 Hello and welcome back friends. So, we were discussing Newton's laws of motion and we were going through different types of fundamental forces. In the previous session, what did we discuss? We discussed about gravitational forces, isn't it? So, it was Sir Isaac Newton who gave to the world the concept of gravity and gravity was one of the four fundamental forces available in nature now. What we are going to do is we are going to discuss the next fundamental force and that is called electrostatic or other electromagnetic force as well. So, we will discuss about first what is electrostatic force and then we will talk about electromagnetic force. So, we are basically linked to each other and the genesis of these forces is nothing but a very fundamental concept or fundamental property of matter which is called charge. So, you know that you would have been doing this experiment in your childhood as well where you take a comb and then rub it against your dry hair and then bring the comb near to the tiny bits of paper and you will see that the bits of paper getting attracted towards the comb or the scale ruler which plastic scale which we otherwise use. So, that is charge. So, what is charge? My friend charges a basic property of matter. So, other basic property of matter was mass you know. So, charges another basic property of matter. Now, by virtue of which they attract or repel each other. So, do this experiment if you get time. So, you take a small balloon and you know inflate the balloon and then rub it against dry hair and then if the balloon is small enough you will see that when you take it to the you know near a wall it will get stuck to the wall. So, that is the you know the because of the difference in the amount of charge present on the balloon in the wall you will see there is a force of attraction. Now, we know by our previous knowledge there are two types of charges positive charge as well as negative charge and like charges repel and unlike charges attract each other. This is what we have observed in nature and we also measure charge in terms of Coulomb. So, this is the unit Coulomb. So, Charles Augustine Coulomb was the person who gave the law of electrostatics or the force of attract electrostatics relation. So, hence in his honor we have named the unit of the charge as Coulomb. Now, you can see there are two charges positive as well as negative and they attract each other. So, let us say if there are two negative charges then they will repel each other and if there are two positive charges then they will again repel each other. So, like charges repel unlike charges attract. So, with this property of charge now let us consider two charge particles. So, one is this one the left one the other one is this right one one has a charge capital Q the other one has small Q and let us say they are separated by a distance d between them. So, they are separated by a distance d between their two centers. Now, this was Charles Augustine Coulomb who came up with this observation that these two charged particles if they are unlike in nature that is one positive and one negative. So, let me say this is my negative charge and this is let us say positive charge. So, they will attract each other if they are both of same nature that is positive and positive they will repel. If they are again both negative and negative then they will repel again. But in this case one is positive one is negative. So, they are going to attract each other. So, you can see FAB is the amount of force by which the positive charge you know A is attracting the charge B like that vice versa right. So, one force is this force is being applied by whom this force is applied by this charge here and it is applied on whom on this negative charge. So, this negative charge is being pulled towards the right towards the positive charge similarly this positive charge is being pulled towards the left. So, this is one fundamental force which is existing in nature. So, the moment there is charge on two particles then they attract each other or repel each other. Now, this is charge on body A this is charge on body B. So, this is the description and this is D distance between A and B and this will be force on A by B and this will be force on B by A. Now, let us now understand the relation between the two forces. So, hence like what we saw in gravitational force if you recall there also the two forces with which the two bodies of some mass attract each other was equal is it not the forces on each other applied was same. Similarly, here the force applied by one particle charge particle on the other charge particle whether attractive or repulsive is going to be same in magnitude. So, direction definitely opposite but magnitude of the two forces are going to be the same and like what we saw in gravity again here as well force is directly proportional to the product of the charges. So, whatever is the charge on one and charge on the other one multiply it. So, force is directly proportional to that what does it mean. So, if you double this charge and double this charge as well. So, let us say if you have 2 q and small 2 q then what is going to happen the force is going to be 4 f right. So, this 2 and this 2 multiplied together. So, force will become 4 fold that is what is the directly proportional. So, here again like gravitational charges gravitational force sorry f is directly proportional to 1 upon d square or f is inversely inversely proportional to square of proportional we learned this right in gravitational as well similar behavior quite fascinating isn't it inversely proportional to d square right. So, hence as you increase the distance the force of attraction or repulsion as the case maybe is going to go down and if you bring them closer the attraction is going to go up. So, you would see you know when you bring the dry ruler and when you you know rub it against your hair dry hair and then when you bring it closer to the particles closer enough to be able to be pulled right otherwise if you have a large distance between the ruler let us say this was the ruler and it is let us say it is positively charged let us say and there is something which is negatively charged particles here then you have to bring them closer right bring the ruler closer to the bits and pieces of that paper otherwise if the distance is too high it is not going to attract. So, that is what we had we have observed in our experience right. Now if you combine all of them together. So, like again there is a quite similarity with gravitational law. So, f is directly proportional now to product of the two charges. So, multiply the two charges divided by the distance squared right this one is similar to law of gravity and you will be astonished to know it is you know the this relation will was first of all between distance between them and force of attraction repulsion between them was given by Charles Augustine de Coulomb. So, Charles Augustine de Coulomb was the person who gave this law and combining we can write it like that. So, f is k into q q by d square where k is this constant proportionality constant whose value is 9 into 10 to the power 9 Newton meter square per Coulomb square this is also called Coulomb's constant why because again this particular law was given by Charles Augustine de Coulomb. So, hence again in his honour this particular value of k is called Coulomb's constant again you can easily find out the units easily find out the unit. So, if you can see k if you do the calculation k will be f into d square by q into q isn't it if you just do the cross multiplication now clearly f has a unit of Newton d square will be meter square and divided by Coulomb's square and hence you can see this is the unit I hope you understood this part. Now electrostatic forces act when the charges are at rest. So, whatever we learnt in the previous case that there are two charges and they are attracting or repelling that happens only when they are rest but when the charges start moving then not only the electrostatic force but there is something called electromagnetic force also start acting. So, what I am saying is still the charges are stationary there will be only electrostatic force of attraction only electrostatic right. So, hence the word electrostatic static anyways means at rest stationary right something which is fixed right not changing. So, hence when the charges are at rest this is when they are at rest that is they are not moving right but let's say they start moving then they also start applying on each other there is something called magnetic force right. So, that is what we will be studying deeply or you know in more detail when we do electricity and magnetism but for the time being please understand electromagnetic force again there are two forces one when the charges are stationary then electrostatic force which is given by that Coulomb's law will be applicable and when they start moving for example in case of an electric current let's say there is a wire and it is carrying a current i let's say what is current current is nothing but flow of electrons which are charges and let's say there is another electric current which is you know flowing nearby then they will start you know experiencing force over and above the Coulomb's force which is now called magnetic force and you would have heard that you know when you know there was a very famous experiment which actually revolutionized this entire you know field of physics when you know Orsted a Danish scientist found out that when you bring a compass magnetic compass near conductor which is carrying current then the magnetic compass observed or magnetic compass was deflected right so compass started deflecting the moment you bring it closer to a wire which carries electric current you can do this experiment at home also to see the deflection in a magnetic compass when it is brought near an electric current now this was given by Hans Christian Orsted a Danish scientist in 1820 he figured out and later on this particular discovery itself became the you know link between electricity and magnetism now taking a cue from Orsted's discovery Michael Faraday another 19th century famous physicist you know gave the world a lot of theories including electromagnetic induction theory and today the electricity which we use and the all the instruments or the equipments which run on electricity is you know the genesis of all that was this discovery where electricity electric current carrying conductor deflects a magnetic compass so this is another force which is available in or which is you know existing in nature one of the fundamental forces so we saw two fundamental forces so far one was gravitational force another is electromagnetic force and now we are going to discuss two other fundamental forces which are called nuclear weak forces and nuclear strong forces