Electric charge and field with handwritten notes


So electric charge and field is not the term we used to hear in our daily life but if your are a class 12th science student just like me then you are familiar with these terms. So today I am going to share with you all the things about electric charge and fields which I have learnt.

Charge is basic entity of matter. Electric charge is  the basic identification of different entity like electrons and protons and other subatomic particles. Electrons are negatively charged whereas protons are positively charged particle. Electric charge is denoted by symbol 'q' or 'Q' and its a scalar quantity.

Now you know about electric charge is now let's understand about electric field.

Electric field is the area around electric charge where you can feel the effect of electric charge. So here further I will share with you about electric charge and electric field in more detail.

What is electric charge ?

So as I said earlier, electric charge defines the basic identification of different atomic entities such as electron proton and other sub-atomic particles as well.

 Electric charge:electron
Electric charge:electron

Well every thing in physics is denoted by some symbols same like electric charge is denoted by symbol 'q' and electric charge is a scalar quantity which means that electric charge has only magnitude (value) and doesn't have any directions. 

As you know: 

  1. Electrons are negatively charged particles.
  2. Protons are positively charged particles.
  3. Whereas neutrons are neutral as their name suggest they doesn't have any charge in them.


One phrase which I heard often is like things repel each other and unlike things attract.

Well this phrase is correct for electric charges as well. Like electric charge repel each other when brought closer whereas unlike charges attract each other. 

For example:

Positive - positive or negative - negative will repel each other and on the other hand positive - negative or negative - positive will attract. 

Basic properties of electric charge:



Well you know about electric charge but how to recognise one? So here I will share the basic properties of electric charge you need, to identify one.

3 basic properties of electric charges are:
  • Additivity of charge
  • Quantization of charges
  • Conservation of charges
So now lets discuss about each of these charges

Additivity of charges:

This property of charge says that charges can be added or subtracted by simple rules of algebra. 

For example: 2 positive charge + 1 negative charge will be equal to 1 positive charge. 

Quantization of charges:

Total electric charges present on a body is not present in the continuous quantity rather than it is present in the integral multiple of one electron.

Well I know it must be bit confusing for you to understand so let me explain it to you more briefly.

Suppose there are 5 electrons in a body carrying 2 negative charges in each of them. So what you will notice is you can extract either 2 negative charges from a body by removing one electron or either 4 negative charges by removing 2 electrons. 

But what would you do if you need to remove 3 electric charge? Well it's not possible because each electrons carries two negative charges and if you remove one then you will be left with 1 less electric charge you required and if you remove 2 electrons then you are left with 1 extra electric charge. 

But removing exact 3 electric charges from a body is not possible and this is why number of charges present is not continuous it's integral multiple of no of electrons.

Q=Ne (where n is the number of electrons)

Conservation of charges:


Well, the basic property is simple to understand. This property means that even if you remove an electron from a body or add an electron in a body, number of proton in universe is always going to be same to the number of electron present in universe and this is conservation of charges.


No. of  proton in universe = No. of electron in universe

There are some point you need to understand:

1. An isolated electric charge can neither be created or destroyed what you can only do is to transfer electric charge.

2. The charges in pair can be destroyed and by this law of conservation of charges is maintained because number of proton is same as number of electron in the universe.


Conductor and insulator:




So what I have learnt in my research is that all the elements are basically divided into two categories in electrostatics on the basis of whether electricity can flow or not through them. So let's find out about these elements:

1. Conductor: Conductors are basically metals which allows electricity to pass through them and we all know that.

2. Insulators: Well next on the list are insulators, insulators are basically the elements which don't allow electricity to pass through them. For example: woods and rubber. 

Well we all know about this since our childhood but interesting thing which I find is why conductors allow electricity to pass and insulators don't?

So here's where charges play role. Well electrons carry electric charge and we all know that and when in a body electron is allowed to move (even a little bit!) then that body allows electricity to pass whereas those body which don't allows this electron to move are called insulators and electricity can't pass through them.

Well, it's not that only conductors and insulators are present in this world. In fact there are other thing such as super conductor and semi conductor are also present. Super conductor passes electricity without any blockage and semi conductor passes electricity only in certain conditions.

