so this video is for those of you who want to get an introduction into electronics so one of the first components that we're going to talk about is resistors because typically you'll find them in almost every electronic circuit resistors are designed to limit the amount of current flowing in the circuit and you could calculate the amount of current flowing in a circuit using ohm's law v equals ir v stands for voltage measured in volts i represents the current measured in amps r is the resistance measured in ohms the current flowing through a resistor is the voltage across the resistor divided by the resistance itself so let's say we have a battery attached to a resistor the long side is the positive terminal of the battery the short side is the negative terminal of the battery let's use a 12 volt battery conventional current is going to flow from the positive terminal of the battery through the resistor back to the negative terminal of the battery conventional current flows from a high potential to a low potential but in actuality electrons flow in the other direction from the negative terminal to the positive terminal so that's really what's happening in the circuit electrons are the charge currents they're the ones that are moving through the metallic conductor but now let's make a table that shows the relationship between resistance and current so let's say the resistance is 3 ohms using this formula if we take 12 volts divided by 3 ohms we're going to get a current of 4 amps now what if we increase the resistance let's say from three ohms to six ohms what's going to be the current well using the same formula if we divide 12 volts by six ohms we're going to get two amps and so note what note the relation between resistance and current if we increase the resistance in a circuit the current decreases and so we could use this device to control the amount of current that is flowing in a circuit if you want to increase the current you can decrease the resistance if you wish to decrease the current increase the resistance now sometimes you may need to decide what resistor to use in a circuit and one important consideration that you need to take into account is the power rating of a resistor so let's say we have a battery attached to a resistor and let's say this is a 100 ohm resistor and the power rating is one half watts power is equal to voltage times current it's also equal to the square of the current times the resistance and it's equal to v squared over r so you could use any one of those three formulas to calculate power now this is important because if you apply too much power to a resistor or too much voltage the resistor can burn up it can get hot and that could create a lot of problems so you need to control how much power you deliver to a resistor because it can only dissipate so much without damaging itself and other nearby components so what we're going to do in this example is given the resistance and the power rating of the resistor what is the maximum voltage that we should apply across this resistor so we're going to use this formula p is equal to v squared over r now because we're solving for v we're going to rearrange the equation i'm going to multiply both sides by r so i get v squared is equal to the power times the resistor taking the square root of both sides we get the formula to calculate the maximum voltage that should be applied across the resistor so v is equal to the square root of p times r so here we have a one half watt resistor and the resistance is 100 ohms so half of a hundred is 49 and the square root i mean it's 50 well half of a hundred is not 49 it's 50. but you know what it is easy to take the square root of 49. that's seven so if you want to get a ballpark of your answer you know it's close to seven the square root of 50 is about 7.
07 volts so that is the maximum voltage that should be applied across this resistor if you don't want it to overheat now the next thing that we need to talk about is how you can connect multiple resistors together if you wish to increase the resistance in a circuit you want to add resistors in series because the total resistance of resistors connected in series is the sum of each individual resistor so the more resistors that you add the greater the total resistance will be in a parallel circuit the more resistors you add in parallel to each other the total resistance actually goes down to calculate the total resistance you can use this formula it's 1 over r1 plus 1 over r2 plus 1 over r3 raised to the minus 1. and i'm going to give you an example of that shortly but right now i want you to understand something in a series circuit there's only one path for the current to flow and this is the only path that it can flow from a to b in a parallel circuit there's multiple paths for the current to flow going from a to b the current can flow through r1 it can flow through r2 and it can flow through r3 and so that could help you to distinguish whether if two components are connected in series or in parallel if there's only one path for the current to flow it's connected in series if there are multiple paths for the current to flow it's connected in parallel now there's some other important things to take into account here because in a series circuit the current flowing through each resistor is the same so the current flowing through r1 is called i1 the current flowing through r2 is i2 each of these currents they're the same current i1 is equal to i2 and that's equal to i3 now in a parallel circuit the voltage across each resistor is the same so the voltage across r1 is v1 the voltage across r2 is v2 because they're connected across the same two points so that's another thing you need to be aware of when dealing with series circuits and parallel circuits in a series circuit the current flowing through each resistor is the same in the parallel circuit the voltage across each resistor is the same now let's say we have two 10 ohm resistors there's a lot of things we can do with that with those two 10 ohm resistors we can get three different resistor values we could use one of the two resistors to get a resistance value of 10 ohms or if we wish to increase the resistance we can connect the two resistors in series and so the total resistance across these two points let's call it point a and b is now r1 plus r2 so we get 20. now if we wish to decrease the resistance we can connect the two resistors in parallel now whenever you connect two resistors in parallel if the two resistors have the same value if they're identical the total resistance of the two identical resistors will be half of their respective values so it's going to be 5 ohms so thus if you want to increase the resistance connect resistors in series if you wish to decrease the resistance connect them in parallel now for those of you who want to see the calculation it's going to be 1 over r1 r1 is 10 plus 1 over r2 raised to the minus 1.
