PHYSICS: FORM TWO: Topic 8 – TEMPERATURE

The Term Temperature

Define the term temperature

The SI Unit of Temperature

State the SI unit of 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.

Physical properties that change with temperature are called thermometric properties of a thermometer which include the following:

1. Expansion of liquid when heated. E.g. Alcohol and mercury
2. Expansion of strip of two metals.
3. Generation of an electric current when heated
4. Change in resistance of a wire

When 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 fixed points.

The lower fixed point is the temperature of pure melting ice while the upper fixed point is the temperature of the boiling water. Therefore lower fixed point is referred to as the ice point and the upper fixed point is referred to as the steam point.

Since water freeze at 0 degrees centigrade and boils at 100 degrees of centigrade, it follows that the lower and upper fixed points are 0⁰C and 100⁰C respectively.

The 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

1. Mercury in glass thermometer
2. Alcohol in glass thermometer

These thermometers have bulbs, which are reservoirs of liquid, and stems with fine bodies through which liquid rises and falls during the variation of temperatures.

The liquid used in these thermometer types is called thermometric liquids. A thermometric liquid is the one which show significant changes with the change in temperature and hence used in thermometers. A thermometric liquid must be very sensitive to temperature changes.

Comparison between the thermometric liquids:

Mercury	Alcohol
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

The temperature of the human body is measured by a special kind of mercury thermometer known as clinical thermometer. It has a large bulb, thin tube to make it sensitive to temperature changes of human body. The tube of the clinical thermometer has a constriction necessary for preventing the back flow of mercury after the thermometer has been removed from the human body.

A 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.