# Heat and Temperature | Physics grade XI :

## Heat :

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

## Temperature :

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 :

SN Heat SN 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 .

i.e.