it is likely that they have ever seen a resistor like this in a circuit or even in a heater but how does a resistor works internally? first we must clarify the concept of electricity and for this we are going to use this piece of cable this cable has electrons, which are sub-atomic particles of negative charge and that they can move freely. when a potential difference or voltage is applied between the two ends of the cable the electrons are forced to move and it is to this movement that it is known as electric current.
however, even though the electrons can move through the wire, not everything is so easy there is a resistance that Opposes the flow of electrons this is: the electrical resistance, and its unit of measurement is the Ohm. In fact, since our own body is capable of conducting electricity We could say that we ARE a resistor, and even that in these two circuits the resistance is equivalent but clearly one of the two options is more viable to implement. One of the main differences is that each one has a coefficient of resistance determined by the material of which they are made in fact, there are multiple classifications depending on this coefficient of resistance If a material has a low coefficient of resistance it is said that this is a conductor, such as copper or gold.
On the contrary, if its coefficient is higher, it is said to be an insulating material such as glass or plastic but there are also materials with intermediate values, and these are known as semiconductors, which, in some cases, vary their conductivity depending on other external factors as we saw in the previous video about diodes. and finally, although they are an extreme case, there are also superconductor materials which when they are below a certain temperature decrease their coefficient of resistance drastically we now know roughly how the coefficient of resistance affects the behavior of the material And if one day we find two cables of exactly the same dimensions but made of different materials, such as: one copper and one iron. after having seen a table like this, we could say with complete certainty that the resistance of the iron cable is greater.
but, we could not say what is the value of the resistance of any of the two cables because only that information is not enough to know. let's focus on the cable copper. The resistance of this cable will be equal to the coefficient of resistance multiplied by the length of the cable and divided by the area of its cross section let's make some analogies to understand more easily how each variable affects.
let's imagine that this cable is a pipe to which we will add two tanks, one filled with particles that represent the electrons and an empty one, to also represent the potential difference or voltage that will be applied to the cable, when we close the circuit and let the current pass we can see how the particles pass without problems towards the other tank this, because copper has a fairly low resistivity coefficient Now, if we change the copper for another material like iron the pipe will have obstacles inside, in this way, when we let the current pass, it will be more difficult, but not impossible it reaches the other end. that is, it will have a greater resistance. having an insulating material would be like having the pipe completely covered the next two variables are easier to understand if we have a greater distance to travel in the pipeline it is logical that more time and effort is required to get to the other extreme that is, the longer the cable, the greater the resistance.
On the other hand, if we increase the diameter of the pipe, even when the resistivity coefficient continues to affect the entire volume There are going to be more possible ways for electrons to pass. in other words, the greater the cross-sectional area of the cable there is less resistance to the passage of the current. Now let's go back to the real version before my computer melts by doing these simulations.
although this formula is usually used for its simplicity you should know that the temperature can also affect the value of a resistance since the coefficient of resistivity you find in the tables like this specify the temperature at which that value is correct. although most metals increase their resistivity coefficient by increasing their temperature this is not true for all materials It's important to mention that in reality almost never will have only one cable between the two poles of a power source as this could generate a short circuit, that is, a sudden increase in the amount of current that passes through the conductor I say almost, because when you want generate heat, as for example in a heater, basically that is what is done. To this phenomenon by which a conductor emits heat when a current passes through the It is known as the Joule effect.
and we can calculate the energy dissipated in the form of heat as the multiplication between the voltage applied to the driver the intensity of the electrical current that is happening and the time during which this occurs. The limitation of doing this is that the material could be melted or oxidize extremely fast, leaving it unusable That is why for such cases, alloys such as Nichrome are usually used which, in the first place, has a melting point of 1400 ° C and also has a high coefficient of resistance this last characteristic is precisely the reason why only that section is heated and not the cable that we plugged in. going back to the main topic.