Charging by induction:

So we all charge our smartphones everyday but do you know that which physics phenomena is actually working when you charge your phone? Lets find out!

Well for that we need to know what charging by induction means.


 Charging by induction
Charging by induction

Suppose a body has some negative charge and when that charged body comes into a physical contact with another one the charges on both the body are distributed evenly on both of them. So if there are two bodies:

Body A and body B, body A is negatively charged and body B is neutral (means number of electron is equal to number of proton).

When both bodies come in contact total charge of both the bodies is now equally distributed and due to this negative charge on A is less than before and we see that body B is now not neutral and has some negative charge as well.

Here charge flows from body A to body B

Well this is what charging by induction means. Body which was neutral sometime back has now a negative charge.

So how is this related to your phone charging?

Your phone battery is neutral and when you plug in your charger, charge transfers into phone battery and this way your phone get's charged.

What is Coulomb's Law ?




We learnt good amount of info about electric charge. So now let's dig little deep into this and let's introduce coulomb's law of electrostatics.

So let me get you a basic idea what coulomb's law is before digging more into it. Coulomb's law helps us to find the force between two charges. Well sounds pretty unbelievable because charges are so small that we can't even see them.

Then how can we find the force between the two?

Let's solve this mystery by learning each step coulomb took before coming to a conclusion.

Inverse square law:

Well this law is pretty simple. If we put this law in simpler format then inverse square law says that more is the distance between two object less effect they will experience due to each other. Sounds legitimate ?

Well this law also says that more is the distance less is the effect but by how much distance exactly this effect will be reduced?

Well the answer to this is present in the heading as well. Effect or intensity will be reduced by the square of the distance between two objects. 

Suppose distance between two objects is increased by 4 units, then effect will be reduced by 16 times and if distance is increased by 8 units then effect will be reduced by 64 times. 

So putting all these in one basket we can came to a conclusion that inverse square law stats that effect or intensity between two objects is decreased by the square of distance present between them.

But wait don't get off track!! How will this inverse square law exactly helps us in finding force between the two charge? Well  think it's pretty clear now that how coulomb found that.

Coulomb's law of electrostatics:

So coulomb actually implemented this inverse square law to find the force between two electric charge. Coulomb states that force between two charges is varied inversely as the square of the distance between the charges and was directly proportional to the product of the magnitude of the two charges.

Well I know, I went little too technical so let me put it into simpler words. So above statement only means that force between two charges depend upon two factor:

1. Product of both the charges
2. Distance between them.

From inverse square law we know that intensity will reduce to the square of the distance between them.

So on the basis of above point coulombs derived these formula:


Well to remove the proportionality sign we need to introduce a constant here also known as coulombs constant.

So before removing proportionality lets understand about this coulombs constant once.

Coulombs constant (C):

So first basic rule of physics is to remove proportionality sign in any formula and for that you need to introduce a constant in it. Therefore, Coulomb's constant is introduced in the above formula.

What are these constants actually? Well these constants are the fixed value for certain formula but in Coulombs constant something is different.

Coulomb's constant is different for different medium, so Coulomb's constant is not same for all the medium and if charges are present in any other medium you will find that the value is different.

But, why? So to find that we actually need to understand what is Coulomb's constant.

Coulomb's constant is denoted by 'C' and you can say that this C is actually the obstacle present between two charges. Space between two charges also provides some resistance between force between two charges.

This resistance or obstacle between force of two charges is Coulomb's constant and as power of this resistance will vary for different kind of medium hence Coulomb's constant also differs for different medium.

Coulomb's constant 'C' is equal to:


What does this formula denote? and how does above formula help us to denote the obstacle between the force?

So 𝞮 (epsilon) defines permittivity

Permittivity is a characteristic of a medium between two charges which affect the force between them.

So well from the definition of permittivity it's clear that how the above formula denotes the obstacle and resistance between two charges.

So now when you know all these so now Coulomb's law for force between the two charges is:


Where F is the force between two charges q1 and q2 and r is the distance between the two charges where 𝞮 (epsilon) is permittivity. 

What is Electric field ?




