What is KCL (Kirchhoff’s Current Law)

The first kirchhoff’s law is Kirchhoff’s current law in short known as KCL and the second Kirchhoff’s law is Kirchhoff’s voltage law in short known as KVL and these two laws were given by Gustav Kirchhoff a German physicist and in this lesson will discuss KCL and in the next lesson will talk about KVL. 

Now according to KCL, “the algebraic sum of the currents entering any node is zero”. Let’s try to understand the meaning of this statement. whenever you calculate the algebraic sum of the currents which are entering a node then you will find the algebraic sum is equal to zero. So what do we mean by algebraic sum is the aggregation of two or more quantities taken with regard to their sign. So here we are calculating the algebraic sum of the currents. This means we will calculate the sum of currents with their signs and when you calculate the sum of currents with signs you will find it is equal to zero at any node. Now you have to follow one convention according to the convention: the current which is entering or we can say the entering current will have the positive sign (+) and the leaving current will have the negative sign (-). So this is the convention that will follow in case here and this convention is opposite in nodal analysis but for now, just remember this convention that the entering current will have the positive sign and the leaving current will have the negative sign and I will take one example. 

How to apply KCL

In this example, we are having this node and you can see that five currents are meeting at the node.

Current I1 is the entering current. Current I2 is the leaving current. Current I3 is the entering current I4 is also the entering current and I5 is the leaving current. So two currents I2  and I5 are the leaving currents and the remaining three currents I1, I3 and I5 are the entering currents. Now we will calculate the algebraic sum of the currents this means we will add all the currents

Entering Current  =  +

Leaving Current  =  –

We can see that, 

sum of entering currents = sum of leaving currents 
So remember this point that the sum of entering currents will be equal to the sum of leaving currents. This is one important point. Now let’s understand why we are getting the algebraic sum is equal to zero at node. We know node is not a circuit element and therefore it cannot store the charge and also destruction and generation of charge is not possible according to the law of conservation of charge. So this particular statement is based on the law of conservation of charge + the fact that node is not a circuit element. Because of these two points node will not be able to store the charge. It will not be able to generate the charge and also it will not be able to destroy. Now the current entering means the charges are entering and the number of charges entering to this node must be equal to the number of charges leaving the node.