Heat

Concept of Heat

• Heat may be defined as energy in transit.
  
• Word heat is used only if there is a transfer of energy from one thermodynamic system to the another.
  
• When two systems at different temperatures are kept in contect with each other then after some time temperatures of both the syatems become equal and this phenomenon can be described by saying that energy has flown from one system to another.
  
• This flow of energy from one system to another on account of temperature difference is called heat transfer.
  
• Flow of heat is a non-mechanical mode of energy transfer.
  
• Heat flow depends not only on initial and find states but also on path it's.

Calorimetery

• Calorimetery means measurement of Heat.
  
• Calorimeter is the device used to measure heat and it is cylinderical vessel made of copper and provided by a stirrer and a lid.
  
• This vessel is kept in a wooden block to isolate it thermally from suroundings.A theromometer is used to measure the temperature of the content in the calorimeter.
  
• When bodies at diffrent temperature are mixed in a calorimeter,they exchange heat with each other.
  
• Bodies at higher temperature loose heat while bodies at low temperature gain heat.Contents of the calorimeter is continously stirred to keep temperature of contents uniform
  
• Thus principle of calorimetery states that the total heat given by hot objects is equal to the total heat received by cold objects.

Change of phases

• There are three phases of matter i.e, Solid, Liquid and Gas.
  
• Substance for example, H₂O exists in solid phase as Ice,in liquid phase as Water and in gas state as Steam.
  
• Transtion from one phase to another phase are accompained by absorption or liberation of heat and usually by change in volume even at constant T.
  
• Change of phase from solid to liquid is called melting , from liquid to solid is called fusion and from liquid to gas is called vaporisation
  
• Once the temperature for phase change is reached( e,g melting or boiling temperature) no further temperature change occurs until all the substance has undergone phase change.
  
  Melting Point:-Temperature at which solid and liquid phase are in thermal equilibrium with each other
  Boiling point:-Temperature at which liquid and vapour phase are in thermal equilibrium with each other
  
  
• Change of phase from solid state to Vapour state without passing through liquid state is called sublimation for e.g Dry Ice and Iodine sublimes.
•  
• Note **Heat can do two things when Heat energy given to the body It can :-
•  Either Rise the Temperature of body or Change the Phase of body Example above Graph.

Latent Heat:

• The amount of heat per unit mass that must be transfered as heat when a sample completely undergoes phase change is called latent Heat of the substance for the process.
  
• Thus when a sample of mass m completely undergoes phase change,the total energy transfered is
  Q=Lm
  where,
  L- Latent heat and is characterstics of a substance
  
• Its unit is JKg^-1.
  
• Latent Heat for a solid liquid change is called Latent Heat of Fusion L_f.
  
• Latent Heat for a liquid gas change is called Latent Heat of Vaporisation L_v.
• Latent Heat of Fusion is 80cal/g
• Latent Heat of Vaporization is 540cal/g
• 1 cal = 4.2 Joules 

Specific Heat Capacity

• If a system undergoes a change of temperature from T to T+ΔT during the transfer of ΔQ amount of heat then heat capacity c of the system is defined as the ratio of
  c=ΔQΔT
  
  
• Thus Heat capacity per unit mass of a substance is its specific heat capacity.
  c=ΔQmΔT
  
  where,
  m - mass of the substance
  ΔQ - Heat absorbed or rejected by the substance
  ΔT - Change in Temperature
  
  
• Specific heat capacity depends on the nature of substance.
  
• It is constant characterstics of the substance and is independent of the ammount of substance.
  
• It also depends on the temperature of the substance
  
• Its unit is J Kg^-1 K^-1
  
• If the amount of substance is specified in terms of no of moles n instead of mass m then the heat capacity per mole of the substance is
  =ΔQnΔT
  
  and is known as Molar Specific Heat capacity.
  
• It is constant characterstics of the substance and independent of the ammount of substance
  
• It depends on the nature of the substance ,temperature and amount of heat supplied
  
• Its unit is J mol^-1 K^-1
  
• In case of gases , when a gas is heated, ordinarly there is change in volume as well as pressure in addition to change in temperature
  
• For simiplicity either volume or pressure can be kept constant.Thus gas have two specific heat capacities
  1) Specific heat capacity at constant volume C_V
  2) Specific heat capacity at constant pressure C_P
•  _{Specific Heat of Water is 1 cal/g}
• _{Specific Heat of Ice is 0.5 cal/g}