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We all have heard the word “Current” and “Electricity” but ever wondered how this current is created? In electrostatics, as the word suggests we were considering all the electric charges in a static state means in rest but when these electric charges are in motion they create an electric current.

When electric charges are in motion they create an electric current, this motion of electric charge also occurs in many natural phenomena for example lightning. In lightning, charges flow from the cloud to atmosphere resulting in electric current which is what we see as lightning.

In this article, we will be providing you current electricity from class 12th physics and at the end of the article, my personal handwritten in-depth notes are also given with all the derivations of this chapter’s current electricity.

What is electric current ?

We all know that electric current is created due to the motion of charges in one direction. So to understand electric current more clearly imagine a small area which is normal (90°) to the direction of flow of electric charge.

Electric field applied across a conductor

Now both the positive and negative charges may flow in forward or backward direction across the area. Now suppose for time interval ‘t’ the net amount of positive charge flowing in a forward direction is ‘q+‘ and the net amount of negative charge flowing in the backward direction is ‘q‘.

So the net electric charges ‘q’ flowing across an area in time interval ‘t’ would be:

Electric current is defined by the symbol ‘I’ and the value of electric current is:

So this is the current across the area in the forward direction.

P.s:- If this value is negative then ‘-‘ sign means that current is flowing in backward direction.


Ohm’s law:

Ohm’s law is the basic law regarding the flow of current in a conductor. So what is ohm’s law and most importantly what is the use of ohm’s law?
Ohm’s law states that “In a conductor, potential difference (V) between the end of the conductor is directly proportional to the amount of current flowing in them.” which simply means that more is the potential difference between the end of conductor more will be the amount of current flowing through that conductor. 

Limitations of Ohm’s law:

Ohm’s law is a useful law and even though it applies to a large number of materials there are still some limitations of ohm’s law. So let’s discuss the three limitations of ohm’s law:

Limitation of Ohm’s law
V ceases to be proportional to I mean some material don’t obey ohms law after a certain time.
The relation between V and I depends on the sign of V.
There can even be more than one value of V for the same amount of current I.

Drift velocity:

So we know that if an electric field is not present then electron present inside conductor moves in random direction due to thermal energy and collide with each other but once the electric field is applied these electrons start moving in the same direction creating a flow of current.

Drift velocity

So one thing, which we need to take care of here is while moving with thermal when an electric field is applied all the electrons move in the same direction resulting in the current formation. While moving these free electrons experience drift in the direction of the force.

So when an electric field is applied across a conductor, free electrons inside conductors experience a force in direction of higher potential and while free electrons keep moving with thermal velocity they drift in direction of the force. This phenomenon of drifting and velocity is drift velocity.

This drift velocity is responsible for “Current” formation because unlike thermal velocity it does not get cancelled out and all the free electrons move in the same direction. This drift velocity for a single electron is very small to the power of 10^-1 mm but still is able to create current and effective because the no. of electron carrying is very large.

Drift velocity derivation with its relation with electric current is been provided in our notes provided in this article so download them for derivation.

Resistance and resistivity:

So from the ohms law, we know that in a conductor electric potential (V) is directly proportional to the current (I). So in physics whenever this kind of situation arises where two terms are proportional to each other, simply introduce a constant to find the value of the two.

This constant has fixed vale for given temperature, pressure or any other general condition.

Same way, in this case, we will also introduce a constant between potential (V) and current (I).  This constant is called Resistance and is denoted by symbol ‘R’. So now this equation looks like this:

The S.I unit of resistance (R) is ohms denoted by symbol ‘Ω’.

So what is the conceptual meaning of resistance?

Well, Resistance is the opposing force in a conductor which stops current to flow. So the amount of opposition experienced by a current in a conductor is a resistance of a conductor.

The resistance of the conductor depends on the dimension of the conductor and therefore resistance (R) is directly proportional to length (l) and inversely proportional to the cross-sectional area (A) of a conductor. 

P.s: Directly proportional means value of resistance (R) will increase with the increase in length(l) and inversely proportional means the value of resistance will decrease with the increase in cross-sectional area (A). In the same way, if the length (l) will decreases resistance will also decrease but if cross-sectional (A) area is decreased resistance will increase. 

So the formula for resistance will be like this:

Handwritten notes:

This article is a brief summary of chapter 3 current electricity for all the derivation related to this chapter and other important things I have prepared my special handwritten notes. These notes have all the derivation of this chapter including different devices such as a potentiometer, galvanometer, and meter bridge and Wheatstone bridge which are important topics for boards.

So do read these notes if you are a class 12th student and preparing for your board exam. This are easy to understand notes. To download them CLICK HERE.

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