Free Physics NEET Video Lectures for Current Electricity

00:00:01 [Music] [Music] hello welcome all of you to this course of lecture what I will be doing in the current and the next few lectures is to discuss about current electricity let me first point out that so far you have discussed charges at rest and what we are going to do in this chapter is to look at charges which are in motion and that is what we call as current now before I go to physics of currents let me also tell you that current occurs in nature as well the most common one being lightning which occurs due to electric

00:01:20 discharge during an electric electrical storm see what happens is this that find drops of water go and reach the cloud at fairly high height they essentially become like ice clouds and when different parts of these clouds they collide with each other they usually lead to generation of electric current and such discharge which you call as lightning could occur between electrically charged regions of a cloud or between two clouds or between a cloud and the ground now of course you all know that the power of lightning could

00:02:13 be quite significant because an average bolt of lightning transfers about 15 Coulomb of electric charge now you wish to tell you that a Coulomb ascender Coulomb is a fairly large amount of electric charge because you might recall that the charge of an electron is 1.6 into 10 to the power minus 19 coulombs so therefore when I talk about 1 Coulomb it means 10 to the power 19 times the charge in an electron and if you are talking about a very large Lightning then the amount of charge that is transferred

00:03:00 could be as high as something like 300 to 400 coulombs and a typical potential difference in a lightening could be as high as 200 thousand to five hundred thousand volts and that can can go even up to as high as thirty million volts in addition to this another thing that occurs in nature and with really beautiful consequences is that Sun emits gases and particles which move towards the space at a fairly high speed and part of which also reaches the earth and some of these which reach the atmosphere

00:03:53 particularly at the higher atmosphere they I happens to ionize things and they sort of work in or circulate in the region of atmosphere known as the ionosphere and and this current they create some beautiful sites known in the northern hemisphere as aurora borealis are also called the Northern Lights and very similar things also happened in the southern hemisphere which we call as the southern light or also known as aurora australis any fact I have shown you a image of the aurora borealis in the northern

00:04:42 hemisphere from NASA public website and but they occur in different types of colors now in addition in nature there are certain fish actually six categories of fish prominent among them being eels and catfish which also emit electric charges now remember that our muscle cells have electric potential and but in some phase they sort of evolved into cells which are called electrolyte cells and which can generate potential differences of the order of 800 to 1000 volts or so and a much smaller scale even within a human

00:05:29 body we know that many of our body functions for example the pumping of blood into our hearts they take place by signals which arrive there from the brains and these signals are also electrical in nature and of course they are much smaller we'll be talking about their magnitude where but but basically these examples that I gave you they're examples of situations where the current is not steady so what we are interested are in situations where the current remains steady and this lecture and the

00:06:15 subsequent ones we would be primarily interested in talking about currents which are steady and later on we'll see that steady currents also become sources of magnetic field very loosely speaking electric current is nothing but flow options you this is the formal definition and let's try to make this a little clearer now let let me say that I have an arbitrary surface some area doesn't matter and suppose I have charges let us say positive charges which I will call Q plus and negative charges group there Q

00:07:21 minus and they are passing through this surface now so whatever enters into that surface comes out so therefore so this is the charge in and they are coming out on the other side of that surface so this is my charged out obviously that's also equal to Q plus and this is the Q mess now so the net amount - which is flowing in is Q plus - Q - now supposing these charges were steadily flowing through that surface then the amount of charge that is going to that surface would be proportional to the time T so this is proportional to

00:08:24 time T during which we make our of the vision so in other words the rate at which the charge flows that I will call as or define formally as my current this is equal to Q divided by T now this of course assumes that my flow is steady but supposing it is not now then I would take the formal definition in a slightly different way and I will say that take a small time interval delta-t now during which the amount of charge that is flows through that surface that I talked about is Delta Q then my current I add that instant around which

00:09:27 I have taken that delta T can be defined as limit of delta T going to 0 I take as small as time interval as possible Delta Q divided by delta T you recall from your calculus that this is nothing but my definition of DQ by DT so this is my formal definition of current let's look at what are the units of current so note that current is defined as charge divided by time so therefore obviously the unit that I expect is Coulomb per second actually so this is given a name ampere actually in SI units the ampere is

