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

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

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

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