Temperature - science lessons for life

Wednesday, January 4, 2017

Temperature

Daily weather report transmitted through television channels may be familiar to you. Do you remember hearing that the lowest temperature was reported from Nuwaraeliya while the highest temperature was reported from Trincomalee on a particular day? Can you remember that it is difficult to dry washed clothes on rainy days and that they dry fast on warm sunny days?

Try to recall the coolness experienced in eating an ice-cream and the warmness felt in drinking a hot cup of tea.

The physical quantity that describes each of the instances above is the temperature.

Temperature can be specified as a fundamental property of any material object. An ice cube has a very low temperature. Temperature of warm water is higher than the temperature of cold water.

Our body too has a temperature. Therefore we can say whether the temperature of a certain object is higher than or lower than the temperature of our body by touching the object.

Temperature is measure of the mean kinetic energy possessed by the particles that form an object.

Measuring temperature
By touching various objects we can get a rough idea about their temperature. However, since the temperature felt by touching is not so accurate and cannot be expressed as a numerical value, it is not a suitable method for measuring temperature. Therefore, the scientists in the past had realized the necessity for constructing a device for measuring temperature.

Thermometers
The device employed to measure temperature is known as the thermometer. World’s first thermometer was invented by Galileo Galilei around 1600 A.D.
Thermometer constructed by Galileo
Galileo Galilei
Various types of thermometers are in use at present. We will only be focusing on the glass - mercury thermometer and the glass-alcohol thermometer in this chapter.

Glass-mercury Thermometer
The glass – mercury thermometer is constructed by connecting a narrow glass tube to a bulb containing mercury. When the temperature rises, the mercury in the bulb expands and moves up along the narrow tube. The temperature can be read from the scale marked on the tube according to the length of the mercury column.

Although the volume expansion due to a small temperature difference is small, the length of the mercury column rises up by a clearly visible amount as the diameter of the narrow tube containing mercury is very small. A glass – mercury thermometer is shown below Figure.

A glass-mercury thermometer
Mercury is commonly used in thermometers as it has a uniform expansion over a broad range of temperatures, is a good thermal conductor and is a liquid over a broad range of temperatures (from - 39 oC to 357 oC). However, due to the toxicity of mercury, use of glass-mercury thermometers is on the decline.

Glass-Alcohol Thermometer
Glass-alcohol thermometer is constructed in the same manner as the glass - mercury thermometer, but replacing mercury by ethyl alcohol (ethanol). Since the melting point of ethanol is -115 oC, it is suitable for measuring low temperatures much below 0 oC. Ethanol is a suitable liquid for thermometers as it has a high expansion relative to most other liquids and as the expansion increases uniformly with temperature. Since purified ethanol is a colorless liquid, it is colored with a coloring material in order to see the alcohol column clearly.

Digital Thermometer
In addition the thermometers mentioned above, digital thermometers from which the temperature can be read directly are also commonly used today. In constructing digital thermometers, an electrical property such as the resistance which depends on the temperature is used instead of the expansion caused by an increase in temperature.
A Digital thermometer
Temperature Scales
There are three temperature scales widely used for temperature measurements. They are the Celsius, Fahrenheit and the Kelvin scales.

Celsius Scale
The Celsius scale has been formed by taking the temperature at which pure ice melts into liquid water under the pressure of one atmosphere as the zero temperature (0 oC) and the temperature at which water vaporizes into steam under the same pressure as 100 oC.

These two temperatures have been chosen for the Celsius scale as the temperature at which ice melts into water and the temperature at which water boils can be easily obtained and as these temperatures have fixed values apart from the variation with pressure.

The definite temperatures used in forming a temperature scale are known as fixed points. For the Celsius scale, these two fixed points are divided into 100 divisions.

Fahrenheit Scale
For the Fahrenheit scale too, the melting point of ice and the boiling point of water are used as the two fixed points. However, here the melting point of ice is taken as 32 oF and the temperature range between the two fixed points are divided into 180 divisions. Accordingly, the boiling point of water is 212 oF.

Kelvin Scale
The zero values of the Celsius and the Fahrenheit scales have been chosen according to the wishes of the people who introduced them. However, the British scientist Lord Kelvin later showed that there is a minimum value to the temperature that any object can reach. This temperature is known as the absolute zero temperature.

The temperature of an object is a measure of the mean kinetic energy of the particles that constitute the object. The temperature of the object decreases when the kinetic energy of the particles
decreases. When the kinetic energy of all the particles become zero, the temperature of the object reaches the absolute zero. Its temperature cannot be decreased below this value. This temperature has been found to be - 273.15 oC according to the Celsius scale.

Lord Kelvin
The Kelvin scale is defined so that its zero (0 K) is at the absolute zero temperature. However, in this scale, a temperature difference equal to 1 K is chosen to be equal to a temperature difference of 1 oC.

Accordingly, the melting temperature of ice is 273.15 K and the boiling temperature of water is 373.15 K. These temperatures are approximately taken as 273 K and 373 K respectively.

The international unit of measuring temperatures is the Kelvin (K).

Relationship between Celsius and Kelvin scales

Celsius and Kelvin scales
Since the difference between the Kelvin and Celsius scales lies only in the temperature chosen for their zero values, in order to convert a temperature measured in Celsius into the Kelvin scale one only needs to add 273. In order to convert a temperature measured in Kelvin into the Celsius scale one has to subtract 273.

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