HEAT 1

Heat cont… Heat Consist of the following (i) Thermometry (Thermometers) (ii) Heat Transfer -Thermal Conduction -Thermal Convection -Thermal Radiation (iii) Thermodynamics 2

Heat cont… Definition: Heat is the form of energy which can flow from one point to another depending to the change of temperature. INTRODUCTION Heat is the form of energy or we simply call thermal energy which can flow in a given system depending to fundamental quantity called Temperature TEMPERATURE The temperature is a quantity of the degree of hotness or coldness being common to two systems being in thermal dynamics equilibrium. The dimension of temperature is Q Temperature is described by a number (chosen Scale such that when two bodies are in Contact heat flows from higher temperature to lower temperature 3

Temperature measurements The only apparatus used to measure temperature is the thermometer. A Thermometer is a chosen system or body which has some properties continuously varying degree of hotness. The properties are called Thermometric properties. 4

Temperature measurements cont… Thermometer 5

Thermometric properties Thermometric property is the property of the substance which varies linearly with the temperature. 6

Thermometric property examples i. The length of liquid column in a glass to be ( This is for mercury in –glass Thermometer) ii. The e.m.f of a Thermo couple (for thermocouple Thermometer) iii.The electrical resistance of platinum wire wound in to a Coil.(for platinum resistance thermometer) iv.The pressure of a gas whose volume is kept constant (for constant volume gas Thermometer) 7

Thermometric property examples cont… (v) Volume of the gas at constant pressure. (for constant pressure thermometer) (vi) Colour of radiation emitted by hot body 8

Thermometric Property Examples cont… (i) Length of a liquid column L Mercury in glass thermometer Alcohol in glass thermometer Laboratory thermometer Clinical thermometer (Barometer) / Thermometer 9

Thermometric Property Examples cont… 10

Thermometric Property Examples cont… (ii) Electromotive force (e.m.f) of a thermocouple. 11

Thermometric Prop. Examples cont… (iii) resistance of the wire 12

Thermometric Property Examples cont… 13

Thermometric Prop. Examples cont… 14

THE FIXED POINTS OF A THERMOMETER. Fixed point is a single temperature at which it can confidently be expected that a particular physical events always takes place. The two fixed temperatures point must be known and required for setting the temperature range of any thermometer. Examples of fixed points: (i) Melting point of pure ice (ice point) or lower fixed point (ii) upper fixed point (steam point) 15

THE FIXED POINTS OF A THERMOMETER. cont… i.The lower fixed point, or ice point, is the temperature of pure melting ice at normal atmospheric pressure. ii.The upper fixed point, or steam point, is the temperature of pure boiling water at normal atmospheric pressure. 16

THE FIXED POINTS OF A THERMOMETER. cont…. 17

THE FIXED POINTS OF A THERMOMETER. cont…. 18

HOW TO DEFINE A TEMPERETURE SCALE. Essential steps are needed. a)Choose the thermometric material b)Select the thermometric property of the material c)Select the two fixed point (lower and upper) d)To allocate a value such as t m to degree of material. 19

Determination of temperature by using thermometric property.. 20

Determination of temperature by using thermometric property. cont… 21

Determination of temperature by using thermometric property. cont…. 22

Determination of temperature by using thermometric property. cont… 23

Determination of temperature by using thermometric property. cont…. 24

Examples of thermometric property. 25

Examples of thermometric property. Cont…. 26

Example 1 A given platinum has resistance R o and R 100 at the ice point and steam point respectively i. What is its fundamental interval? ii.If it has resistance R at unknown temperature t p. what is the expression for t p ? Solution 27

Solution i.Fundamental internal is the difference between the thermometric value of upper fixed point and lower fixed point. ii. 28

Example 2 Write down an expression which calibrates a thermometer using a thermometric property X to read temperature in 0 C is. Hence find the temperature of system when its pressure is 4.5 P a given that at lower and upper fixed point the pressure is respectively 1.5 P a and 3.0 P a. 29

Solution. NB: If the scale of the degree of hotness is to be in k, then t m =T, t =Ok, N= k

Example3 The resistance of a resistance thermometer at Celsius temperature as measured by a constant volume gas thermometer is given by Calculate the temperature as measured on the scale of the resistance thermometer which corresponds to a temperature of 60 0 C on the gas thermometer 31

Solution. 32

Solution. Cont…. 33

Thermometers compared Types of thermometer Thermometric property AdvantagesDisadvantages Mercury in glass thermometer Length of mercury column in capiraly tube -portable -we read directly -relatively small -Not very accurate -its error is larger compared to others Constant volume gas thermometer pressure-very wide range of temperature -very accurate -very sensitive -very cumbersome a bulb may have a capacity of Very slow to use 34

Thermometers compared. Cont… Types of thermometer Thermometric property AdvantagesDisadvantages Platinum resistanceElectrical resistance-wide range -the most accurate -not suitable for varying temperature thermocouple-emf of two wires of different metals -wide range -very small and used for both local and varying temperature -is the most accurate. Not as accurate as platinum resistance. 35

HEAT TRANSFER Heat may be transferred from one point to another depending in the difference in temperature between the two points. When heat travels it involves some molecular vibrations or complete movement Therefore Conduction of heat can be explained in terms of the kinetic theory of matter. 36

CONDUCTION In conduction a temperature difference causes the transfer of energy from one region hot body to another region of the same body which is at a lower temperature. The flow tends to equalize the temperature within the body. Conduction takes place in solids, liquids and gases. 37

In solids and Liquids Free electrons in the lattice structure of solids or liquids move randomly as molecules in the gas. These transfer energy from the hot region to the cold region of the body. In solids the ions are coupled (joined together to form a shape of solid or liquids. The atoms in a heated region vibrate with high K.E to the coupling of atoms they easily transfer this energy to neighboring atoms. 38

In gases It is are suit of collision of fast moving molecules and slow moving molecules and slow moving molecules during a collision gain K.E From kinetic theory the fast moving molecules are from the heated point. 39

Temperature distribution in heated conductor Consider a cylindrical bar of uniform cross section area A which is heated at one end. 40

Temperature distribution in heated conductor cont… NB: Temperature gradient may be constant or continuously changing depending on the heat loss to the surrounding before it reaches the other end. 41

Temperature distribution in heated conductor cont… 42

Temperature distribution in heated conductor cont… 43

Temperature distribution in heated conductor cont… If the bar is lagged no loss of heat by any way –conduction, convection or radiation. Temperature gradient determines the rate of flow of heat in a body. This indicates that the rate of flow of heat is constant the lines of flow of heat are parallel. The temperature gradient along the un lagged bar is not a constant, it decreases with distance from the heated end 44

Temperature distribution in heated conductor cont… 45

Temperature distribution in heated conductor cont… NB: In both cases, the temperature gradient is negative indicating that the heat flows in the direction of increasing distance between decreasing temp. For un lagged bar the lines of flow of heat are diverged 46

THERMAL CONDUCTIVITY 47