Now that we know how to get a quantity of Ohms to our liking in theory we could create our own resistance using an extremely thin material with a high coefficient of resistance but in reality they are not like that if they were just a wire It would be extremely difficult to get an accurate resistance in such a small size. There are different ways to create a resistance depending on how many Ohms you want to get and how accurate is its value. The first way is using a nickel wire wound in a ceramic tube which by the way is an insulator in this way, you can control the total resistance modifying the length of the cable, but maintaining a compact size.
the second option is using a composite material in a defined volume but that by varying the elements that make it up you can vary its coefficient of resistance and therefore the resistance of the resistor. and the third option, is by means of a ceramic cylinder covered by a carbon film, which is cut in a spiral until obtaining the desired resistance. in other words a carbon wire is gradually created to increase the resistance to the desired value Aa generally these resistances are so small It would be quite difficult to print what their value is, that's why was invented a code of colors by which we can know its value in Ohms, even without numbers for example, this is a resistor of 200 kilo Ohms, The way to read its value is as follows: the first and second bands correspond to digits from 0 to 9 in this case red is 2 and the black is 0 then, the third band corresponds to a multiplier to avoid the need to put many black bands when representing large values in this case, the yellow band means that you have to multiply by 10000 so 2 0 per 10000 gives us 200000 ohms or 200 Kilo omhs.
and the last band that is left corresponds to the tolerance of this value Well, as we saw earlier, a lot of precision is required to generate an exact value. in this case, gold means that the tolerance of the value is ± 5% of the defined value. may happen that at some point you will find a resistor with more bands and its reading will vary slightly, but the logic is the same If you search for "color resistance code" on Google you will find many options to download.
but I also wanted to make one for you and you can download it for free on my Patreon page As you can imagine, it is unlikely that there are resistors of all values, so there are different ways to mix known resistors to get a desired value the first way is connecting resistors in series whereby the value of the equivalent resistance is equal to the sum of the connected resistors. this is very easy to remember if we think about the formula that we saw at the beginning by connecting two equal resistors in series the only thing we are doing is multiplying the length of the cable by two and therefore its value will be double On the other hand, if we want to reduce the total resistance of a set of resistances what we can do is connect them in parallel. if we connect two equal resistances in parallel and think again in the formula, we will realize that what we are doing now is simply multiplying the cross-sectional area by two that is, we are going to have half the resistance.
for more complex systems the way to calculate it is something like this but the important thing is that you understand why it is like that. At this point I think you are ready to start talking about what a resistance is for. the use of a resistance with a static value in a circuit allows us to regulate the voltage that will be generated in different components suppose we have a battery of 12 Volts and an LED that works at a maximum of 3 Volts If it exceeds this value, it will burn.
in this extremely simple circuit just by putting a resistance of the right value we can make exactly 3V pass through the LED In this video I do not want to go into much detail about how to calculate the value of the resistance that we would need but, if you want to continue on your own, I summarize 3 extremely important laws the first is Kirchhoff's voltage law, which tells us: that if we add all the voltages following a closed path in a circuit the value must be zero or in other words, the sum of the voltage in each of the components in this trajectory must be equal to the voltage in the power source that is supplying them. the second, is the current Kirchhoff law, which tells us that in each node, that is, where there is more than one possible path for the current the current that enters must be equal to the one that comes out. and finally the third law is Ohm's Law which tells us that the voltage in a component is equal to the current that passes through the multiplied by its resistance which by the wayz allows us to calculate any of the three variables as long as we have the other two.
as I said, a resistance allows us to control the voltage that passes through other components, and there will be times when we want to modify that voltage during the use of the circuit, not only during the design stage. and this is where the Potentiometer appears. the way a potentiometer works is using an arc-shaped resistor which, by adding a point of contact with another terminal right between its ends, act as if they had two resistors in series.
in this way, if we measure the resistance between the first terminal and the intermediate one, we will obtain a value, whereas if we measure the third terminal and the intermediate we will obtain another value. however, by measuring the resistance between the first terminal and the last This will always be the same, since being in series must be added.
