Concept of temperature
Temperature is the degree of hotness or coolness of a body. The hotter the body is higher is its temperature.
The SI unit of temperature is kelvin (K). For day to day applications, Celsius (˚C) is used. Temperature is measured with a thermometer.
There are three scales of temperature.
i. Fahrenheit scale
ii. Celsius or Centigrade scale
iii. Kelvin or Absolute scale
In Fahrenheit scale, 32 ˚F and 212 ˚F are the freezing point and boiling points respectively. Interval has been divided into 180 parts.
In Celsius scale, also called centigrade scale, 0˚C and 100 ˚C are the freezing point and boiling respectively. Interval has been divided into 100 parts. The formula for converting a Celsius scale to Fahrenheit scale is:
The formula for converting a Fahrenheit scale to Celsius scale is:
Kelvin scale is known as the absolute scale. On the Kelvin scale 0 K represents absolute zero, the temperature at which the molecules of a substance have their lowest possible energy. The solid, liquid, gaseous phases of water can coexist in equilibrium at 273.16 K.
Kelvin is defined as 1/273.16 of the triple point temperature.
The formula for converting a Celsius scale to Kelvin scale is:
K = C + 273.15
The formula for converting a Kelvin scale to Celsius scale is:
C = K - 273.15
The temperature at which the pressure and volume of a gas theoretically reaches zero is called absolute zero. This is shown in Figure 1.8.
For all gases, the pressure extrapolates to zero at the temperature -273.15 ˚C. It is known as absolute zero or 0 K. Some base line temperatures in the three temperature scales are shown in Table 1.1.
i. 25 ºC to Kelvin
ii. 200 K to ºC
i. (TK) = (T°C) + 273.15
(TK) = 25 + 273.15 = 298.15 K
ii. (T°C) = (TK) - 273.15
(T°C) = 200 - 273.15 = -73.15 ºC
Convert the following
i. 35°C to Fahrenheit (°F)
ii. 14° F to °C
i. T(°F) = T(°C) × 1.8 + 32
T(°F) = 25° C × 1.8 + 32 = 77° F
ii. T(°C) = (T(°F) – 32)/ 1.8
T(°C) = (14°F – 32)/ 1.8 = -10° C