Thermal & Chemical Effect of Current

Thermal & Chemical Effect of Current

• Joule's laws of heating effect :
  When some potential difference V is applied across a resistance R then the work done by the electric field on charge q to flow through the circuit in time t will be w = qr = vit = i² RT = Joule.
  This work appears as thermal energy in the resistor.
• Electric power :-
  The electric power of a device
  
  
  Resistances in series :-
• As the current i is same, the power (Heat) developed is proportional to the resistance greater the resistance, larger will be the power consumed.
• An electric bulb of low wattage will glow more in series because its resistance is more than a high wattage bulb.
  Resistances in Parallel :
• When resistances are connected in parallel, V is same , ,ie., power (Heat) is inversely proportional to the resistance ie, P₁R₁= P₂R₂or H₁R₁= H₂R₂.
• More power is consumed in smaller resistance of the Combination.
  Note : The price of electricity consumed is calculated on the basis of electrical energy and not on the basis of electrical power.
• Thermocouple
• The electrical conductivity in a metal is due to motion of electrons. Different metals have different free electron densities.
• When two dissimilar metals are joined at the junction a potential difference is developed due to diffusion of electrons from one metal to the others. The rate of diffusion depends upon temperature of the junction. When the two junctions of loop are at the same temperature, the two junctions will be at the same potential and no current passes through it.
• If the two junctions are kept at different temperatures, their potentials will be different and a current passes through it. The electricity developed is called thermo-electricity. The emf is called Thermo-emf. The current is called thermoelectric current. The phenomenon is called Seeback effect. The arrangement is called thermo-couple.
• Seebeck arranged the metals in an order that can form a thermocouple. This order is called thermoelectric series.
  
  Variation of emf with temperature :
• When the cold junction is at 0°C and hot junction is at t°C, the thermo emf developed is given by
  E = t + where and are thermoelectric constant having units are volt/°C and volt / °C² respectively (t = temperature of hot junction)
  For E to be maximum at t = t_n, i.e., +t_n = 0
  t_n =
• Neutral temperature is independent of the temperature of cold junction.
• Temperature difference between neutral temperature and cold junction is equal to the temperature difference between neutral and inversion temperature i.e., t_i - t_n = t_n - t_i
  
  t_n = .
  
  Thermo-electric power :- (Seebeck coefficient)
• The rate of change of thermo emf with temperature is known as thermoelectric power (P) or Seebeck Coefficient (S).
  P =
  .
  
  • When current is allowed to pass through a thermocouple, heat is evolved or absorbed at one junction and heated up, and heat is absorbed at the other junction and it is cooled. This phenomenon is called 'Peltier effect'. If the direction of flow of current is reversed, the hot and cold junction are also interchanged. Peltier effect is reversible.
    
    Peltier Coefficient :- ()
  • H q or H = q = it. Where is a constant known as Peltier coefficient.
  • Peltier Coefficient is defined as numerically equal to the amount of heat evolved or absorbed at a junction when 1 A of current is passed through it in 1s (or) 1 coulomb of charge passes through the junction.
  • S.I. unit : Joule/ Coulomb.
    Dimensional formula is M¹L²T^-3A^-1
    
    Thomson Coefficient : ()
    In Thomson's effect it is found that heat released or absorbed is proportional to Q
    H Q
    H = s Q
    where
    = Thomson's Coefficient. It's unit is Joule/Coulomb °C or Volt/°C.
  • It can be proved that Thomson's co-efficient of the material of conductor
    = = T ×
    = Thermoelectric constant = .