1. Concept of Heat and Temperature
- Familiar sensations of hotness and coldness are described with adjectives such as hot , warm, cold, cool etc.
- When we touch an object, we use our temperature sence to ascribe to object a property called temperature.
- Temperature of a body determines whether it feels hot or cold to the touch. The hotter you feel on touching the body higher is its temperature.
- So we can say thet Temperature is relative measurement of hotness or coldness of a body.
- An observation about hot and cold bodies in contact is that, when they both are in contact temperature of cold body increases and that of hot body decreases. This happens because energy is transferred from hot body to cold body when they are in contact and this is a nonmechanical process.
- This energy which is transferred from one body to another without any mechanical work involved is known as HEAT.
- Heat is a form of energy and heat transfer from one body to another takes place by virtue of temperature difference only also heat transfer takes place from body at higher temperature to body at lower temperature.
- S.I. unit of heat is Joule(J) and that of temperature is Kelvin(K).
2. Measurement of temperature
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- Measurement of temperature can be obtained using a thermometer.
- Construction of thermometers generally require a measurable property of a substance which monotonically changes with temperature.
- Examples of some common type of thermometers
(1) Mercury in a glass thermometer.The height of mercury in the tube is taken as thermometric parameter.
(2) Constant Volume gas thermometer-Gas in bulb is maintaned at constant volume.The mean pressure of gas is taken as thermomtric parameter.
(3) Constant Pressure gas thermometer-Gas in bulb is maintaned at constant pressure.Volume of gas is taken as thermomtric parameter.
(4) Resistance therometer-Electric resistance of a metal wire increases monitonically with the temperature and may be used to define temperature scale. Such thermometers are resistance theromometers.
- Thermometers are calibrated to asign a numerical value to any given temperature.
- Defination of any standard scale needs two fixed refrence points and these points can be corelated to physical phenomenon reproducible at the same temperature.
- Two such standard points are freezing and boiling points of water at same pressure.
- Two such familiar scales used for measurement of temperature are Celsius and Fahrenhite scale.
- Temperature in celsius is measured in degree.
- Fahrenhite scale has a smaller degree then celsius scale and a diffrent zero of temperature.
- Relation between Celsius and fahrenhite scale is
TF=9/5 TC + 32°.
TF - Fahrenhite Temperature.
TC - Celsius Temperature.
Letters C & F are used to distinguish measurements on two scale thus
0° C= 32° F
this means that 0° C on celsius scale measures the same temperature as 32° F on the fahrenhite scale.
- On fahrenhite scale melting point of ice and boiling point of water have values 32° F and 212° F and that on celsius scale are 0° C and 100° C.
- If we now talk of Kelvin scale ,the melting point of ice and boiling point of water in the scale are 273.15 K and 373.15 K respectively.
- Size of a degree in celsius and kelvin scale are same.
- Relation between Celsius and kelvin scale is
TC = TK - 273.15 K
TK - Temperature in Kelvin
TC - Temperature in celsius
- Another modern fixed point of temperature is the triple point of water.
- Three phases of water i.e, ice,water,water vapour co-exist at this value of temperature and pressure where
Ptr= 0.46 cm of HG.
Ttr & Ptr are triple point temperature and pressure.
- Now if Ptr is pressure of an ideal gas thermometer at triple point temperature Ttr and if P is pressure at some other temperature T then corresponding temperature is
T= P( Ttr / Ptr)
provided Ttr & Ptr are low.
- Electric resitance of metal wire increase monitonically with temperature and may be used to define the temperature scale
- If R0 & R100 are resistance of metal wire at ice and steam point respectively then temperature t can be defined corresponding to resistance RT as follows
- A platinum wire is oftently used to construct a thermometer which is known as platinum resistance thermometer.
- Gas thermometer can also define Celsius scale
- If P0 is pressure of gas at ice point and P1000 is pressure of gas at steam point then temperature T corresponding to a pressure P of gas is defined by