00:10:34 defined not as Coulomb per second because Coulomb is not a fundamental unit ampere is it is defined in terms of its magnetic effects but that will be we'll be talking about in later parts so ampere is a fundamental unit in you so let's go back to the cases that we talked about for example typical household appliances in India you know that Indian electricity supply is in the range of 220 to 240 volts and typical current values for household appliances is of the order of a few amperes so let's say up the order of five

00:11:41 amperes or so if you want to compare this with for instance the strength of the current that takes place in and lightning this could be typically several thousand nuggets you or other formulas that I talked about could also go to millions of our beers now on the other side I talked about certain fish like catfish and eel which give out electric currents there again the natural things they are typically about N and a deer on the lower end the human nervous system gives its mouth micro amperes you

00:13:01 so now that we have talked about the definition of electric current let me now try to tell you how exactly we look at the flow of currents though it is not a an exact similarity but there is a lot of similarity with water flow throw of water for instance through a pipe now remember that for example at your homes you have a water faucet and typically there is a pipe which is going through and supposing at this end there is a water faucet and I know that if you open the tap the water immediately starts coming out now what happens is

00:14:04 this we have also observed that there is no time difference between almost no time difference between the time when you per open the tap and the water comes out now actually what is happening is this that water is being pushed from one end but because there is a closed faucet on the other end it is not able to go and it stopped and and so there is no movement of water physically from here to there because their pipe is already full of water now when you open the faucet then basically all that happens is the water gets

00:14:47 pushed but almost as soon as you open it because there is already water at this end the water starts flowing in there so the incoming water is pushing the water which is there in this and of course then the water comes out now almost very similar thing happens in electricity for example at your home now you switch on for example a light and you'll find that there is no perceptible time difference between the time you switch on the light and the light coming up and the main reason is again the same

00:15:30 thing they in the wire which is there there are extra charges already there so what you did when you push the switch is to essentially please provide that push like what I showed here so therefore the what we need to do because the electrons are already there as a part of the matter and what we need to do is to essentially push it so what we will be talking about is how this mechanism of pushing the electrons is arrived at and you know how does charges actually flow so let us look at how is current created

00:16:30 so first thing that you have to recognize is that the ability of a material to conduct electricity depends on the property of the material we know that there are two types of charges there are positive charges and negative charges now what we require is a separation of charges in order to ensure there is a flow of charge now for instance we could create a static charge by rubbing together two materials this you have learnt in your first lecture on electricity where we talked about static electricity for instance if you rub a

00:17:24 piece of amber with animal fur then of course I generate static electricity and then if we touch amber to the ground the current immediately assess through and the static list is lost of course this current that we'll talked about doesn't last for very long and like the other cases that I mentioned these are also not currents which can be utilized in any useful way so so this is known the way one talks about generating electric current so let us now come to what type of materials are suitable for

00:18:23 generating electric current the first thing that we know is all material consists of atoms and molecules and so behavior of a material electrical property or any other property for that matter depends upon the atoms and molecules which constitute a matter and we're interaction ability to conduct also depends upon physical condition of the material for example pressure temperature etcetera we will be talking about one or two of these things later but but this is this is roughly what happens but for the

00:19:27 purpose of electric current the class of materials that we are interested in are known as conductors these are typically substances such as silver copper aluminum etcetera mostly in some state but of course you also have mercury which at normal temperature is a liquid and is an exception so these are materials which are readily conduct electricity you now that of course happens if these things are brought into a closed circuit and an electric field is applied so the reason it happens is that the

00:20:34 atoms in a conductor they have the ability to very easily give up when I say very easily it means with where it will cost a little cost of energy give up one or more valence electrons and and then this valence electrons which are contributed by the atoms which comprise that solid now they all belong to the material as a whole and not to that particular atom or atoms to which it belonged and very frequently we use the world electron gas so these are all so-called free electron gas they're free in the sense that they are

00:21:34 not tightly bound to an atom but they belong to the solid as a whole so belong to solid as a whole now this electron gas that we have talked about when you apply an external electric field they are free to move because as we know charges accelerate in an electric field now in our lectures on electrostatics we have learned that inside conductors I cannot have an electric field so electrostatics you know inside is you what we will see now is that under dynamic condition this statement will not remain valid so let me I will come

00:22:46 back to it again but we will say not to under dynamic condition now we have been just talking about current in mostly solids and in this case since that is the most common form of electrical conduction we have been talking about motion of electrons but let me tell you that conduction or transport of charges do not necessarily happen only with electrons it can happen even with positive charges and the typical example is what is known as an electrolyte you might recall your elementary chemistry a little bit for example if I

