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Grouping of the cell's





Grouping of the cell's

  • A limited amount of current can be drawn from a single cell or battery
  • There are situations where single cell fails to meet the current requirement in a circuits
  • To overcome the problem cells can be grouped in series and in parallel combinations or mixed grouping of cells is done in order to obtain a large value
    of electric current

(A) Series combination

  • Figure below shows the two cells of emfs E1 and E2 and internal resistance r1 and r2 respectively connected in series combination through external resistance

    Series combination of cells

  • Points A and B in the circuit acts as two terminals of the combination
  • Applying Kirchhoff's loop rule to above closed circuit
    -Ir2-Ir1-IR+E1+E2=0
    or
    I=E1+E2/R+(r1+r2)
    Where I is the current flowing through the external resistance R
  • Let total internal resistance of the combination by r=r1+r2 and also let E=E1+E2 is the total EMF of the two cells
  • Thus this combination of two cells acts as a cell of emf E=E1+E2 having total internal resistance r=r1+r2 as shown above in the figure

    Equivalent E of the series combination of cells

(B) Parallel combinations of cells

  • Figure below shows the two cells of emf E1 and E2 and internal resistance r1 and r2 respectively connected in parallel combination through external resistance
    Parallel combinations of cells

  • Applying Kirchhoff's loop rule in loop containing E1 ,r1 and R,we find
    E1-IR-I1r1=0 ------------------------(1)
    Similarly applying Kirchhoff's loop rule in loop containing E2 ,r2 and R,we find
    E2-IR-(I-I1)r2=0 ------------------------(2)
  • Now we have to solve equation 1 and 2 for the value of I,So multiplying 1 by r2 and 2 by r1 and then adding these equations results in following equation
    IR(r1+r2)+r2r1I-E1r2-E2r1=0
    which gives

    Grouping of the cell's|Wheat stone bridge
    We can rewrite this as
    Grouping of the cell's|Wheat stone bridge
    E is the resulting EMF due to parallel combination of cells and r is resulting internal resistance.


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