.            Do not bend the thermometer o bite the bulb
    iii.            Do not use the damaged or broken thermometer as it can cause injury
     iv.            Keep thermometer away from unsupervised children
       v.            Sterilise after use to avoid contamination

Maximum and minimum thermometer
A maximum and minimum thermometer –is mercury-in-glass and an alcohol-in-glass thermometer which can measure the highest and lowest temperatures reached over a period of time.
Ø  This combination is known as six’s thermometer
Ø  It consists of a U-shaped capillary tube with two separate temperature reading, maximum and minimum temperature.
Ø  A minimum thermometer uses alcohol and maximum thermometer uses mercury

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