Page 21 - Basics of Electrical, Electronic and Communication

2.4. Source transformation 31

Example 6
Find V 3 and its polarity if the current I in the circuit shown in figure
2.14 is 0.4 A.

5 Ω V2
+ −
10 V

+
V1 − 50 V 20 Ω

I
a b

V3

Figure 2.14:

Solution
Assume V 3 has the polarity with terminal ’a’ is positive
Applying KVL and starting from left corner of the circuit,

V 1 − 5I − V 2 − 20I + V 3 = 0
50 − 2 − 10 − 8 + V 3 = 0
V 3 = −30V

Negative value of V 3 implies that the terminal ’b’ is positive with
respect to terminal ’a’ (opposite to the assumed polarity).

Example 7
Two coils connected in series have a total resistance of 9 Ω and when
connected in parallel have a resistance of 2 Ω. Find the resistance of
each coil.

Solution
Let the resistance of two coils be R 1 and R 2 .

31