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TEMPERATURE
Temperature is a property of a body which decides which way heat will flow when it is placed in contact with another body i.e. Temperature is the degree of hotness or coldness.
Temperature must not be confused with heat itself.
Heat is a transfer of energy due to temperature energy. The S.I unit of temperature is Kelvin known as the “absolute” or “thermodynamic scale” on this scale temperature is measured on Kelvin. Its symbol is K
Commonly thermometers encounter the unit “Celsius degree” with symbol 0C. Another scale is called “Fahrenheit scale”
MEASUREMENT OF TEMPERATURE
A reliable measurement of temperature is done by using thermometer.
Thermometers use measurable physical properties that change linearly with temperature to give temperature readings
Physical properties that change with temperature are called “thermometric properties of a thermometer” which include;
i) •Expansion of liquid when heated (e.g. alcohol and mercury).
ii) •Expansion of compound strip of two metals.
iii) •Thermometric property change in which when junction of two different metals is heated an electric current is generate.
iv) •Change of resistance of a wire e.g. platinum , resistance thermometer.
•We can investigate some of thermometric properties by studying expansion of mercury in an evacuated tube in the experiment shown below.
EXPERIMENT
To investigate thermometric properties of mercury.
APPARATUS
Water, heater and mercury evacuated narrow tub.
PROCEDURES
1. Put crushed ice in a filter funnel and insert the ice shown below.
2. Put the beaker underneath the funnel to collect the water.
3. Wait until the mercury in the thermometer stops falling.
4. Mark the position of mercury meniscus.
OBSERVATIONS
The marked position of the meniscus is the upper fixed point of the thermometer, in Celsius scale, the upper fixed point is 1000C or 373K.
FUNDAMENTAL INTERVAL OF TEMPERATURE
When you want to construct a thermometer, you must establish two constant temperatures called “fixed points”.
The two temperatures are called upper fixed points and lower fixed points. The fundamental interval of thermometer is the difference between the upper fixed point and the lower fixed point of the thermometer.
The upper fixed point is the temperature of pure steam from water boiling at standard pressure of 760mmHg. Carefully here we use “pure steam” and not boiling water because boiling water has inclusion of impurities and local overheating of the vessel which may alter the boiling point. The lower fixed point is the temperature of pure melting ice.
Note: Impurities lower the melting point of the ice.
Observation
The marked position of the meniscus is the upper fixed point of the thermometer. In Celsius temperature scale, the upper fixed point is 1000C or 373K.
LIQUID IN GLASS THERMOMETER
The working of this thermometer is based on the fact that liquid expands when heated and contracts when cooled e.g. mercury and alcohol thermometers. These two thermometers are called;
1 1. Mercury – in – glass thermometer
2 2. Alcohol – in – glass thermometer
These thermometers have bulbs which are reservoirs of liquids and stems with fine bores through which liquid rises and falls during the variation of temperature.
The liquids used in the thermometers are called thermometric liquids.
TEMPERATURE
COMPARISON OF TWO THERMOMETRIC LIQUIDS
| NO | Mercury | Alcohol |
| 1 | It is a good conductor of heat | It is fairly good conductor |
| 2 | It expands linearly | It expands rapidly (not linearly) |
| 3 | It is clearly seen | It is colorless |
| 4 | It boils at 3600C | It boils at 780C |
| 5 | It freezes at -390C | It freezes at -1120C |
| 6 | Does not wet the glass | It wet the glass |
MODE OF ACTION OF LIQUID – IN – GLASS THERMOMETER
The working of this thermometer is based on the expansion of the liquid in bulb. When the bulb touches a hot body the liquid contained warms up and expand there by rising through the bore and into the stem proportionally to the amount of temperature felt.
When the bore touches something cold the liquid in the stem contracts proportionally to the amount of temperature felt and it falls into the bulb.
By this rise and fall of the liquid level in the steam the different temperature reading can be obtained. The figure below shows mercury – in – glass thermometer that can be found in the Hospitals, Laboratories, and even Homes.