so one over ten plus one over ten is two over ten and when you raise something to the minus one power you basically need to flip the fraction if you flip the fraction the exponent will change sign it will change from being negative one to positive one ten over two to the first power is simply ten over two and ten divided by two is five and so anytime you connect two identical resistors in parallel the total resistance will be half of their respective values now the next type of device that we need to talk about is the light bulb the light bulb converts electricity into light energy and this particular light bulb does so through a process known as incandescence in incandescence what happens is when a metal gets very hot it's going to become red hot first and as it gets hotter the color changes it'll turn yellow it may even be white hot and so when a metal is very hot it begins to emit not only heat energy but light energy and so incandescence is a process where you can generate light by heating up a material in this light bulb there is a tiny wire called a filament so this is going to be green color and the reason why it has to be so thin is because if you have a lot of electrons flowing through a very thin wire it's going to be very easy to heat up that metal wire and as the metal wire gets hot it's going to emit light if it if it reaches a high enough temperature and so that's incandescence when you apply an electric current through a very tiny piece of metal it can get hot to the point where it emits light now with these types of light bulbs they're not very efficient in generating light energy they work but a lot of the energy is wasted as heat energy they generate a lot of heat and is another type of device that we could use that can also generate light energy and this is known as an led a light emitting diode let's use a green led a light emitting diode also converts electricity to light energy but it does so more efficiently it doesn't use the process of incandescence in fact leds are monochromatic light sources they generate light of one specific wavelength and as a result these devices are very efficient at converting electricity into light energy very little heat is generated they don't use up that much current a typical green led has a voltage drop of 2 volts and the amount of current that usually flows to an led can vary between 0. 1 milliamps to 20. if you put too much current in an led it can burn out so there are limits when you're buying an led you need to look at the manufacturer specifications in terms of what the maximum forward current should be but this is typically the range of most leds that you encounter in electronics now let's say we have a 9 volt battery and we want to determine the value of the current limited resistor that we should use in order to get a current of 10 milliamps flowing through this led because you don't want too much current to flow in it so this is when it's useful to add a current limited resistor how can we determine the resistance value that we need well for one thing we need to determine the voltage across the resistor the led will use up two volts out of the nine volts that the battery is providing to the circuit so the resistor is going to take up the other seven volts and this is the basic idea behind kirchhoff's voltage law the sum of the voltage drops in the closed loop or in a closed circuit must add up to zero the battery has because it increases the energy to a circuit we're going to put a positive voltage to it the resistor and the led they absorb energy from the circuit so we're going to put a negative value if you add 9 negative 7 and negative 2 you'll get 0 which satisfies kirchhoff's voltage law now in order to calculate the resistance that we need we could use ohm's law v is equal to ir so solving for r is going to be the voltage divided by the current so we have 7 volts across the resistor and we want a current of 10 milliamps across it by the way if you take if you divide volts by amps you're going to get the resistance in ohms if you divide volts by milliamps you'll get the resistance in kiloohms so just be aware of that so if we take 7 volts divided by 10 milliamps this is going to give us 0.
7 kilo ohms now one kilo ohm is a thousand ohms so . 7 kilo ohms is 700 ohms so we need a 700 ohm resistor in order to get a current of approximately 10 milliamps in a circuit now the next type of device that i want to talk about is something known as a potentiometer now you might be wondering what is a potentiometer a potentiometer is a variable resistor it's a device where you can adjust the resistance of a circuit and that's pretty useful because you could use that to control the voltage across an element or you could use it to control the amount of current in the circuit but let's talk about the potentiometer so here is the electrical symbol of a potentiometer you can draw this way you can draw this way it's a variable resistor but let's use this version of it i want you to understand how it works so let's call this point a point b and point c now let's say this is a 100 kilo ohm potentiometer if c is at the middle of a and b the resistance between a and c will be 50k the resistance between b and c will be 50k but the resistance between a and b is always 100k and that's important so let's say we move up point c closer to a and this is still a 100k resistor so the 100k potentiometer will still have a resistance of 100 kiloohms between point a and b that's not going to change what changes is the relative resistance between a and c and b and c so now if the resistance between a and c is 25 k the resistance between b and c will be 75 kilo ohms the sum total will always be 100 kilo ohms and so sometimes you don't need to use all three points of a potentiometer you may only need to use two of the three points and so if we only use a and c we can adjust the resistance of a and c from zero kilo ohms to 100 kilo ohms now there's many applications of potentiometers we won't be able to cover all of them in this video but let's talk about the basics of it one useful application of the potentiometer is using it for brightness control let me draw a better circuit sometime my drawing skills is not what it should be so in this example i'm only going to use two of the three parts of the potentiometer so let's use a 9 volt battery and we're going to use a green led with a voltage drop of 2 volts let's call this r1 and r2 and let's say that r1 is a 10 kilo ohm potentiometer let's call this point a b and c so between a and b the resistance is always 10 kilo ohms but between a and c it can vary between 0 and 10 kilo ohms now if r1 goes to zero kiloohms that's dangerous for the led because without r2 there we could have too much current flowing through the led so r2 is it serves to protect the led in the event that r1 is tuned to zero kiloohms so the maximum current of a typical led is 20 milliamps to determine our r2 value let's assume r1 goes to zero so r2 is going to be the voltage which if we take the difference between 9 and 2 volts the maximum voltage across r2 will be 7 volts divided by a maximum current of 20 milliamps now if you want to convert from milliamps to amps divide by a thousand one amp is a thousand milliamps so 20 divided by a thousand that's point zero 0. 02 so 20 milliamps is 0.
