0Hello,
In this lesson I will discuss a little about resistance and resistors. First, let's be clear on the main difference between the two, then I will explain the details a bit. Resistance is a property, i.e., it is a "quality" of the electronic and electrical components, whereas Resistors are electrical components that are used to introduce the resistance in a circuit, wherever we require. The formula that we study of (voltage x current), is the formula for finding resistance. The unit of resistance is "Ohm" after the scientist who gave the formula.
Now any electrical component or circuit or equipment, even your ammeters and volmeters will have certain resistance in them. There's nothing to be shocked or surprised about the fact that a component not named resistor can have resistance. Even the human body has some resistance of its own.
Now, the value of this resistance will vary from component to component. In the labs, we have an ohmmeter to measure the resistance of a resistor.
Now, what is this Resistance?
It's taken from the english word "resist". It simply means to oppose. Now in the electrical world, it retains this meaning. Resistance is simply the property of any component to avoid the flow of current. Current, in turn, is flow of electrons (negative charge) in any electrical component.
Conventionally, direction of flow of current is taken to be opposite to direction of flow of electrons.
There are some substances, specially some metals (that we study about in Chemistry), that allow current to flow freely and so, are said to have very low resistance, because they don't resist/ oppose to it.
Similarly substances that oppose flow of electrons very strongly are said to have very high resistance. They are called Insulators. There are some in between substances, that don't oppose the flow of charge as strongly but don't let them flow as freely. Such components are termed as Semi-Conductors.
There's another formula of Resistance based on physical property of the material rather than current flowing through it. This one seems more scary than the easy VxI formula (also called Ohm's formula), but is more helpful when you want to select a resistor for your circuit. Now before we go to it, you might ask that is everything has a resistance then why do we have these resistors? Now it might happen while you are studying a circuit, that the components do not provide enough resistance as you would like. So we use these specially made resistors to provide for the scientific study.
Having that little confusion out of the way, let's now look at this new formula that seems really easy to forget during the exams.
Mathematically, it is given as
R=ρl/A
where ρ= Resistivity of the material, it depends on the particular substance that is used to make resistor
l= length of the resistor
A = cross- section area of resistor, i.e. area of resistor seen from any end of resistor, for practical purpose you may understand it as the area where electrons flow.
We understand the resistivity as a property of material. Length is also pretty much clear. We generally find it hard to understant cross section area at an early age.
Now imagine you are walking on the bridge. If the bridge is wider, you can walk with more friends together. Otherwise, some of you will be left behind and some will be walking in front of you.
Now how many friends can walk together is determined by the width of the bridge. This is similar to cross- sectional area of wire through which electrons are trying to pass.
If cross sectional area is large, more electrons can flow together, and therefore, opposition to electrons is less. So we say resistance will be less.
If length of the bridge is more, more people will be on it at any given time, even if they don't reach the otherside at same time. Same is true for resistor wih longer length.
If bridge is weak, then less people will travel on it. Same is true for resistor with higher resistivity.
Given the information, if any one of the variable is missing, it can be found from the formula.
I hope you gained some perspective of Resistors and Resistance. If you enjoyed reading it and would like me to teach you then please feel free to understand. Doubts are most welcome. See you next time.
