Materials lesson 02

Class:  Engineering       sub: Engg materials                                           LESSON 02

The material we as students of electronics are interested in are called SEMI-conductors. Having 4 valence electrons they are neither good conductors nor bad conductors. In fact when they join with other atoms to form molecules they become perfect insulators. WHY and WHEN?

By doing something at the time of making them we can CONTROL the electrical properties. By electrical properties we mean resistive values; we can make good, better conductors using the same SIZE of the material brick.

We know what an atom is. What an ion is.

When does an atom become an ion?

We told you that an atom is ELECRICALLY NEUTRAL, meaning that if we touch it, we do not get electrical shock. WHY? Because number of electrons are equal to number of protons. Lighter electrons are held together by force of attraction.

Suppose we remove one electron from an atom. Then the atom is less negative now. It is POSITIVE. It can accept one electron. It became an ion. REMEMBER this.

At ambient temperature ( room temp) different materials will be needing less or more energy to make an electron FREE. If there are less number of free electrons available then less charge will be carried across, some materials may not have a single free electrons (insulator).

In our lesson on atom we raised a question. When an electron becomes FREE where does it go? It cannot go outside the material brick. WHY?

And we know there are so many free electrons in a piece of material. Where do they go?

Imagine you go to a party like marriage function. You see number of tables lined up with various types of food items like vegetables, poories, dahi etc. And there are some number of chairs. You fill up your plate and sit at some chair. Most people sit at chairs with their plates. Suddenly you get up to take a second helping of achaar. You get up and go to the food table. But when you come back you find that someone else is sitting on your chair. You keep standing till somebody else gets up from his chair to fetch poorie and then you go and sit on his chair. This is what is happening all the time. Some number of guests are always STANDING because there are no chairs empty. That is for SOME small period of time you are FREE, you have no chair to sit. Now suppose a policeman orders that all persons who are FREE must leave the party and meet him outside, you become MOVING electron, carrying your plate full of food (charge) and you disappear. More guests not knowing what happened come inside for food and meet the same fate. The process continues. Guests come, pick up food and go out. This is CURRENT.

You can imagine children playing the game KHO-KHO. It is the same thing.

But we want to carry large currents. We cannot fully depend upon the electrons made free from the semi-conductor materials we found. We need more electrons. How to get them?

Again suppose we think of the party you went to. If there are large number of guests, there must be large number of waiters who can lay food table, serve the food. NOTE that for us electron is an electron. Imagine waiters wearing similar good clothes as the guests. We cannot tell who is the waiter and who is guest; both are looking smart. Waiters were supplied by the catering contractor. Since we cannot tell waiters from guests even some of these waiters can start sitting on the empty chairs like guests. Also when the police comes anybody holding food plate goes out. In this case in addition to people getting up to take chatni, there were large number of waiters also looking like and behaving like guest and carrying food plates. So there are available large number of electrons ( made free + free supplied by contractor). Thus, for the same policeman large number of guests go out. More current.

We have made a material better conductor!

Now you see there are two types of FREE electrons:

  1. Those released from atoms
  2. Those added from outside.

This is possible because we were able to add more FREE electrons from outside. It is like adding a bucketful of colored water into the tub on Holi. It makes the color darker.

HOW TO DO THIS? – in the next lesson.

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