Hello students, welcome back. So, we were discussing certain properties of charge as well as methods of charging. Now, let’s apply them in some examples. So, let’s start with example number one.

A glass rod is rubbed with a silk cloth. Glass rod acquires a charge of plus 19.2 into 10 raised to minus 19 Coulomb. You have to find the number of electrons lost by glass rod, charge gained by silk and is there a transfer of mass from glass to silk? We are given mass of electrons. Now, students, this is a case of glass rod being rubbed with a silk cloth. Now, in this case we know when rubbing takes place that is charging by friction must be taking place. So, this is a case of charging by friction. Now, when charging by friction takes place there is a transfer of electrons from one body to another, in this case from glass to electrons. Now, how much mass, how many electrons are being transferred can be determined with the help of quantization of charge which says that Q is equal to ne. Now, we know capital Q is 19.2 into 10 raised to minus 19 Coulomb, we know small n is integer and e is 1.6 into 10 raised to minus 19 Coulomb, that is charge on one electron. So, we apply the values we get the value of small n to be 12, that is 12 number of electrons are lost by glass rod. Now, since electrons lost by glass rod will be same as electrons gained by silk as it is the case of charging by friction. Therefore we can say that charge on silk will be minus n into e, where n is 12 and e is 1.6 into 10 raised to minus 19. So, we apply the value we will get Q to be minus 19.2 into 10 raised to minus 19 Coulomb. Now, we know the charge on silk, we know charge on glass. Let’s find will there be a transfer of mass.

Now, students we know when charging takes place it is due to transfer of electrons. It is due to transfer of some finite mass because electrons have some finite mass. Hence we can say yes, there is some mass transfer because an electron has a finite mass. Now, how much mass transfer will take place? That can be given by mass of 1 electron into number of electrons transferred which will come out to be 1.08 into 10 raised to minus 29 kg. Now, students, this was a case of charging by friction and quantization of charge clubbed together. Many problems are formed with the help of these two concepts.

Let’s discuss one more example where charging by friction is clubbed with which body will gain which type of charge. Now, if an object made of substance A is rubbed with an object made of substance B, then A becomes positive charged and B becomes negative charge. If however an object made of substance A is rubbed with an object made of substance C, then A is becoming negatively charged. We have to tell what will happen when B and C are rubbed with each other. Now, students, what are we given, we are given A and B when rubbed with each other, A is acquiring positive charge and B is acquiring negative charge. When A and C are rubbed with each other than A is acquiring negative charge obviously C will acquire positive charge. What will happen, when B and C are rubbed with each other? Now, let’s solve. Now, from information one when A and B are rubbed with each other we see A is acquiring positive charge and B is acquiring negative charge. As a result we can conclude that B has a higher tendency to gain electrons as compared to A. From second information when A and C are rubbed we can see that A becomes negative charge and C becomes positive charge. So, this can help us determine that out of A and C, A has a higher tendency to gain electrons as compared to C. Now, from step 1 and step 2, we can see that out of A, B, C, B has highest tendency to gain electrons as compared to A and C. So, when B and C are rubbed with each other then it is very obvious C will lose electrons and B will gain electrons. As a result we can say that C acquires positive charge and B acquires negative charge. So, students, we can see charging by friction and which body will be charged negative and which body will be charged positive, we have learned one example illustrating this particular concept.

We will come back in next module till then, students, thank you.