If you think about laws and theories related to Practical Electronics, the simplest ones, yet powerful and most commonly encountered ones are what we know as Ohm’s Law and Kirchoff’s Current Law.

Lets understand these and try to internalize them forever as they indeed enable us to walk the path in a very effective, correct and calculated manner.

OHM’s Law

Simply said, Ohm’s law talks all about resistance and if you want to recall all about resistance, you can take a detour here and go back to THIS LECTURE to quickly brush it up.

George Ohm

Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

Important point here is to note that, it is current which is dependent on Voltage and not the other way around.

Let us understand this law intuitively.

We have already learned that current is nothing but the flow of electrons and the intensity of flow depends upon how much the electrical pressure (Voltage) is between the two points of of a conductor, just like our old water in pipe analogy. More the pressure more will be the flow, but also the flow will be restrained by the resistance offered by the pipe which depends on size and length of the pipe and hence in the case of conductor, it is restricted by the resistance offered by the material, size and length of the conductor.

1. The more the resistance less the current hence the current is inversely related to resistance
2. The more the voltage the more will be the current and hence current is directly proportional to voltage.

Using this observations George Ohm in 1826 postulated the following law.

Current (I) = Voltage (V) / Resistance (R)

Practical Illustration:

Imagine a situation as below:

If we connect a battery terminal by a conductor with resistance R then the conductor because of the voltage across the battery terminals will develop a current equal to V/R.

Practically a conductor has a very less resistance that is why its called a conductor and wires are made out of it else it will resist the flow of current and we loose the power which we do not want. So what happens is since the conductor has a very less resistance a large current flows and its so much so that it causes what we know as shoret ciurcuit.

At this stage its important for us to learn that since reistance resitricts the flow of current it must consume the electric power and loose that power in heat as there is no other mechanical work happening.

This power loss is equal to Voltage²/Resistance, if we use the formula given by Ohm then same can also be re-written as Current² x Resistance, same can also be written as Voltage x Current.

This also tells us some quick facts and thumb rules, like if current increases by 2 times then since power depends on square of current, it increases by 4 times. This is a very important rule as it tells a lot to us and also tells us what happens in a short circuit. The current increases multiple times because of low resistance of conductor and hence power increases square times of that and it causes so much heat that the conductor itself melts.

Let us take a hang of some practical current and resistance values.

Imagine a common household light bulb. If its power consumption is rated at 100W and we know that the household suppy voltage is 220V then it means that the current will be 100/220 ~ 0.5 Ampere.

That means the resistance of the bulb will be 220/0.5 = 440 Ohms.

Now that we know how Ohm’s law works.

From our practical illustrations it is also understood that Resistance works like a current limiter and hence voltage limiter as well. This is how we make use of them. By installing resistors of fixed values we limit current or voltage intended for a particular use & application.

Now we are ready to understand the another important law called Kirchhoff’s Law.

Kirchhoff’s Law

This is a very simple and yet powerful law and also easy to understand if we pay attention to the fact that in a circuit with passive elements like resitors, capaciors and/or inductors, there is always conservation of charge, in the sense that there is no element generationg or consuming the charge hence the charge should remain same and hence, the law sates that,

At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node.

Just like number of vehicles entering a Toll Booth from many directions will always equal to the number of vehicles going out.