Voltaic Cell Basic Construction and Working of Voltaic Cell, Voltaic Cell Working, Polarization of Voltaic Cell, Local Action of Voltaic Cell

1. Presented by
Dr. R. RAJA, M.E., Ph.D.,
Assistant Professor, Department of EEE,
Muthayammal Engineering College, (Autonomous)
Namakkal (Dt), Rasipuram – 637408
MUTHAYAMMAL ENGINEERING COLLEGE
(An Autonomous Institution)
(Approved by AICTE, New Delhi, Accredited by NAAC, NBA & Affiliated to Anna University),
Rasipuram - 637 408, Namakkal Dist., Tamil Nadu, India.
Voltaic Cell Basic Construction and Working of Voltaic Cell

2. Voltaic Cell Basic Construction and Working of Voltaic Cell
 A simple voltaic cell is made by immersing one zinc plate and one copper plate
inside a water diluted sulfuric acid solution.
 As shown in the figure, if the copper plate and zinc plate are connected
externally with an electrical load, an electric current starts flowing from copper
plate to zinc plate through the load.
 That means there is some electrical potential difference developed between the
copper plate and the zinc plate.
 As the current flows from copper to zinc, it is obvious that the copper plate
becomes positively charged and the zinc plate becomes negatively charged.
10/23/2020 2

3. Voltaic Cell Working
10/23/2020 3
 The working principle of voltaic cell depends upon the principle that, whenever
two dissimilar metals are immersed inside an electrolyte solution, the more reactive
metal will have a tendency to dissolve in the electrolyte as positive metal ions,
leaving electrons behind on the metal plate.
 This phenomenon makes the more reactive metal plate negatively charged.

4.  Less reactive metal will attract positive ions present in the electrolyte, and
hence these positive ions get deposited on the plate making the plate positively
charged.
 Here in this case of simple voltaic cell, the zinc comes out in the sulfuric acid
solution as positive ion and then reacts with negative SO4
− − ion of the solution
and forms zinc sulfate (ZnSO4).
 As the copper is less reactive metal, the positive hydrogen ions of the sulfuric
acid solution have a tendency to get deposited on the copper plate. More zinc
ions coming out in the solution means more number of electrons leave in the
zinc plate.
 These electrons then pass through the external conductor connected between
zinc and copper plates.
10/23/2020 4
Contd..

5.  On reaching on the copper plate, these electrons then combine with the
hydrogen atoms deposited on the plate and form neutral hydrogen atoms.
 These atoms then combine in pairs to form molecules of hydrogen gas and the
gas lastly comes up along the copper plate in form of hydrogen bubbles.
 The chemical action taking place inside the voltaic cell is as follows,
However, this action stops when the contact potential between Zn and dilute
sulfuric acid reaches the value of 0.62 Volt.
 During operation of a voltaic cell, the zinc late is at a lower potential with
respect of the solution film adjacent to it as shown in the figure below.
10/23/2020 5
Contd..

6. 10/23/2020 6
 Similarly, when Cu plate is placed in contact with the electrolyte, then the positive
hydrogen ions in the solution have a tendency to get deposited on it until its potential
rises nearly to 0.46 V above that solution. Hence, the electrical potential difference
developed in a voltaic cell is 0.62 − (− 0.46) = 1.08 Volts.
 In a simple voltaic cell there are mainly two drawbacks, referred as polarization and
local action.
Contd..

7. Polarization of Voltaic Cell
 It is observed that in this cell, the current gradually gets reduced and after a
certain time of its operation, the current may cease altogether.
 This decrease in current is due to the deposition of hydrogen on the copper
plate. Although the hydrogen comes out from the cell in form of bubbles, still
there is a formation of a thin layer of hydrogen on the plate surface.
 This layer acts as an electrical insulation, thereby increase the internal
resistance of the cell. Because of this insulating layer, further hydrogen ions
cannot get electrons from the copper plate and get deposited in ion form.
 This layer of positive hydrogen ions on the copper plate exerts a repulsive force
on other hydrogen ions which are approaching the copper plate. Hence the
current gets reduced. This phenomenon is known as polarization.
10/23/2020 7

8. Local Action of Voltaic Cell
 It is found that even when the voltaic cell is not supplying any current, zinc
continuously gets dissolved in the electrolyte.
 This is due to the fact that some traces of impurities like iron and lead in
commercial zinc form tiny local cells which are short-circuited by the main
body of zinc.
 The action of these parasitic cells cannot be controlled so that there is some
wastage of zinc. This phenomenon is known as local action.
10/23/2020 8

9. Thank You
10/23/2020 9