Heat and Temperature | Physics grade XI :
Heat is the form of energy which simulates our sense organs so that we feel hot or cold . It follows from a body of higher temperature to one of lower temperature .
Its unit is joule(J) in SI and is calorie in CGS .
1 calorie (cal) = 4.2 J
The degree of hotness or coldness of body is called temperature . It determines the direction of flow of heat .
Its unit is Kelvin(K) in SI and is degree Celsius (°C) in CGS .
Difference between heat and temperature :
|1||It is the form of energy.||1||It is a degree of hotness and coldness of a body.|
|2||In, SI units, its unit is joule and in CGS system its unit is calorie.||2||Its SI unit is Kelvin and is degree celsius in CGS system.|
|3||It is a cause .||3||It is a effect.|
|4||Two bodies can be in thermal equilibrium without having same amount of heat .||4||Two bodies can’t be in thermal equilibrium if they are at different temperatures .|
|5||It is a measure of total kinetic energy of all molecules of a body.||5||It is measure of average kinetic energy of all molecules of the body.|
|6||Flow of heat is independent of amount of heat energy contained in the bodies in thermal contact .||6||Flow of heat dependent on temperature of two bodies.If flows from a body at higher temperature t a body at lower temperature.|
Thermal Equilibrium :
Two bodies are said to be in thermal equilibrium if they have same temperature.
Zeroth law of thermodynamics :
If two bodies are separately in thermal equilibrium with a third body,then the first two bodies must be in thermal equilibrium with each other . This is known as Zeroth law of thermodynamics .
Consider three bodies A , B and C as shown in fig. 1 . Here A and C and also B and C are in thermal equilibrium . So A and B must be in thermal equilibrium with each other .
Lower Fixed Point (LFP) :
The temperature at which pure ice melts under standard atmospheric pressure is called Lower Fixed Point on a chosen scale.
Upper Fixed Point (UFP) :
The temperature at which pure water boils under standard atmospheric pressure is called Upper Fixed Point on a chosen scale .
Temperature Scale :
(i) Celsius Scale :
In this scale , LFP is 0°C and UFP is 100°C . The length between LFP and UFP is divide into 100 equal parts . Each part of division represents 1°C .
(ii) Farenheit Scale :
In this scale , LFP is 32°F and UFP is 212°F . The length between LFP and UFP is divide into 180 equal parts . Each part of division represents 1°F .
(iii) Kelvin/Absolute Scale :
In this scale , LFP is 273K and UFP is 373K . The length between LFP and UFP is divide into 100 equal parts . Each part of division represents 1K .
(iv) Reaumer scale :
In this scale , LFP is 0°R and UFP is 0°R . The length between LFP and UFP is divide into 80 equal parts . Each part of division represents 1°R .
Relation between temperature scales :
(C – 0) / (100 – 0) = (F – 32) / (212 – 32) = (K – 273) / (373 – 273)