Heat & Thermodynamics

Thermodynamics

  • (F – 32 / 180 = C/100)  At -40*C both scales show identical readings
  • No upper limit of temp. is defined but have a definite lower limit i.e. absolute 0 k or -273*C
0*C = 237 K 100*C = 373 k Rise of temperature is equal in both the scales

In cold countries we use alcohol in thermometer instead of mercury, because freezing point of mercury is -39*C & of alcohol is -115*C


Electronic Thermometer

  • Basic component thermistor, whose resistance changes with temperature
  • Measures resistance & covert it to temp. as shown digitally

Fahrenheit_Celsius_Digital_Thermometer

 


Water – Min Volume & Maximum Density at 4*C

  • When temp. falls, top layer of water contracts & become denser & sinks to bottom
  • This process goes on until whole water of the pond reaches 4*C i.e. at max density
  • If temperature falls any further, top layer expands & remain at the top till it freezes and water under it remains at 4*C, leading to survival of aquatic life

 


Heat Transfer Processes

Conduction

  • Occurs in solids, when one end is heated then the other end also heats up
  • Mainly due to transfer of heat though solids particles

 

Convection

  • Occurs in liquids & gases, via circulation of particles to form convectional currents
  • For ex. heating elements in geyser are fitted near bottom & cooling units in freeze are fitted near top

Conduction-convection-radiation

Radiation

  • No medium is required (heating by electromagnetic waves)
  • Good absorbers are good emitters too, for ex : Dark colors & rough surfaces
  • Highly polished surfaces are poor radiators

 


Newton’s law of cooling

  • Rate at which hot body loses heat is directly proportional to surrounding temperature.
  • For ex. a body will come from 90*C to 80*C in less time then 40*C to 30*C
  • Rate of cooling of hot water > rate of cooling of cold water in refrigerator

Cloudy nights are warmer than clear nights as clouds act as a blanket & reflect radiations emitted by earth back to the earth


Specific Heat Capacity

  • Heat required to raise the temperature of a unit mass of substance by 1 k (unit joule/kg*kelvin)
  • Specific heat of water > specific heat of land (1 cal = 4.2 j)

Water is used as a coolant in car engines mixed with ethylene glycol which acts as a lubricant, as a coolant & lowers the freezing point of water


Air conditioner

  • Maintains temperature & humidity of a place
  • 1 ton means transfer of 12000 BTU of heat from room in an hour

 


Boiling point of water vs atmospheric pressure

  • Boiling point of water depends on external atmospheric pressure.
  • When atmospheric pressure is at 76 cm of mercury, water boils at 100*C, but when external atm. pressure is increased, boiling point of water is also increased
  • Increased boiling point allow water to hold more heat, which cooks the food faster
  • At higher altitudes, atmospheric pressure is reduced, lowering the boiling point of water & food takes much longer to cook
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1 Comment

  1. hello sir , plz provide paper 2 IR portion notes.

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