Thermal properties of Matter

Heat is a form of energy transferred between two(or more) systems or a system and its surroundings by virtue of temperature difference.(definition)

Temperature of a substance is a physical quantity which measures the degree of hotness or coldness of the substance(definition)

Heat energy flows from a hot body to a cold body.

The direction of energy exchange between two bodies  in contact is shown by a physical quantity known as temperature.

Thus,temperature of a body is a physical quantity which shows whether heat energy flows from one body to another or not.

UNITS OF HEAT: (i) In CGS system, unit of heat is calorie. It is written as cal.

To measure the temperature of a body using a thermometer, different temperature scales are defined by which a thermometer can be calibrated (measured). 

The various temperature scales used to calibrate a thermometer are:1. Celsius scale2. Fahrenheit scale3. Reumer scale

When a hot body is placed in contact with a cold body, the energy of the hot body decreases while that of the cold body increases till the temperature of both the bodies become equal.

Thus,Energy transferred from one body to another due to their temperature difference is known as heat.

(ii) In SI system, unit of heat is joule (J).

1 kcal = 1000cal = 4186 J

1 kJ = 1000 J

1 cal = 4.186 J ~ 4.2 J

When a hot body is placed in contact with a cold body, energy flows from the hot body to the cold body. 

Each temperature scale has two fixed points called lower fixed points and upper fixed points.

The branch of science that deals with the measurement of temperature of a substance is called thermometry

The device used to measure the temperature of a body is called thermometer.

A substance that displays thermometric properties and is used in a thermometer is called a thermometric substance.e.g. alcohol, mercury

There are 4 main thermometric properties:

(i) When the temperature of substances like solids, liquids and gases is increased, these substances tend to expand (i.e. their volume increases).

(ii) The pressure of gases increases when gases are heated at constant volume.

(iii) The resistance of conductors increases when conductors are heated.

(iv) Thermo e.m.f. is produced in thermocouple circuits.

1. CELSIUS SCALE:  In celsius scale, the lower fixed point is the melting point of pure ice, at standard atmospheric pressure.It is marked as 0 degree C

The upper fixed point is the boiling point pure water at standard atmospheric pressure, also called steam point. It is marked as 100 degree C.

The interval between the melting point and steam point is divided into 100 equal parts, each 1 degree C.

2. Fahrenheit scale:In Fahrenheit scale, the lower fixed point is the melting point of ice at standard atmospheric pressure and is marked as 32 degree F.

The upper fixed point is boiling point of pure water and is marked as 212 degree F.

The interval between the two fixed points is divided into 180 equal parts, each 1 degree F.

F =  (9 / 5)C + 32

i.e. (9 C + 160) / 5

C = (F - 32) 5 / 9

C / 100 = (F - 32) / 180

IDEAL GAS: At low pressure and high temperature, all real gases behave as ideal gases.

Ideal gas obeys the following gas laws:

Boyle's law: When temperature(T) is constant,        PV = constant

Charles' law: When pressure is constant,        V/T = constant

Ideal gas law: As PV = constant and V/T = constant,       PV/T = constant

Ideal gas equation: PV = nRT

In a constant - volume gas thermometer, temperature is read in terms of pressure.

The pressure temperature graph is a straight line, which when extended  reaches 0 pressure at the absolute temperature, which is: -273.15C. 

Absolute zero is the foundation of the Kelvin temperature scale or absolute temperature scale.

In this scale, -273C is taken as the zero point, i.e. 0 K. 

K = C + 273

NOTE: The temperature in Kelvin scale is always positive and never negative.

The ice point and steam point usually change with addition of impurities to water.

To overcome this problem, triple point of water is used as the single reference point of temperature scales.

Triple point of water is a temperature at which all three states of water co-exist in thermal equilibrium.

Triple point of water is 273.16K (not 273.15K) under a pressure of 4.58mm of mercury.

It is 0.01C in celsius scale.

The reason for assigning the value 273.16K to triple point of water is that it makes division on  Kelvin scale equal to that on the celsius scale.

T = 273.16 (P / Pt )

The increase in the dimensions of a body due to the increase in its temperature is called thermal expansion.

l/L = at

a/A = 2a = Bt

v/V = 3at = rt

(i) Linear expansion: When a solid is heated and its length increases, the expansion is called linear expansion.

We have,

l (proportional to)L1t

or,l = aL1t

or,a = 

Heat and Temperature

Measurement of temperature

Absolute temperature

triple point of water

thermal expansion