Understanding Ohm’s law

Do you really know it quite well…???

Every year, when I start teaching Ohm’s law, to the students of my fresh batch of XI standard, I get almost the same kind of responses! Not verbally, actually! But the faces of students, which I can read, tell me the same thing!

But I am always quite sure, that these students only know the verge of this beautiful law and when it comes to the applications of Ohm’s law, the students turn awful.

So not going into the details of this law, I shall concentrate on its applications. And while talking about applications I shall step by step try to explain the basic concepts regarding it.

Basic concept

As the definition goes, “so long as the physical state of the conductor remains the same, the amount of current flowing through a conductor is directly proportional to the potential difference applied across the two ends of the conductor”.

Image Ohm's law

The current while flowing through a conductor experiences ohmic opposition, but the PD (Volt) compels it to flow through…!

Now about the proportionality constant (R). We know R means opposition. It is the opposition experienced by the free electrons while flowing through the conductor. When these electrons are in motion, their trajectory path changes randomly and thus it results in the electrical parameter known as resistance.

This R itself changes from metal to metal. This is because the value of (ρ – rho) i.e. the coefficient of resistivity of particular material also decides the value of resistance. It also depends only many other factors like the Length (l) of the conductor, Area of cross section of the conductor, through which the electric current is flowing. Hence the equation for R is very meaningful.

R = ρ.(l/A) …. expressing opposition, which the free electrons experience, while flowing

Well, so far so good! Looking at the cartoon (above) gives us quite a good idea about the electrons having such bad experience! Now we shall go into the details of applications of this law.

Understanding basic application of Ohm’s Law

To start with we consider a very simple circuit, as given below.

image potential divider

A simple circuit of potential divider

Here the battery voltage (V) is divided by the two resistors R1 and R2, hence the circuit is also known as potential divider circuit.

Now if we apply Ohm’s law to this circuit, to calculate the current (I), then it would be simply given as –

I = V/ (R1+R2)

Now what about the current flowing through these two resistors? If you consider, the conventional current flowing from (+) terminal of battery towards the (-) terminal, then will this current get reduced within each of the resistors while flowing?

What students imagine….?!

As the current is flowing through the two resistors, first it will go through R1 and IT WILL REDUCE. Then the reduced current will flow through R2 and WILL REDUCE FURTHER…!!!

But of course, this is not true! How it could be? Well, I will explain it as follows…

Suppose for the time being, we say that the above imagination is correct. Ok.

Now in this case, suppose we consider electron current, while flowing through R2 and then R1 (electrons will start from -ve terminal of battery). Suppose 10000 electrons start their journey from (-) flowing through R2 and then R1 and then to (+).

Now the problem is, according to the above imagination, if current were to reduce while flowing through R2 & R1, then say, out of these 10000 electrons, some of them will be “consumed” by R1 and R2.

This creates another problem! If a resistor were to “consume” free electrons, then its conductivity should increase…! This will definitely reduce the resistance of R1 and R2 after some time, as the resistors will “eat up” the flowing electrons…!

So this all comes to an absurd situation! Hence this imagination is false…

Then what happens in reality?

 In reality, the value of current which starts flowing from (+) terminal of battery or say the number of electrons starting their journey from (-) terminal, is already DECIDED by the amount of RESISTANCE (R) present within the path, on which this current will flow. Hence, according to Ohm’s Law, I said initially that I = V/ (R1+R2).

It means that what value of current should flow through the circuit is already decided, itself by the amount of resistance R1+R2 present on that path…!

This is such typical misconception, which I have witnessed, not just for students but for some teachers also. Hence I thought to write this post. Please share this article with your friends to propagate the basic understanding to avoid such misconceptions in future.

You can read more interesting details which working on applications of this law, on this link. Its really a good article to read on.

2 comments… add one
  1. Sameer

    Nice explanation
    Thanks

  2. Anjelica

    What a simili…!
    Thanks sir.
    You are great…!

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