00:23:57 have an electrolytic solution let us just take a simple situation like a common salt solution now I know that these are typical common salt these are what are known as ionic compounds the way to happened is this better these sodium atoms and chlorine atoms have this property that be outside the valence electron in sodium item for example was extremely loose in the sense nature was very weakly bound to sodium atom and as a result sodium pudding we easily lose that and when sodium lost an electron this is one example which you

00:24:50 might have seen there so basically what happens is that if it actually loses initial electron and thereby becoming a sodium plus so I should probably write it as this minus electrons so sodium plus plus an electron but this electron that is there is easily attracted by chlorine so chlorine very readily agrees to accept this electron in its valence M and so the chlorine accepts the electron and becomes a negatively charged chlorine ion so therefore what happens in the sodium chloride solution is that we have

00:25:39 instead of sodium chloride together as atoms we have na plus cell lines now so suppose I had a situation like this this is electrolytic solution so for my typical example I am talking about sodium chloride so let us look at that now I insert two electrodes and I will see that I will need to sort of connect them with a battery the positive end connected to one of the electrodes the negative end is connected to the other electrode so this in our usual language this is a positive electrode also called

00:26:25 an anode this is a negative electrode called a cathode now look at this since the cathode is negative the electrode is negative it has the ability of at writing the sodium plus ions so the sodium plus ions will start moving in this direction now likewise the chlorine ions would move in the reverse direction and of course if there is a current this would in there is a closed circuit this would give rise to a current but basically the reason I talk to you was that though generally we talk about electrons being

00:27:17 the agent of conducting electricity we also have the sodium and the chlorine ion for example the positive ions can also conduct electricity and why in fact we will see later on in our lectures on semiconductors that even the absence of electrons there is a vacancies which behave like positive charges they contribute to the electrical current so now what what is the situation in an insulator in an insulator unlike the case of a conductor or the case of an electrolyte for instance the electrons are tightly bound and another result

00:28:02 they are not free to move so tightly bound electrons unlike in the case of a conductor where we have said that there cannot be under static condition electric field inside a conductor in case of insulators different distribution of charges are and the inside the electric field need not be zero at this moment I postpone my definition or the properties of what are called semiconductors because that requires a detailed discussion by itself but we will come back occasionally to this so let us look at first the metals or the

00:29:07 conductors metals are of course conductors and let us look at what happened in electrostatics now suppose I had a piece of metal and I let's suppose that this is a sample and these are my two ends and let us suppose artificially I provided a positive charge plate here and a negative charge here now look at what actually happens the free electrons which are here they would start moving towards thee play to the positive charge and and you see initially before this happens because one end has a positively charged

00:30:08 plate the other end has a negatively charged plate I had essentially created an electric field but in this is a good conductor as soon as this happens this will attract the the positive plate will attract the electrons and of course the as a result the negatively charged nature of the other plate will disappear and the motion of the electrons inside would soon stop soon stop so this is what would happen in static condition but let's look at it slightly differently suppose I had a situation like this but

00:30:49 I did not have a static condition because I had applied an external electric field so let us let us draw the external electric field and see what happens so I have the same situation there and let's suppose I have an external electric field in which this has been placed now I realize immediately electrons would start moving opposite to the direction of the electric field so that will charge up this side and so that would make backside positive the result of this is to create an effective field

00:31:47 inside the material so let us call it in and this will go on till the field inside which was external to begin with that has been exactly canceled and there is no longer any field around there now this is this is precisely what happens and and as a result the picture that emerges out of this is like this a conductor in a an external electric field the field lines are like this no field inside so I have negative charges here the positive charges there this is my external so basically what is happening is this

00:32:46 that it is the electrons which are moving from the right to the left creating an effective electric field and that exactly cancels the external field that we had put the material in and then let filled inside in a static condition is equal to zero now suppose I do something more what I do is I have a method of removing these electrons which come here happiness so basically what I am creating is a pump so let me go back to the same picture again but create dynamic condition so the same picture I had alerted field

00:33:39 and we have seen that this electric field had made this side negative this side positive and what I do is this I have a mechanism we discuss more about what mechanism it is later but supposing there is a method of taking out these electrons and feeding it back there so creating something like a charge pump if you like how we do it I'm not given in this picture but you might have realized that what we do is to join it by a battery and things like that but with this charge pump remove the electrons almost