02 amps 7 divided by 0. 02 is 350. so we want r2 to be 350 ohms so if r1 is set to 0k the most or the maximum amount of current that we're going to get in the circuit is 20 milliamps which means the led will be super bright but now if r1 is tuned all the way to 10k then the total resistance in the circuit will be 10k plus 350 ohms so thus it will be 10.
35 kilo ohms keep in mind 350 ohms is 0. 35 kilo ohms so the max current which will be voltage divided by resistance that's going to be we're going to have 7 volts across these two resistors equivalently if this goes up to 10k so the total voltage across the two resistors will be no more than 7 volts due to the 2 volt drop of the led and the total resistance is 10. 35 kiloohms now remember if you divide volts by amps you get the resistance in ohms if you divide volts by kiloohms you're going to get the current in milliamps 7 divided by 10.
35 kilo ohms is going to give us a current of 0. 676 milliamps so with this potentiometer the current in the circuit can vary between 0. 68 milliamps and 20 milliamps so when r1 is set to 0k the current is going to be very high which means the led will be super bright if we adjust r1 to 10k the current flowing in the circuit will be very low which means the led will be very dim so thus we could use a potentiometer for brightness control the potentiometer can help us to adjust the resistance in the circuit thereby adjusting the current flowing in a circuit thus controlling the brightness of the led so that's one of the many applications of potentiometers in electronics now before moving on to the next topic i want to mention a few things for those of you who want more information on potentiometers leds how to calculate the current in series and parallel circuits and even other things i'm going to talk about later in this video i'm going to post some links in the description section below this video so feel free to take a look at that if you want access to other videos regarding electronic circuits in addition i have a playlist on electronics that you could find at my website www.
videodashtutor. net so if you want to see a playlist with electronics topics feel free to take a look at that and for those of you who want to see what equipment i've used when building electronic circuits and other science and tech stuff you can check out my amazon store it's amazon. com slash shop slash the organic chemistry tutor so if you want to see what type of multimedia i've been using or what type of electronic components that i've experimented with and if you want to test it out yourself you could find it all at this link so feel free to take a look at that when you get a chance now the next thing that we're going to talk about is the voltage divider network using resistors because the potentiometer is very useful in this case so let's say we have two resistors and both of them is 10 ohms and let's say we have a 12 volt battery the voltage across the 10 ohm resistor will be half of the 12 volt battery and let me give you the formula associated with this type of circuit let's call this r1 and r2 and let's call the voltage of the battery v and we're going to say the voltage across r2 is the output voltage the output voltage is going to equal the input voltage times r2 divided by r1 plus r2 now let's adjust this circuit now this circuit works by reducing the voltage through power dissipation the energy provided by the battery some of it is dissipated in the form of heat and because we're losing energy we can decrease the voltage but now let's see how we can incorporate a potentiometer in this circuit so this is one way in which you can do it so let's say this is r2 and the potentiometer is r1 let's say r2 is a 1 kilo ohm resistor and r1 is let's say let's make it a 10 kilo ohm resistor and let's use a 12 volt battery so what we're going to do is we're going to calculate the range of the voltage across r2 so r1 can vary between 0k and 10k so let's calculate the output voltage when r1 is set to 0k so we know the output voltage is going to be the input voltage which is 12 times r2 over r1 plus r2 so when r1 is 0k you can just eliminate r1 out of the equation it's going to be r2 over r2 which cancels to 1 so the output voltage will be 12.
so let's say v out is equal to 12 volts now what about if r1 is set to its maximum value in this case r2 is still 1k r1 is 10k so it's going to be 12 times 1k divided by 11k so the minimum voltage of this circuit will be 1.
