PHYSICS: HEAT

PHYSICS: HEAT

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Come On Let us dive into the ocean of General Science,

Overview

• Heat is the process of energy transfer from one system to another
• Units of heat: Joules (J), Calories, British Thermal Unit (BTU)
• Temperature is a measure of internal energy (enthalpy)
• Heat transfer can happen spontaneously only from a warmer to a colder body. Reverse heat transfer can only happen with the aid of an external source such as a heat pump.

Mechanisms of heat transfer

• Conduction is the most significant heat transfer mechanism in solids. It occurs as hot high energy molecules interact with neighbouring and transfer heat to them. Eg: heat transfer from one end of a metal rod to another
  
• Convection is most significant in liquids and gases. This happens when hot molecules move and transfer energy to other molecules. Eg: boiling of water. When water is heated on a stove, hot water from the bottom rises and displaces colder liquid which falls. 
  
• Radiation is the only form of heat transfer possible in the absence of a medium. Heat is transferred in the form of electromagnetic radiation. Eg: heat from the sun reaching the earth.
  

Heat transfer in everyday life

• Copper is used in construction of boilers and cooking utensils because it is a good conductor of heat
  
• Air is a poor conductor
  
• Wool and cotton are good insulators i.e. poor conductors. Their insulation arises mainly due to air spaces between molecules
  
• Double-walled glass doors with air between them are better insulators than windows with a single thick glass layers
  
• Eskimos live in snow huts because snow is a poor conductor of heat, and hence protects them from the extreme cold outside.
  
• Land and sea breeze, ocean currents are arise due to convection
  
• The boiling point of water at sea level and atmospheric pressure is 100C. When extra heat is added, it changes the phase of water from liquid to gas (water vapour). 
  

Thermometers

• Thermometers can be divided into two groups: 
  
  • Primary thermometers: measure temperature directly based on the property of matter. They are relatively complex and not used commonly. Eg: thermometers based on velocity of sound in gas, thermal noise of an electrical resistor etc.
    
  • Secondary thermometers: measure temperature relative to a pre-calibrated quantity. They are easy to use and used commonly.Alcohol thermometer, mercury thermometer, medical thermometer are all secondary thermometers
• In cold winter places, alcohol thermometers are used instead of mercury thermometers because the freezing point of alcohol is lower
• For extra-low temperature measurements (-200 C), Pentane is used
  
• Water is not suitable for use in thermometers because it freezes at 0 C and has irregular expansion
  
• Mercury is used for common medical thermometers because
  • It does not cling to glass and hence reading is easy
    
  • It is opaque and easily seen
    
  • Its expansion is uniform and hence calibration is easier
    
  • It is a better conductor of heat than alcohol and hence responds more rapidly to changes of temperature
    
  • It has low specific heat capacity and hence is more sensitive
    

Common appliances based on heat

• Solar cooker: is a box made of insulating material such as wood, cardboard etc. The box has a glass cover to retain heat inside by greenhouse effect. The inside of the box is painted black to increase heat absorption.
  
• Pressure cooker: Pressure cooker increases the boiling point of water by increasing pressure. When the boiling point of water increases, food cooks faster. Pressure cookers are especially essential in hill stations because at higher altitudes the boiling point of water decreases due to lower atmospheric pressure
  
• Refrigerator and Air-conditioner: are heat pumps that transfer heat from inside to the external environment. They use a refrigerantwhich is a compound that undergoes reversible phase change from gas to liquid. Common refrigerants include ammonia, sulphur dioxide, carbon dioxide and methane. The use of chlorofluorocarbons has been phased out due to concerns regarding depletion of the ozone layer.

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JOHAR