Question

How can i apply the Kirchoffs law?

Answer 1

(i) Starting from the positive terminal of the battery having highest emf, distribute current at various

junctions in the circuit in accordance with ‘junction rule’. It is not always easy to correctly guess the

direction of current, no problem if one assumes the wrong direction.

(ii) After assuming current in each branch, we pick a point and begin to walk (mentally) around a

closed loop. As we traverse each resistor, capacitor, inductor or battery we must write down, the voltage

change for that element according to the above sign convention.

(iii) By applying KVL we get one equation but in order to solve the circuit we require as many equations

as there are unknowns. So we select the required number of loops and apply Kirchhoff’s voltage law across

each such loop.

(iv) After solving the set of simultaneous equations, we obtain the numerical values of the assumed

currents. If any of these values come out to be negative, it indicates that particular current is in the opposite

direction from the assumed one.

Answer 2

Kirchhoff's junction law says that the sum of currents entering a junction must equal the sum of currents leaving the junction. Current is never used up in a circuit, so it makes sense that all the current that goes into the junction must come back out, too.

Kirchhoff's loop law says that the sum of the changes in voltage around and closed loop in a circuit must always be zero. This means that if you add up the voltage across each circuit element all the way around the loop, you would get zero.


The first step is to determine how many branches there are in the circuit and define a current variable for each branch. You need to find only one current in each branch, because every point in a branch will have exactly the same amount of current; but different branches may have different currents.





In this circuit, you can see that we have three branches, and we have labeled the three currents and defined their direction. Now, how did we know the direction of the current in each branch? We didn't! Most of the time in a complex circuit like this, you will not be able to tell which direction current will flow in a branch of the circuit just by looking at it.

That's okay! You just need to pick a direction and make sure that all your calculations are consistent with it. If you picked the wrong direction, when you calculate the current, it will be negative. This will let you know that the current is actually going in the opposite direction from the way you predicted.

You can now use the junction law to write the first equation that we will use to analyze this circuit.

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