okay hello and um welcome again to um another lesson in ee 1100 and in in this lesson here that you are doing where the objectives are going to be um the resistors using ohm's law and and we will talk also about power so so then here are the objectives are we going to discuss which we did with resistors the using ohm's law which kind of like that's what you will be using throughout this semester and whenever using circuits ohm's law is going to be used so this is something that you have to embed in your brain about ohm's life you want to be dealing with electrical circuits so what is resistance that we talked about what's a resistor it's just it is a circuit element that resists the flow of charge think about it like if you are driving on the highway and there's too many cars in front of you it's just you can't go in high speed it just kind of like slows you down it's the same thing here and similar obviously like if you have resistance we have a current that is flowing through that resistance the the higher the resistance is going to slow more and more the current till it becomes of course the uh the case in which let's say resistor is very high infinite then that's almost no current will be flying through of course usually uh in in practical cases current will flow but very very very small amount so then what we can say that the higher the resistance the lower the current and this is again from ohm's law if we have um of course using v equal r times i if we want to do a look at it in terms of i then it's going to be v over r so here as we can see the more we keep increasing r so if r keeps increasing that's v and then v is constant the same value then r is going to be increasing keep increasing until it becomes very very large i is going to be reduced less and less and less so that's what this one here is basically that's what we are saying so here we have the resistance this is the resistance element that we use and so this is r and so we say the voltage across is always if we have the current going in this direction so that is the i of this r then always the current will when it reaches that resistance it's we say this is the it reaches the positive terminal of the resistance and goes out of the negative terminal of the resistance you will see that when it's a voltage supply it's the opposite it comes goes in to the negative terminal of the power supply and out of the positive terminal of the power supply so here we have r in this case is just the resistance and the units are in what we call ohms and sometimes we use the greek letter to represent ohms we will use this uh kind of like omega um and this so that's the uh what we use okay so typical of course values for uh r for resistance for resistance or the ohms we use kilo ohms mega ohms becomes really very large so it's one once it goes into the mega that's kind of large uh resistance in which the current is going to be really very very small so maybe like um you know on the uh maybe very very small so um so we use the uh of course current scan can be one ohm two ohms one kilo ohm two kilo ohms 2.2 kilo ohms so there's usually certain values that are manufactured so that's usually there's like kind of a standard to it so what is uh so this is the resistance just an element two terminals it has a positive terminal negative terminal depending on how it doesn't really matter i mean it's it's how you uh connect the resistance then how the current will flow that's always the case in which where like i said it flows into the resistor then that's the positive terminal going out of the resistor that's the negative terminal so now if we want to use uh ohm's law what is ohm's law it's that person that's his names arms that is related to his name so basically what it does it relates and i mentioned that in here the voltage equals the current that's ohm's law relates the voltage and current and a circuit so we will say the voltage and an ohm's law is equivalent to v so in general v times equals to r times the current um so then if we want to say with respect to that resistance here if we will say this is the voltage v sub r that's the voltage across the resistor then we will say that v sub r is equivalent to r times the current going through that r and so if we want to use like uh one unit so one ohm is equivalent to 1 volt over 1 amp so something like that so then usually the current like we said the voltage in ohm's law as the again the resistance times the uh current that we have so actually i wrote it here so we can say this is the ohm's law that is in general is the resistance times the current and so this is like an algebraic formula if we want to find the uh current so we will say from this formula i then it's just the take the r to the other side reciprocal so that will become v over r so that's how we can find r i mean high the current which is then related to this equation v over r if we want to find the resistance from this here we see that r is equivalent to v over i so that's how they are related these uh three values one way of knowing it's sometimes people use it using the pyramid in which we have um like that we have the voltage and then here we have r and we have i so then we can say that the voltage is equivalent to r times i or i is equivalent to v over r or r is equivalent to v over i so this is one way you can kind of like uh one way of remembering it in an easy um in an easy way so let's do an example on this okay so that i have enough room so an example for doing this we have a 2.2 kilo ohm resistor that has 5 volts across it then what is the current going through it so here if we have this resistance this resistor this resistor is 2.2 kilo ohm that's the value and the voltage across is 5 volts that's what's given we need to find the current so this is the current since the voltage here is positive negative then the current is going to flow this way because flow is from the positive terminal uh going into the positive terminal and out of the negative terminal so that is then i so here again using ohm's law in which we have v is equivalent to r times i here v is 5 volts r is 2.2 kilo ohms times i then just algebraically that is equivalent to five the volts over the 2.2 kilo ohms and if you do that on your calculator that should give you 2.72 milliamps just like a small current all right of course the um this is this is if we want to look at it this is like the base the voltage that we have over the 10 to the 3 that's the kilo ohms 10 to the 3 and that's going to be if we take it that's going to be equivalent to the base times 10 to the minus three so that's so that's an example of how we can find the uh current so once the given to us the voltage and the resistance we can find the current if the current i mean the resistance given and the current is given we can find the voltage using ohm's law and so forth any two values given we can find the third using the ohm's law all right another one in this lesson that we want to talk about is the power so what is the power we want to know that always the case when we are using resistance the power and resistance is always the resistors absorb the power so that's very important to know that resistors only or can only absorb power so then the power formula for the power is the voltage times the current and so if we want to find power across a resistor or a power of a resistor we have the voltage from the there we have the voltage equivalent to what r times i so if we substitute for the voltage here then the p is equivalent to v which is r times i and here times i again so that's going to be r i squared or i squared times r so if we do the another substitution like we can say um so doing or doing a similar substitution we can get so in this case here we want to do it here we have the power equal v sum times i from ohm's law i is equivalent to what is equivalent to v over r right so if we substitute for i in here that is going to be v or b p it's going to be v times v over r that's for the current in here which means that it is equivalent to v squared over r so that's two ways we can use it in which we can say the pi is the same thing the power depending on what is given to you if you are given you know what's the voltage you know what's the resistance so then you can say across the uh resistor i mean the voltage across the resistor then the power is v square over r and it is always observant power the power is being observed meaning that how is it observed it's going to be consumed into heat usually that's how it is usually consumed and or it is wasted using that it goes into heat so then then the power in general for a resistor the power v times i or i squared times r or v squared over r this is always these are the three ways we can find the power across a resistance all right so um and again so that is the case always this is the three ways you can see it use then so let's do an example on on this and then we can end this lesson so an example so a resistor of value for 70 ohms has 3 milliamps of current going through then the question is what is the power being observed by the resistor so in in doing this so we have a resistor that's its value of 470 ohms and the current that is going through so this must be positive negative since it's going in this direction is i is equivalent to three milliamps here this is the or and the so we have the two values we can find the voltage and we can use so we can use that of the formula since we have the current and we have the resistance so we can use the uh as we saw up here we can use the i squared times r to find the power because that's the easier way to to do it so or we can use find the voltage and then so that's two steps finding the voltage and then saying v square over r so then the power of the easiest way since we already know that since we know the current we know the resistance so we can say we use the uh the formula i squared times r for the power in this case this is 3 milliamps squared times 4 70 ohms and if you do that on your calculator that will give you 4.23 milliwatts all right of course if we want to find the voltage of your ass to find the voltage it's just going to be what the i times are which is then the resistance which is 470 ohms times the current which is 3 milliamps and that will give us a value of 1.41 volts you can use the same thing and now since we find the voltage so we can use the power is equivalent to v times i that's another way of doing it so the voltage here we found it to be one for 1.41 volts and the current is times three milliamps and it's going to give us the same value of 4.23 milliwatts so you can see this many ways we can use any of these three uh ways to find the power and uh resistor all right that's the end of this lesson thank you for listening and take care