Heat

Heat

LEARNING OBJECTIVES 
* Heat and temperature.
* Thermometric scales.
* Heat capacity, water equivalent and specific heat.
* Change of state.
* Latent heat.
* Principle of Calorimetry.

Real time Applications:
*    In medical field thermometers are used by the doctors to know the temperature of a patient.
*    In industries with the help of heat energy many metals are going to convert into thin sheets and wires.
*    In mechanics heat plays a wide role in running a motar vehicle.
*    In factories the temperature of a resultant mixture can be found by using calorimetry principles.
*    Calorimetry is used to determine the calorie content of a food.
*    In chemical laboratories heat change is a fundamental to understand chemical reactivity and physical property of a given material.
 

Important Formulae:
1.\( \frac{{C - 0}} {{100}} = \frac{{F - 32}} {{180}} = \frac{{R - 0}} {{80}} = \frac{{K - 273}} {{100}} \)
2.    Q = ms
3.    Q = m.C.\(\Delta{t}\)
4.    Q=mL
5. (Water Vapours)
6. Principle of calorimetry
Heat energy given out by hot body = Heat energy absorbed by cold body.

Heat and Temperature:
1.Heat is a form of energy which causes in us the sensation of hotness (or) coldness. 
2.Temperature is the degree of hotness (or) coldness of an object.
3.Following points sum up the flow of heat energy from one body to another body.
i) Heat energy always flows from a body at higher temperature to a body at lower temperature till equilibrium is reached.
ii)At equilibrium temperature, the heat energy does not flow.
iii)The rate at which the heat energy flows from a hot body to a cold body depends upon the difference in temperatures of both the bodies.  
iv)The amount of heat energy flowing into the cold body is always equal to the amount of heat energy supplied by the hot body. 
 i.e Heat energy lost (given out) = Heat energy gained OR absorbed. 

Effects of heat energy :
i)Heat energy brings about change in temperature. 
ii)Heat energy brings about change in dimensions. 
iii)Heat energy brings about change in the states of matter.
iv)Heat energy bring about chemical changes in matter. 

Units of heat :-
        system         Unit             Symbol
        C.G.S        calorie             Cal
        S.I              joule                 J
         1 Calorie = 4.18 J    Kilo calorie = 4180  J   
 
Units of temperature :
Generally temperature is measured in degree Celsius ( ⁰C). 
In S.I system temperature is measured in kelvin (K).

Thermometer:
* It is a device used for measuring the temperature of different objects.
* First thermometer was invented by Galileo in 1593 AD. In this thermometer air was used as the thermometric substance,as the air rapidly expand on heating and  contract on cooling.

1.Clinical Thermometer : 
1) It is used for measuring the temperature of the human body. 
2) It is graduated in between 35 - 42⁰C (or) 95 - 108F
3) In this, the bend (constriction) in the bore does not allow the mercury to fall back  in the bulb after its use, unless the thermometer is vigorously shaken. 
4)The normal temperature of a human body is 98.4⁰F or 36.9⁰C.

2.Maximum and minimum Thermometers :
For recording the maximum and minimum temperatures reached on any particular day,  maximum and minimum thermometers are used. They are:
i)Rutherford’s maximum Thermometer. 
ii)Rutherford’s minimum Thermometer. 
iii)Six’s maximum and minimum Thermometer.
The other Thermometers are
 i)Laboratory thermometer     ii) Alcohol Thermometers,etc.
 
 Reasons for using mercury in Thermometers: 
1) Mercury needs very little heat to expand. Thus, it can easily record the temperature of a body without changing the temperature of the body. 
2) The temperature at which mercury boils is 357⁰C. Thus, it can be used to record temperatures as high as 357⁰C. 
3) The temperature at which mercury changes into solid state is - 39⁰C. 
4) Mercury does not stick to the sides of the capillary tube. Thus, it helps   in recording  accurate temperature. 
5) Mercury is opaque and shining. Thus, it can be easily seen as a fine thread in
    the capillary tube. 

Reasons for using Alcohol in Thermometers:-
1)Freezing point of alcohol is less than - 100⁰C. 
2)Alcohol expands more than mercury for the same rise (or) fall in temperature.
Thus, alcohol thermometers can measure more accurately than mercury. 
3) Alcohol can be brightly coloured and hence,  can easily be seen in the glass capillary tube. 

Standard points (or) fixed points:- The fixed temperatures marked on the stem of a  thermometer by the common consent of scientists. 
Lower standard point (or) lower fixed point : The melting point of pure ice at a pressure of 76 cm of mercury. 
Upper standard point (or) upper fixed point :The boiling point of pure water at a pressure of 76 cm of mercury. 

Thermometric Scales :
1) Celsius (or) Centigrade Thermometer:
* This scale was introduced by Celsius. 
* On this scale0⁰C represents the melting point of ice and 100⁰C is the steampoint.
* This scale is divided into 100 equal parts. 

2) Fahrenheit Scale :
* This Scale was introduced by Fahrenheit. 
c. Each division on this scale is called 1⁰F.     

3) Reaumur scale:
* This Scale was introduced by Reaumur. 
* On this Scale 0⁰R represents the melting point of ice and 80⁰R the steam point. 
* The length in between the standard points is divided to 80 equal parts.Each division on this scale is called 1⁰R. 

4) Kelvin scale: 
Temperature on kelvin scale= Temperature in  ⁰C+273.
Relation between different temperature scales: \( \frac{{C - 0}} {{100}} = \frac{{F - 32}} {{180}} = \frac{{R - 0}} {{80}} = \frac{{K - 273}} {{100}} \)   

Note : The relation between centigrade scale and Fahrenheit scale can also be written as  .(   \( F = \frac{9} {5}C + 32 \)  ,  \( \because \frac{C} {{100}} = \frac{{F - 32}} {{180}} \Rightarrow \frac{C} {5} = \frac{{F - 32}} {9} \Rightarrow \frac{{9C}} {5} = F - 32 \Rightarrow F = \frac{{9C}} {5} + 32 \))