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KIRCHHOFF'S LAWS IN AC CIRCUITS | COUPLED INDUCTORS |

In the previous chapter,
we've seen that the use of Kirchhoff's laws for AC circuit analysis not only
results in many equations (as too with DC circuits), but also (due to the use of
complex numbers) doubles the number of unknowns. To reduce the number of
equations and unknowns there are two other methods we can use: the *node
potential* and the *mesh (loop) current*
methods*.* The only difference from DC
circuits is that in the AC case, we have to work with *complex impedances (or admittances)* for the passive elements and *complex
peak or effective (rms)* values* *for
the voltages and currents.

In this chapter we will demonstrate these methods by two examples.

Let's first demonstrate the use of the node potentials method.

**Example 1
**

Find the amplitude and phase
angle of the current i(t) if R = 5 ohm; L = 2 mH; C_{1} = 10 mF; C_{2} = 20 mF; f = 1
kHz; v_{S}(t) = 10 cos wt V and i_{S}(t) =
cos wt A

Here we have only one
independent node, N_{1} with an unknown potential: j = v_{R} = v_{L} = v_{C2} = v_{IS
}. The best method is the node
potential method.

**The node equation:
**

**
**

Express j_{M} from the
equation:

Now we can calculate I_{M } (the
complex amplitude of the current i(t)):

A

The time function of the current:

i(t)
= 0.3038 cos (wt + 86.3^{Â°}) A

Using TINA

{Solution by TINA's Interpreter}
om:=2000*pi; V:=10; Is:=1; Sys fi (fi-V)*j*om*C1+fi*j*om*C2+fi/j/om/L+fi/R1-Is=0 end; I:=(V-fi)*j*om*C1; abs(I)=[303.7892m] radtodeg(arc(I))=[86.1709] |

**Now
an example of the mesh current method**

Find the current of the
voltage generator V = 10 V, f = 1
kHz, R = 4 kohm, R_{2} = 2 kohm, C = 250 nF, L = 0.5 H, I =
10 mA, v_{S}(t) = V cosw t, i_{S}(t)
= I sinw t

Although we could again use the method of node potential with only one unknown, we will demonstrate the solution with the mesh current method.

Let's first calculate the
equivalent impedances of R_{2},L (Z_{1}) and R,C (Z_{2})
to simplify the work:
and

We have two independent meshes (loops).The first is: v

The two mesh equations are:
V_{S} = J_{1}*(Z_{1} + Z_{2}) + J_{2}*Z_{2}
J_{2} = I_{s
}

You must use complex values for all the impedances, voltages and currents.

The two sources are: V_{S}
= 10 V; I_{S} =
-j*0.01 A.

We calculate the voltage in volts and the impedance in kohm so we get the current in mA.

Hence:

j_{1}(t)
= 10.5 cos (wÃ—t -7.1Â°) mA

Solution by TINA:

{Solution by TINA's
Interpreter}
Vs:=10; Is:=-j*0.01; om:=2000*pi; Z1:=R2*j*om*L/(R2+j*om*L); Z2:=R/(1+j*om*R*C); Sys I Vs=I*(Z1+Z2)+Is*Z2 end; I=[10.406m-1.3003m*j] abs(I)=[10.487m] radtodeg(arc(I))=[-7.1224] |

Finally, let's check the results using TINA.

KIRCHHOFF'S LAWS IN AC CIRCUITS | COUPLED INDUCTORS |