00:34:35 as soon as they arrive here and then fix it there so this has the effect that no electric field internal electric field will be created internal equal to zero for conduct hood you mean that current could slow regularly and this is the way charges would flow so this is the way the current appears so hence charges in flow so now that we have talked about that current is basically a flow of charge and we have seen that though there are situations where both positive and negative charges could flow most of the

00:35:50 time we are restricted to talking about electrons so let me talk about what about the direction because we all know that for example when I gave the example of water flowing we also know that light has a direction because it depends upon the way they are being pushed so direction of correct now conventionally in spite of the fact that the electrons contribute from the bulk for the bulk of the currents in situations that we are interested in but it has always been defined as the direction of flow of positive charge

00:36:51 so the picture that I gave you a little while back so I have this situation so we said that there are these electrons which are moving to this plate so let me still show that electrons moving in they are being removed continuously so the electron is moving in this direction so according to this definition that I gave you this is a situation where the direction of current is the reverse direction so let me say that that is the direction in which the current is flowing in other words the direction of current

00:37:44 is defined according to a convention as the direction or posit to the direction in which the negative charges flow that is the direction in which the positive charges flow now that is one thing that you would immediately notice that we have said current has a direction however current though it has a direction is not a vector this is something which needs a little a little bit of thinking to appreciate so current i has a direction but is a scaler it is not a vector and this is primarily because it doesn't satisfy the algebraic

00:38:37 law of addition of vectors we will see how currents are added up and in fact we'll be discussing it at a later stage so let us look at I'll go back to our original definition we said DQ by DT in my current so if I take any arbitrary surface so the amount of charge that is flowing through any arbitrary surface is related to current by this relationship integral IDT so we have made a statement that current is not a vector and we have talked about its unit so we define a quantity which is related

00:39:35 to current but turns out to a vector so the current density J is a vector though no current is not affected so let me explain how it works let us suppose I am talking about a current which is passing through an arbitrary sir let me show you a surface here this next suppose is the end of a wire cross section of a wire and the current is entering like this and in living from the surface like any other definition of density I define the magnitude of the current density as the amount of current passing through a unit

00:40:43 8 so let's first talk about the magnitude and then I will come to what is its direction so magnitude of J is I divided by area so this is of course the scalar because current is a scalar area as we understand is the scalar since J is a vector we need to define its direction now you are all aware that if you take an infinitesimal surface area that area can be made as flat as you want by making the surface area smaller and smaller and so that I can associate a direction with this you have learnt even

00:41:46 in your mechanics course for instance so what we are now saying is this that we define D as the direction of the area element in a direction which is perpendicular to the cross-section obviously this definition makes sense only if the area is infinitesimally small now sorry this direction now you realize that every surface has two directions one in which the current is coming in and the other side of the surface is the direction from where the coming current is coming out now we take the direction of the s as that surface

00:42:56 the direction of that surface from which the current is coming out so and we define J to be in the direction of current flow you so what does it mean somebody I have a wire with this cross-section and I have current entering like this so the positive charges are flowing in this direction so that is my direction of J and when the current comes out it is this surface from where we come out and so therefore it is outward normal to this surface so this is your direction of use this implies that J dot DS is

00:43:56 positive for positive correct so current is not a vector but current density which is the point definition by point definition I mean that I take a small enough area I can make that area as small as I like so that density is defined as a point function so at every point in the material I define that is so we have seen that under electrostatic condition there is no field inside a conductor now what happens in such a situation so the electrons are still free and so let us write it down static condition

00:45:00 so no field inside the material so what happens is this that in such a situation the free electrons themselves have water normal thermal velocity now typical thermal velocity of electrons in a metal is on the order of 10 to the power 6 meter per second fairly large velocity now what happens inside the metal is this that these electrons move inside the metal in a random fashion and let me explain to you what actually happens so in a metal there are these ions or atoms which are located there is a periodicity type of

00:46:01 thing but let us not worry about that and going essentially a pattern here so what happens is this if you take a particular electron this electron is continuously colliding with atoms or ions and then a bouncing back back from the atom and then collides with another one so typically a situation could be like this if this is an electron it could go here then here then here so all sorts of things can happen there is no particular pattern and let me explain why this collision is essentially elastic in nature so these

