Heat and Temperature
Definition of Heat
Heat is a form of energy, entry or exit of which correspondingly increases or decreases internal energy of a body when no work is done on the body or by the body.
Definition of temperature
Temperature is the thermal state of a body or a region of space that determines whether the body or the space will give out or absorb heat energy from a neighboring body or a space in thermal contact.
Different thermometer scales
(a) The centigrade or Celsius scale: It is introduced by Celsius, is usually used in scientific laboratories. In this scale the lower fixed point or the ice-point is 0°C and the steam-point 100°C. The fundamental interval is divided in 100 equal parts; each part is called 1°C (one degree Celsius).
(b) The Fahrenheit scale: It is suggested by Gabriel Fahrenheit, is usually used in clinical and meteorological purposes. Here, the ice-point and the steam-point are correspondingly marked as 32°F and 212°F, and the fundamental interval is divided into 180 equal divisions. Each division is called one degree Fahrenheit. (1°F).
(c) Absolute scale or Kelvin scale: It is designed by Lord Kelvin, is used internationally in modern scientific world. In this scale the ice-point is marked 273K and the steam-point 373K, the fundamental interval is divided into100 equal divisions, like that in the Celsius scale. Each division is read as one degree absolute or one Kelvin (IK). In fact, Kelvin scale of temperature starts from the temperature corresponding to -273°C, taken as zero Kelvin.
Relation between Celsius, Fahrenheit and Kelvin scales
Since the range of temperature from ice-point to steam-point is equal in all the three scales, 100 centigrade degrees = (212 - 32) or 180 Fahrenheit degrees = (373 - 273) or 100 absolute degrees.
We consider three thermometers in the above three scales are dipped simultaneously in a liquid of certain temperature. Let the temperatures recorded in the Celsius, Fahrenheit and Kelvin thermometers respectively be C, F and K. Now it can be proved that
C / 5 = F- 32 / 9 = K- 273 /5
Units of Heat
(a) C.G.S. unit of heat is Calorie. (b) The M.K.S. or S.I. unit of heat is Joule.
1 calorie equals 4.18 or 4.2 joules approximately.
The quantity of heat absorbed or given out by a substance during a thermal change depends on (a) mass, (b) difference of temperature and (c) specific heat of the material of the substance.
(a) Mass: A larger mass of a substance absorbs or gives out more heat than a smaller mass of it for a certain change of temperature. So, if 'H' be the quantity of heat absorbed or given out by a substance of mass 'm’ for a given change of temperature, H = m.
(b) Temperature: The heat absorbed or released by a certain quantity of a given substance increases or decreases accordingly as the difference between the initial and final temperatures is large and small. Thus, H= (T-t) where, (T-t) is the difference between the initial and the final temperatures of the body.
(c) Specific heat: This is a fundamental property of matter. An equal mass of different materials absorb or give out different quantities of heat, even if they are heated or cooled through the same range of temperature.This is also called specific heat capacity (abbreviated as SHC) in S.I. system.
Heat (H) absorbed or given out by body of mass (m) for rise or fall of temperature through t is given by H = mst.
Principle of Calorimetry
Heat lost by the hotter bodies = Heat gained by the colder bodies.