Temperature is property of a body, which decides which way heat will flow when it is placed in contact with another body.
The SI Unit of Temperature
State the SI unit of temperature
Temperature refers to the degree of coldness or hotness of a body. The SI unit of temperature is Kelvin (K) known as absolute zero or thermodynamic scale.
Commonly in thermometers we encounter the unit Celsius degree (0C) another scale is called Fahrenheit scale.
Measurement of Temperature
Measurable Physical Properties that Change with Temperature
Identify measurable physical properties that change with temperature
A reliable measurement of temperature is done by a thermometer.
Thermometers use measurable physical properties that change linearly
with temperature to give temperature reading.
properties that change with temperature are called thermometric
properties of a thermometer which include the following:
Expansion of liquid when heated. E.g. Alcohol and mercury
Expansion of strip of two metals.
Generation of an electric current when heated
Change in resistance of a wire
The Fundamental Interval of a Thermometer
Define the fundamental interval of a thermometer
you want to construct a thermometer you must establish two constant
temperatures called fixed points. The fundamental interval of
thermometer is the difference between the upper fixed point and lower
The lower fixed point is the temperature of pure melting ice. Note that impurities lower the melting point of ice.
The Mode of Action of Liquid-in-glass Thermometer
Describe the ‘mode’ of action of liquid-in-glass thermometer
working of this type of thermometer is based on the fact that liquid
expands when heated and contracts when cooled. Examples of Liquid in
glass thermometers are mercury and alcohol thermometers.
These two thermometers are called
Mercury in glass thermometer
Alcohol in glass thermometer
thermometers have bulbs, which are reservoirs of liquid, and stems with
fine bodies through which liquid rises and falls during the variation
The liquid used in these thermometer types is called thermometric liquids.
Comparison between the thermometric liquids:
i. It is good conductor of heat
i.It is fairly a good conductor of heat.
ii. It expands linearly
ii. It expands rapidly
iii. It is clearly seen
iii. It is colourless
iv. It boils at 3600c
iv. It boils at 780c
v. It freezes at 390c
v. It freezes at – 1120c
vi. It does not wet a glass
vi. It wets the glass
Why water is not used as a thermometric liquid
though water is very readily available and it is cheap compared to
mercury or alcohol, it is not used as thermometric liquid because:
Its volume expansion is not linear
It wets the glass
It has high heat capacity
The Temperature of a Body
Measure accurately the temperature of a body
clinical thermometer is a typically a mercury in glass thermometer used
to measure human body temperature. As the temperature rises the mercury
expands and flows up the capillary tube. However, clinical thermometers
have some limitations, they are delicate and can break easily, they may
spread infection if not properly sterilised and they do not necessarily
reflect the core body temperature.