00:46:53 are elastic collision so we are talking about in the absence of any field inside so let us say e inside is zero so elastic collision stiffness now this is something which is very similar to when for instance a very light particle collides against a heavy arrow so for instance a tennis ball bouncing against a wall or a stone bouncing against a truck so we know that a light particle colliding against a heavier mass bounces back almost with an unchanged speed now and only that the direction in which emerge will sort of depend upon

00:47:54 what is the direction in which the collision colliding particles approached the I am and then of course it comes out and the result of collision Andry collision is such that the electron moves around in all directions now if you look at a collection of electrons the because of this random nature the velocities of the electrons different electrons are uncorrelated now so let me write it down the velocities of different electrons are uncorrelated now as a result supposing I say that V I let me put an average sign here

00:49:02 represents the velocity of the is electron the average velocity of V electron if I sum it over I equal to let's say 1 2 let's say n number of particles are there so 1 over m this is equal to 0 there is equal to 0 because the directions of the velocities are hopefully uncorrelated so average velocity of the collection of electrons is zero so let us return to dynamic condition now we have seen that under dynamic condition the electric field inside need not be zero so let's say the inside is

00:50:09 not equal to say now in such a situation the electrons which we said move with a thermal velocity because of the presence of the electric field inside are also accelerated so let's say the electrons are accelerated the velocity or speed with which the moving is still the thermal velocity but they are accelerated what is the result the result is that the picture that I gave you for the static condition roughly holds but because of the presence of the electric field on an average the collection of

00:51:06 electrons would move or drift in a direction opposite to that of the electric field so the collection of electrons or move in a direction opposite to that of the electric field you a simple analogy might help understand this situation supposing you are in a room and there are chairs it's a classroom and a few of you you your eyes are tied and you decide to move around the room now since you cannot see you are likely to bump against the chairs you are also of course likely to bump against yourselves but I assume that for

00:52:15 reasons that I will explain I will assume that you do not collide with each other but collide with the static chips now once you have collided with chair you obviously change your direction and start moving in another direction and again collide with another chair now if this situation where we what happened in case of static condition then after some time if you look at the positions of all the players in the room they would be essentially random which corresponds to the situation the average velocity is

00:52:52 equal to 0 now let me now assume that there is a door to that room where there is a sound signal that is coming up maybe somebody a friend of yours is playing flute there now then since you still cannot see you will be colliding but when after collision you want to change the direction of your motion you are more likely to move towards the direction from where the music is coming so what happens in such a situation after some time that big distribution is still roughly random but on an average the group of students who are there in

00:53:37 that room they will drift towards the door and this is essentially the concept of difference now when I told you that let us assume that you don't collide with each other the analogy is because of this in this case my colliding particles are electrons and the electrons as you know can be regarded as point particle when I compare their dimensions with data bios now since the electrons have negligible volume so the probability that they collide against each other is almost non-existent and because of that

00:54:18 we assume that the electron interaction is neglected now so what happens to the expression for drift velocity in our situation somewhat have returned back to this picture and I assume that I am considering a parallel pipe with a cross-sectional area a and length L now what we are saying is this all the electrons which are within a volume a times L so all electrons within a volume eight times eight we'll pass through the right the face why in this picture this is the direction of the electric field

00:55:42 which is also the direction of the current density and the electrons are on an average moving toward this right and that is the direction of the drift velocity which we represent by VD suppose M is my electron density then the number of electrons which are in that volume is obviously n into L into a so this area is a and the charge that is content is e times n into L into a well this being electronic charge the charge would be negative but in this case I am talking about the magnitude of the charge so in this case my e is the

00:56:41 magnitude of electronic charge Hadj is of course equal to 1.6 into 10 to power minus 19 coulomb so therefore the current density it is given by Q divided by a times T this is the amount that has asked through this surface in time T so therefore this is given by e times n a has canceled out I am left with L divided by T so that is my e times n times the magnitude of ready now sense the direction is opposite to that of the current density I write my current density J is equal to minus e times n times VD

00:57:51 the direction of the current density is opposite to the direction in which the electrons are moving so what I have basically done today is to define what is electric current realize that electric current is simply an alternative name for electric charges which are moving flowing we pointed out that it is not a vector we defined a related quantity called current density and we realized that current density because it's a point function can be regarded a director and we started talking about the concept of

00:58:35 a drift velocity which we will elaborate in more details next time [Music] [Music] you [Music]