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Question
CBSEENPH12037922

State Kirchhoff’s rules of current distribution in an electrical network. Using these rules determine the value of the current I1 in the electric circuit given below.


Solution

For electrical network, Kirchhoff’s rules are as follows:

(i) Junction rule: At any junction, the sum of the currents entering the junction is equal to the sum of currents leaving the junction
                   ∴ Σ I = 0

(ii) Loop rule: The algebraic sum of changes in potential around any closed loop involving resistors and cells in the loop is zero.
           ∴ Σ IR + Σ E = 0

According to Kirchhoff’s rule, I1 + I2 = I3

In the given circuit, 
Applying loop rule to both the lower and upper loops, we get 

40 I3 + 20 I1 = 40          (In loop ABCF)
40 I3 + 20 I2 = 80 + 40  (In loop CDEF)

Adding these two equations, we get
               80 I3 + 20(I1+I2) = 160

                     80 I3 + 20 I3 = 160 
           
                                    I3 = 160100    = 1.6 A 

Putting the value of I3 in the above equation to find the value of I1, we get

          40 × 1.6 + 20 I1 = 40

            20 I1 = 40-64 = -24

                  I1 = -2420 = -1.2 A. 

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