 you have any doubts, this you have not done EMI yet, no doubts, okay, so next we will do is AC generator, AC generator is a device that converts magnetism into electricity, fine, so it is a very useful device like what we discussed last class it is very easy to generate magnetic field, you can you know have a huge amount of magnetic field and if you have a device that converts magnetism into electricity that is like one of the greatest invention of all and this is that one of that invention, okay, so the construction of this AC generator is something like this, you might have seen this in class 10 also, it is North Pole, South Pole, you know right the coil will rotate and you have North and South poles which suppose creates a uniform magnet, it will be fine, a uniform magnet will be created, this coil can rotate with omega angular speed, its area is A, fine, right now the coil is kept like this, getting it, coil is like this, find out the flux through the coil, how much it is, I would have drawn while I was drawing it, why you waited for me, how much is the flux, BA, BA right, the area is like this, magnetic field is crossing this area, so B into A and if this has number of turns as capital N, the total flux will be N, B into A, this is initially but this coil is rotating, suppose this coil rotates little bit, after time t how much it is a rotation, theta will be omega t, right, so angle between area and magnetic field will become how much after time t, omega t, right now it is 0, so that is what V dot A is NVA, after time t it becomes omega t angle, so the flux will become NVA cos of omega t, right, so EMF will be what, minus of d phi by d t, this is equal to NDA omega sin omega t, so you are not changing magnetic field, you are not changing area of the coil, what you are doing, you are changing only angle between area and the magnetic field and you are able to generate EMF out of it, so the current through this coil will be E by R, this is the resistance, this NB A into omega is magnitude of the voltage that is created, can you draw with time the value of flux, how it will vary, this is flux, N is constant, B is constant, it is a cos graph with time, how the cos graph looks like, it will be like this, this is how flux varies, how the EMF will vary, it is a sin graph, so it goes like this, so when flux is maximum EMF is minimum and when EMF is maximum flux is minimum, this is just a basic introduction of AC generator, this actually tells you that you do not need to vary only magnetic field or area, you can change the angle also and you will be able to generate the EMF, right, so let us start the next topic, this is inductance, have you heard of inductance before, have you heard of inductance, what comes to your mind when you hear inductance, capacitor is used to store what, capacitor is used to do what, store charges and electrostatic energy, inductor or inductance is a device to store magnetic energy, right, right down inductor or inductance stores the magnetic energy, now you have found out capacitance like how, you assume that capacitor is given a charge Q and whatever is the potential difference, if you can write in terms of, if you write this equation Q is equal to some constant into V then this constant of proportionality becomes the capacitance, fine, this is how you quantify the capacity of the capacitor, you supply the charge and look at what is the potential difference, if potential difference is high, it means that its capacity is less, it is like you take a bucket, you pour 1 liter of water, if high difference is very high, it immediately picks the height, it means capacity is less, similarly capacitance, you put the charge in it, if potential difference rises to a larger degree, the capacity should be less, reading it, so without increasing its potential to a large degree, it is able to store a lot of charge, that is what capacitance actually means and it is nothing but a constant capacitance, fine, so capacitance also in a way tells you that how much energy you can store, fine, now when it comes to magnetic energy inductance, what is the cause of the magnetic energy, the basic cause of magnetic energy or magnetism, what it is, a dipole, a dipole, right and the basic, the most basic dipole is electron revolving in the orbit, fine and counterpart of that in real life is a loop, okay. Fine or you can just say a current is the cause of magnetism, fine, so suppose you supply current I to any structure, fine and it gives away some magnetic flux, okay in the loop, okay, then the constant of proportionality here, this is inductance, if you have to supply less amount of current to get huge amount of flux, it means that capacity is large, you are able to generate lot of magnetic energy with less current, fine, so it is, I mean there is no other logic other than this that you know that, okay, current can be the cause of magnetic energy and flux can be the effect of magnetic energy, it can denote the cause of magnetic energy, so just like that I mean the constant of proportionality between the flux and the current is assumed to be inductance, fine, now once we have studied the capacitance, this is the generate definition of capacitance, whatever structure you put the charge cube measure the potential constant of proportionality is capacitance, after that what we did, we found out capacitance of some basic structures and the first one was parallel plate capacitor, fine, so now we have definition of inductance, we will try to find out inductance of few basic things, are you getting it, so but then the way this inductance is defined, it can be slightly different from the way capacitance is defined because the flux in one coil can be give to its own current and can also be given because of the adjacent current, is it not, you have a coil like this and another coil like that, if this coil has a current even this coil will have flux, this coil does not have a current in it, getting it, so without having current this coil can have flux, so this current may not be of the coil itself, fine, so the inductance is defined in two ways, one is self inductance, when you define the self inductance, you measure the flux in the first coil in terms of current in that coil itself and you call that as self inductance, you represent it by L, this is self inductance definition, when the flux is because of its own current, then you say that is due to its self inductance, getting it, but if you see that there can be flux because of let us say adjacent current or I2, so this I2 is able to create magnetic energy on the first coil, second coil current is creating magnetic energy on the first coil, fine, so the concept of proportionality is called mutual inductance, this is mutual inductance, fine and mutual inductance, how many mutual inductances are possible, can be as many, right, because this one coil, this can have flux because of all the adjacent coils, so between this and this there will be a mutual inductance between that and this there will be mutual inductance between this and 2, between 5 and 2, between 6 and 2, between 7 and 2, so every pair has a mutual inductance, even these two have mutual inductance, but right now my focus is this with respect to every other thing, fine, so when you try to write down the total flux in the coil, you can write it like this, L into I1 plus M12 into I2 plus M13 into I3 and so on, in our syllabus we will be dealing mostly with just 2 coils, so you will have LI1 plus M12 I2, what if I find out this flux in the second coil because of the current in the first coil, you will get M21, getting it, these 2 will be same, flux in the second coil due to the current in the first coil, this constant of course will be same as this one, mutual inductance will be same, whether you find flux due to the first on the second or flux due to the second on the first, the mutual inductance will be same, sorry, yeah, mutual inductance will be same, self inductance will be different, self inductance is its own property, mutual inductance is the mutual property, fine, so now we will try to find out self inductance and mutual inductance is of some, you know, routine geometry, just like capacitance depends on only its geometry shape and size, similarly inductance also depends on its geometry shape and size.