So we know what electric charge is and electrons and protons have negative and positive electric charges, respectively.

 Electric field due to point charge
Electric field due to point charge


Well every object has some effect in their surrounding and so does electric charges too. They have some effect in their surrounding but what are these effects and can we experience these effects in real life due to small electric charges? So let's learn about this. 

Well, electric charges do have some effect on their surrounding and the area where these effects can be experienced or sensed is called electric field

So basically electric field is the area around an electric charge where its effects can be experienced or sensed.

Formula of electric field is:




Here E denotes the electric field, Q is the source charge and r is the distance between source charge and where force is experienced.

But humans are weird to be honest, we need proof for everything and I am sure you also do! So is there any physical significance of electric field? Can we touch or smell electric field if it has any physical significance?

Let's take a look into this

Physical significance of electric field: 



Well, there is no physical significance of electric field. Electric field is just an area around an electric charge in space where when a test charge or anything is placed they will experience a force due to electric charge.

Electrical field does have a physical significance. I am not being a hypocrite here but there is one plot twist in it. Electric field does not have any physical significance but when we go beyond electrostatics and introduce the concept of time dependent electromagnetic phenomena then their electric field has physical phenomena.

So how exactly introducing a concept of electromagnetic proves electric field has physical significance?

Suppose there are two charges qand q2 and q1 is in motion then we all know that when source charge q1 is in motion force experienced on q2 will change. Speed of light is fastest and we all know that.

So when source charge is in motion, change in effect will take some time to show on q2 as there is nothing which travels faster than the speed of light but what we notice is there is no delay.

Well here electromagnetic phenomena comes into play!

The source charge in motion will create a field which will propagate at the speed of light and reach q2 and will show the force effect on it. 

Now this shows that field actually radiates and transfers energy from one place to another and therefore they have physical significance and this is how, electric field gets physical significance once electromagnetism is introduced but when we are in boundaries of electrostatics electric field does not have any physical significance.

Dielectric strength:



Well we have learnt about insulators before and one thing we learnt was insulators do not pass electricity because there is no space for electron to move.

What if we provide an electron so much power that it even if there is no space for it, it starts moving even a little bit? Sounds possible right?

Well it is possible to do that and if we provide electron with enough energy it will even start moving and then it will behave like conductor.

Here you will see that we need to provide energy to electron to move and it's clear that insulating element was trying to stop this electron to move and this opposing force by insulator is called dielectric strength.

Dielectric strength of an insulator is the maximum electric field force it can withstand before breaking down and starts behaving like conductor.

So more is the dielectric strength better is the insulator.

Electric field lines:



Well now we know about electric field is, the next thing you need to understand that how should we represent electric field lines on paper?  Well we need to represent this cool phenomena on paper and with the help of electric field lines it can be done.

Here electric field lines denote the presence of electric field on paper. Well these lines are imaginary lines and is used just for our convenience to represent electric field on paper.

But these electric field lines do have some rules and characteristic which we need to know before drawing electric field lines.

Characteristics of electric field lines:


  1. Lines will always starts from +ve charge and end at -ve charge.
  2. Field lines are always smooth curves. Tangent at any point gives direction of electric force/field at that direction.
  3. Two electric field lines never cross each other because if they cross that will indicated the two direction of forces which is not possible.
  4. Crowding of electric field lines in a space indicates the relative strength of electric field.
  5. Electric field lines are always right angle to the surface.
  6. Lines behaves like stretched string and trying to decrease their length. This explains that the force of attraction between +ve and -ve charge.


Well these are the rules you need to have in mind before drawing electric field lines.

Electric flux:



Electric flux is kinda different topic to understand. So well we all have water sinks in our kitchen. Well if you have closer look then you will notice that water flows through a hole connected to your house sewage.

Well, now replace water with electric field and that hole with an area. So amount of water passing through the hole or amount of electric field passing through that area is called flux.

So electric flux is nothing but intensity of electric field passing through an area.


Here Δɸ denotes electric flux, E denotes the electric field and ΔS represent area.


Electric dipole:



Electric charge is an interesting phenomena and their are many allotments by which these charges can be helpful and one of them is electric dipole

Interesting and helpful thing about electric dipole is that total charge on electric dipole is zero but total electric field created by dipole is not zero. 

