1

Temperature

2

Temperature

Temperature is defined as the

degree of hotness or coldness of a

body measured on a definite scale

Temperature is the driving force or

potential for heat transfer

3

It is a measure of mean Kinetic Energy of the molecules of the system

A change in temperature of a system accounts for change in molecular

motion and the Kinetic Energy and the molecule

Temperature is a parameter which determines whether or not a system is

in thermal equilibrium with another system

Temperature

4

When work or heat is supplied to a system it is not mandatory that the

temperature of the system will increase. It may or may not increase until

the molecular K.E. increase.

Example: - Temperature of a gas in a container doesn't increase when

we put the container in a train.

Temperature

5

C

A BAdiabatic Wall

Diathermic Wall

6

If two bodies A and B are separately in

thermal equilibrium with third body C then A

& B must be in thermal equilibrium with each

other as shown in figure.

Adiabatic wall is a wall which does not allow

the heat to pass through it.

Both A & B are separated from C through a

diathermic wall which transfer through it.

C

A BAdiabatic Wall

Diathermic Wall

Zeroth law of Thermodynamics

7

Several properties of materials change with temperatures in repeatable & predictable way and this forms the basis for accurate temperature measurement.

Anybody with at least one measureable property that changes as its temperatures changes can be used as a thermometer. Such a property is called a thermometric property and the substance is called thermometric substances.

Measuring Devices Thermometric Property

Liquid-in-glass thermometer Length of the liquid in the capillary tube. Gas thermometer Pressure of the gas Thermocouple emf between two dissimilar metals Resistance thermometer Resistance

Thermometric Property

8

Liquid in glass thermometer Gas Thermometer Electric Resistance thermometer Thermocouple

8

Types of Thermometer

9

It consists of a gas capillary tube connected

with a bulb filled with a liquid such as

mercury & alcohol and sealed at the other

end

The space above the liquid is occupied by

the vapour of the same liquid or an inert gas

As the temp increases the liquid expands in

volume and rises in the capillary

Liquid in glass thermometer

10

The length ‘L’ of the liquid in the capillary

depends on the temp.

Accordingly the liquid is the thermometric

substance and ‘L’ is the thermometric

property.

Mercury freezes at -4°C at standard

atmospheric pressure.

The glass deforms at 595 °C.

Liquid in glass thermometer

11

The constant volume gas thermometer is so exceptional in terms of precision & accuracy that it has been adopted internationally as the standard instrument for calibration.

The thermometric substance is the gas (H2 or He) and the thermometric property is pressure exerted by the gas.

As shown in the Fig the gas is contained in a bulb and the pressure exerted by the gas is measured by an open tube mercury manometer.

Gas Thermometer

12

As the temperature increases the gas

expands forcing mercury up in the open tube.

The gas is kept at constant volume by raising

or lowering the reservoir.

The gas thermometer is used as a standard

worldwide by Bureau of standards.

Gas Thermometer

13

In resistance thermometer as shown in Fig. the change in resistance of a metal wire due to change in its temperature is the thermometric property.

The wire frequently platinum may be incorporated in a Wheatstone bridge.

In a restricted range the following quadratic equation is often used

P/Q = R/S , R= R0 (1+At+Bt2)

Where R0 is the resistance of the platinum wire when it is surrounded by melting ice & A & B are constant.

Electric Resistance thermometer

14

Thermocouple circuit made of from joining two wires A and B of dissimilar metals.

According to See beck effects of the two junctions of two dissimilar metals are maintained at two different temperatures an emf will be generated and that emf is the thermometric property for measurement of temp using thermocouple.

Some commonly used material in thermocouple 1. Copper-constantan (alloy of copper and nickel)2. Chrome- Alumel 3. Platinum- (Platinum-Rhodium)

The advantages are: It comes with thermal equilibrium with the system whose temperature is to be measured quite rapidly because of its small mass.

Thermocouple

15

Ideal gas temperatures scale• Let the bulb of the constant volume

gas thermometer contains any one of the gas like O2, Air, N2, H2 etc.

• Assuming that the pressure of the gas is approximately 1000 mm of Hg when it is in contact with triple point of water i.e.

Pt= 1000 mm of Hg• Let the bulb is surrounded by steam at

atmospheric condition and let corresponding thermometric property is P1

Temperature reading will be

T1 = 273.16 X P1/Pt

= 273.16 X P1/1000

16

Ideal gas temperatures scale

• Again if we remove some gas from the bulb so that when it is in contact with the triple point of H2O, then the Pt value is let 500 mm. Hence the new values of pressure and temperature are to be found out T2= 273.16 ( P2/500)

• If we continue the process by reducing the amount of gas in the bulb then Pt and P will go on reducing and the corresponding temperature can be calculated.

17

Ideal gas temperatures scale• If we consider constant pressure gas

thermometer and a similar series of experiments will be carried out, we will get the same curves for different gases plot between T & V.

• When Vt approaches zero, all gases, through follow different paths will meet at a point give rose to a common temperature T .

• T= 273.16(V/Vt)

• Lim Vt -0

18

Standard scale temperature• In gas thermometer let the

bulb containing the gas is kept contact initially with ice point and the pressure is Pi which is equivalent to 1000 mm of Hg. Then the bulb is in contact with steam point and the pressure is Ps.

19

• Ratio Ps/Pi is determined and called (Ps/Pi) = 1000

• Some gas is then removed such that Pi becomes 500 mm of Hg.

• So (Ps/Pi)500 is obtained.

• This may be repeated for more values of Pi.

• If a graph is plotted between (Ps/Pi) and Pi for all gases (O2, Air, N2, H2, etc.) straight lines are achieved and they meet a common point when Pi approaches to zero.

• On experiment Ps/Pi= 1.366

• Ts/Ti= 1.366……………….(1)

• Ts-Ti = 100 …………………..(2)

• Solving (1) & (2) we get

• Ts= 373.16 K

• Ti= 273.16 K

• T(k)abs = (t°C - tiC)+Ti(K)

• Where ti0C= 0°C

20

Thermodynamic Temperature scale• It is the temperature scale in which temperature value does not

depend on the properties of any particular substance. It is known as Kelvin scale or absolute scale.

• All thermodynamic calculations are based on absolute Kelvin scale. T(K) = T°C+273.15 T(K) = T°C+273 (generally taken)

21

Rankine Scale

• Absolute temperature scale in FPS unit. Relation between relative scale and absolute scale. T(R) = T° (F) +459.67

T(R) = T° (F) +460 (generally taken)