Electrical Engineering: Ch 6: Capacitors (1 of 26) Basics (What is a Capacitor?)

welcome to dialect online here we're starting a new chapter for electrical engineering it's about capacitors and how capacitors work in electrical circuits the first a basic about capacitors what is a capacitor it's in simple terms two metal plates that are side by side with a very small distance between them and typically there's dielectrics between the plates or could simply be air between the plates so the simple version would be that we simply have air between the plates we then connect a battery or a voltage source to either side of the plates we have the positive side of the battery on one side of the plate the negative side of the battery connect it to the other side to the other plate I should say and the potential across the battery drives current to one side of the capacitor filling that capacitor with charge now the convention is that it drives positive charge to the capacitor and all these positive charges then repel the positive charges that are here we then get pushed away because the repulsive force is here and are attracted the negative end of the battery so positive charges then move away from this plate causing this plate than to become negatively charged it turns out that the excess of positive charges on one plate is equal to the lack of positive charges on the other plate therefore the excess of negative charges on this plate so we typically say that this side is positively charged and the side is negatively charged or it has positive charge on this side and it has a negative charge on that side the capacitance of a capacitor can be defined in two ways the first way that it can be defined is that it's simply a ratio of the amount of charge that collects on the plate divided by the voltage that drives a charge onto the plate now what happens here is that that ratio is known as the capacitance of the capacitor if there's a bigger capacitor if the plates are bigger and usually the capacitance does depend upon the size of the plates then you can see that more charge can be collected per the amount of voltage and drive to charge on there therefore you have a larger capacitance so the capacitor is simply the amount of charge you can collect on the plates relative to the amount of voltage you used to drop the charge on there you can also calculate the capacitance of a capacitor using its physical dimensions it turns out that the capacitance can also be defined as some constants e sub naught which is defined right here eight point eight five times ten to the minus twelve coulombs square per Newton Mutants meter squared multiplied times the area of the plates divided by the distance between them so it's a simple formula the capacitance of a capacitor is simply proportional to how big the plates are the bigger the plates the more charge you can collect and proportional to distance between them the smaller the distance the greater the capacitance because then the interaction between the positive negative charges is greater and you can have a stronger electric field between them and yes the capacitance then does depend on the strength of the electric field also notice that in this case since there's air between the plates we use a constant epsilon sub naught now the units for capacitance is called a ferret so unit wise we can define one ferret we say one F standing for ferret that comes after the name Faraday and then is equal to the ratio of one Coulomb per one volt in other words if you apply a potential difference of one volt across capacitor and the capacitor collects one Coulomb of charge due to that then the capacitance of the capacitor will be one farad now one farad capacitor is not a typical capacitor typically capacitor much smaller than that and the Holts fractions of a Coulomb when you apply volts across them and so typically we talk about micro farad's or even nano ferret's or even pico farads so these are the typical sized capacitors you'll find in most circuitry so we talk about micro farad's nano farad pico farads ten to minus six ten to the minus nine or ten to minus twelve ferrets now let's talk about the capacitor in terms of the electric field between the capacitor plates because there's a positive charge on one side and negative charge on the other side we will have an electric field between the plates and of course a direction electric field is from the positive charge to the negative charge now if you think about it this way let's that we have a chart or a plate here that has positive charge on it there will be an electric field emanating away from that plate now the magnitude of the electric field can be defined as the charge density divided by epsilon sub naught now of course the charge density is the charge per unit area so this can be written as the charge on the plate Q divided by epsilon sub naught times the area because charge density is simply the charge per unit area now there's also relationship between voltage and electric field we know that the electric voltage is equal to the product of the strength of the electric field times the distance between the two points for which you want to know the potential difference so if we want to calculate the potential difference here between the plates we can relate that to the strength of the electric field and so the potential difference V is simply e times D and since the strength of the electric field is equal to Q divided by epsilon sub naught times a we can write this as Q divided by epsilon sub naught times a times the distance between the plates now if we go back to the definition of the capacitance here let's go ahead and solve for the ratio of Q divided by V so here we could say that V is equal to Q divided by epsilon sub naught a times D and now one turn that into ratio of Q divided by V so put the V here and we'll put everything else on the other side so we get Q divided by V by being in the free underneath the Q moving this across over here that is epsilon sub naught times a divided by D so Q is Q over V is epsilon sub naught a divided by D which is the same as the capacitance of the capacitor plate so what we've done now is we've shown that the capacitance can be defined as simply the ratio of the charge divided by the voltage across the plates and we can also call it name it in terms of the epsilon sub naught times a divided by D this equation here is the capacitance according to the physical dimensions of the capacitor this definition of the capacitance is simply in terms of the ratio of the charge across the plates divided by the voltage that's the basic concept of what the capacitor is this is how we define capacitance in terms of charge and voltage and this is how to find the capacitance in terms of physical dimensions of the capacitor and that table is done

Channel: Michel van Biezen

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Keywords: ilectureonline, ilectureonline.com, Mike, Mike van Biezen, van Biezen, ilecture, ilecture online, Electrical Engineering, Output, Input, Current, Voltage, Voltage Sources, Capacitors, What is a Capacitor?, Chapter 6, Ch 6, 1 of 20, Farad, Capacitance of the Capacitor, Electric Field

Id: wR1IfIHj-LA

Length: 7min 7sec (427 seconds)

Published: Tue May 10 2016