Weird right? If total electric charge is zero than how can created electric field not be zero?

So to understand how this is possible let's first understand what exactly is dipole. 

An electric dipole is nothing but two equal but opposite electric charges are placed at some distance and both these electric charges are connected by the line in space which said to define the direction of dipole (lines travels from -ve to +ve )

  Electric dipole
Electric dipole


So electric dipole is a pair of equal and opposite point charges separated by some distance and the line connecting these two charges, defines its direction in space.

P.s: Electric field lines start at +ve charge and end at -ve charge but here in case of dipole we are not talking about direction of electric field lines and direction of dipole is from -ve to +ve.

So we now know why total charge on electric dipole is zero but electric field created due to dipole is not zero and therefore in the notes given below we have derived electric field created due to dipole on different position. 

Physical significance of electric dipole:

Well all these cool terms in physics such as dipole is interesting to learn on paper but what about in real and practical world?

Let me ask the same question like before! Does electric dipole have any physical significance or not?

Well let's look into this as well.

Short answer to this is YES! electric dipole has physical significance but how? So let me explain.

In most of molecules you will see that the centre of positive and negative charge lies at the same place. Therefore dipole moment of these kind of molecules is zero 

Wait can we do something through which charges which lie at the same place splits and dipole can be created?

Well YES! If these kind  of molecules are to introduce in electric field their centre splits and now negative and positive charges do not lie on same place and are apart.

So this way by putting these kind of molecules in electric field dipole moment can be created in them.

Some example of these kinds of molecules are CO2 and CH4 . 

There are molecules as well where centre of negative and positive charges don't coincide and these kind of molecules have permanent dipole moment even if electric field is removed. They are called polar molecules. 

Molecules in which centre of negative and positive charges don't coincide are called polar molecules. For example: H2O

Well yes this is the physical significance of electric dipole

Conclusion:

So electric charge are of two types: positive and negative charge. These charges have some basic characteristic such has additivity, quantisation and conservation of charge.

To measure the force between two charges Coulomb gave us formula also known as Coulombs law of electrostatics. Coulomb derived this formula with the help of inverse square law.

Well now electric field comes which is basically the area around electric charge where its effect can be experienced. This electric field can be represented with the help of electric field lines. 

Next is electric flux which is the amount of electric field passing through an area. After electric flux next thing is electric dipole.

Electric dipoles are nothing but when two equal and opposite are placed at certain distance and interesting part about dipole is total charge on dipole is zero but electric field produced by dipole is not zero.

Handwritten Notes:

So this article only covers concept and other details about electric charge and electric field lines. If you want all the derivation and hand written personal notes of electric charge and field Click here.


Don't forget to book mark and subscribe this site because we will update all the subjects and chapter notes soon on this site with all the other interesting details. 

F.A.Q:

Q. Why electric field lines never cross each other? 

Answer: No, electric field lines does not cross each other because if they do it will show two direction of force which is not possible.

Q. What is the direction of electric field lines?

Answer: Electric field lines starts from +ve charge and terminates at -ve charge.

Q. Does electric dipole also have the same direction like electric field lines? (+ve to -ve)

Answer: No, direction of electric pole is from -ve charge to +ve charge which is represented by line between both the charges.

Q. How physical significance of electric field is proved with the help of electromagnetism?

Answer: In electromagnetism you will see that field can radiate and propagate energy which signifies it has a physical quantity.

Q. How physical significance of electric dipole is defined?

Answer: Some molecules have centre of positive and negative charge at same place and they behave like dipoles when electric field is introduced and some molecules such as H2O are called polar molecules in this kind of molecules negative charge and positive charge are not present in the centre and they have permanent dipole moment.


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8 Comments

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    1. Welcome check out other notes as well. I hope these notes help you.

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  2. Sir I take more time to learn some topics. I feel sad .. after I saw your notes I cover a topic in 1hour. before I take at least 3hours ..thank u so much sir ..🤸🤸😀.

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    1. That's really great. I am glad you like the notes. Check other chapter notes as well in case you need it.

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