Hello students, welcome to this beautiful chapter called as current electricity. So, the agenda for the today’s module is Kirchhoff’s Voltage Law. This is very important law as this is going to be the future of this chapter. So, let’s start with it.

So, what exactly Kirchhoff’s Voltage law is? It states that the algebra summation of total potential difference across a loop is equal to zero. My dear friends, to understand this law, we will write the potential difference across our two basic elements, that is battery and resistance. So, let’s start with it.

So, I have a battery of potential difference V. The battery has a high potential represented as positive terminal and it has low potential represented by its negative terminal. So, this is high potential and this is a low potential terminal of a battery. Students, what will happen if I will move a battery from the positive terminal towards the negative terminal. As I will move from the positive towards negative, I will have some loss and that loss is minus V. V is the magnitude of potential drop and the minus sign is due to the loss, as I am moving from high potential towards the low potential. Now, students, what will happen if I will move from the negative terminal of the battery towards the positive terminal of the battery. Obviously, the potential drop will be plus V because from low to high I will gain something.

Let’s understand the potential drop across a resistor. So, I have a resistor R as shown in which the current I is flowing and we know that the current flow flows from high potential towards the low potential. Students, what will happen if I will move in the direction of current? If I will move in the direction of current, I will have a loss because current is flowing from high potential to low potential. The potential drop across the resistor is minus IR. And, students, what will happen if I move against the direction of the current in a resistor. Yes, I am moving from low to high, I will have gain and that gain is plus IR. So, the theory is very simple, when I will move from a positive terminal of battery towards the negative terminal the potential drop is minus V. If I move from a negative terminal towards a positive terminal the potential gain is plus V. If I move in a direction of the current in a resistor the potential drop is minus IR and if I move against the direction of current in a resistor the potential drop is plus IR.

Now, let’s understand the Kirchhoff’s Voltage law with the help of this loop as shown and we will have the different magnitude of the current which are flowing inside this loop. To understand the Kirchhoff’s law, we will have a passage, we will have a path and that path is represented as ABCD in clockwise direction. Students, now I am going to traverse inside this loop and I will write the complete total potential difference which I will encounter in this loop. So, let’s start first with R1, I am moving in the direction of current I1, so, I will have a loss minus I1, R1. Next we have a battery, I am moving from the negative terminal of the battery towards its positive terminal. I will have a gain and that gain is plus V1. Now, the turn of R2, I am moving in the direction of the current I1 through R2, I will have a loss, minus I1 into R2. Now, R3, I am moving downwards B to C and the current I2 is moving upwards. So, I am moving downwards and the current is coming upwards. I am moving against the direction of the current. I will have a gain that is I2 into R3. Now, my dear friends, I am moving from C to D and I am moving from the negative terminal of the battery towards the positive terminal. So, I will have a gain and that gain is V2. Now, my dear friends, again I am passing through resistance R4 and during that passage, I am moving in the direction of the current I3. I3 is passing through resistance R4. So, again I am moving in the direction of current I will have a loss and that loss is minus I3 into R4. Again now I am moving from D to A, if I am moving from D to A, I am moving from positive terminal of the battery towards the negative terminal, I will have a loss minus V3. Now, students, I am moving from D to A in upper direction and through resistor R5 current I4 is coming in downward direction. So, again I am moving opposite to the direction of current. I will have a gain and that gain is I4 into R5. And, students, this is the total potential drop inside the loop and Kirchhoff says that this total potential drop is equal to zero.

That’s all for today’s module, have a good day, thank you.