# The mystery of conductivity

We have all learned the basic knowledge of electricity in science or physics classes, and we all know that some objects are easy to conduct electricity, while some objects are not easy to conduct electricity. We call objects that conduct electricity easily as conductors, such as various metals, pencil leads, salt water, etc.; objects that do not easily conduct electricity are called insulators, such as plastic, rubber, and dry wood. The same objects, why some become conductors and some are insulators, but the conductors and insulators are interchangeable? Let’s take a look at the relevant knowledge scattered in each chapter of the textbook, expand it, and uncover the mysteries together!

Why are the conductivity of different objects different?

This is mainly related to the charge properties inside the object. The ability of an object to conduct electricity requires that the inside of the object have the conditions to form an electric current. In fact, the formation of electric current is very similar to water flow. To form a water flow, liquid water is required first, because they have the conditions to move freely in different directions, but ice cannot. It is solid and loses the conditions for free flow, so it is the same. Water, ice cannot form water flow. Of course, the liquid water in the reservoir can’t form a flow, also because it does not have the free movement to flow in different directions. If the gate of the reservoir is opened and the objective conditions are changed, the water in the reservoir will flow to the gate under the action of the water level difference, forming a flow.

So what is the first condition for forming an electric current inside an object? From a microscopic point of view, the inside of an object contains atoms that we cannot see, even with an optical microscope. Atoms are composed of smaller particles, namely, atomic nuclei and electrons. Compared with electrons, the nucleus has a large mass and volume and is positively charged. It resides in the center of the atom, while the electron is located outside the nucleus and is negatively charged. The electrons in the nucleus and outside the nucleus are attracted to each other because they have equal positive and negative charges, making the electrons rotate around the nucleus at a high speed, just like the eight planets in the solar system revolving around the sun. The positively charged nucleus and negatively charged electrons in the atom are equivalent to the water in the reservoir (as for whether it is liquid or solid, it needs specific analysis). Due to the large mass of the atomic nucleus, its position is relatively fixed and can hardly move freely. The negatively charged electrons outside the nucleus are attracted by the nucleus and move around the nucleus. However, due to the difference in the amount of charge carried by the nucleus and the distance between the electrons outside the nucleus and the nucleus, the attraction of the nucleus to the electrons outside it is also different. . In this way, it appears that some nuclei have little attraction to the electrons on the outside, and some of the electrons on the outside may get rid of the attraction of the nucleus and move at will. These electrons that can move at will are called “free charges” by us, and the free charges are equivalent. It is due to the liquid water in the reservoir; and some atomic nuclei have a strong attraction to the electrons outside it, and they firmly hold the electrons around it. These electrons cannot move at will, which is equivalent to the frozen solid water in the reservoir. So whether an object can conduct electricity depends on whether there is free charge inside the object. With free charge, voltage is applied to both ends of the object, and these free charges will move in the same direction under the action of the voltage, thus forming a current .

The free charge inside the object is too small, and we cannot directly see the conductive mechanism of the object, but we can use a vivid analogy to illustrate it. We compare an object to a school, and an atom to a class in this school. The teacher is equivalent to the nucleus, and the student is equivalent to the electron. For example, some schools are very loosely managed, and it is difficult for teachers to effectively control students during class, so some students will sneak out of the classroom to play while the teacher is not paying attention. These skipped students are equivalent to free charge. At this time, if something delicious and fun appears at the school gate, it is equivalent to the presence of power sources, attracting students who are wandering around the school to move toward the school gate, forming a “crowd flow”. Equivalent to current. On the contrary, some schools are very strict and the class teachers have strong control ability, and no classmate dared to slip out of the classroom during class. Therefore, when something delicious and fun appears at the school gate, no student can slip out of the classroom and run towards the school gate. In this way, there will be no “crowd flow”, which is equivalent to no electricity.

It can be seen that although there are charges such as nuclei and electrons inside objects, whether the object is easy to conduct electricity depends on whether the electrons inside the object can get rid of the constraints of the nucleus and become free charges. If there is a large amount of free charge in an object, it is easy to conduct electricity and belongs to a conductor; if there is no free charge, it is not easy to conduct electricity and is an insulator.

In fact, conductors and insulators are not absolute, they can also be transformed into each other. For example, in a strictly controlled school, students in every class were afraid to skip class, but a group of mischievous friends sneaked into the campus. They “lurked” at the back door of each class and encouraged the students sitting at the door to secretly The ground slipped out, and the students who slipped out in this way became “free electric charges” and had the conditions for “electricity”. For example, glass is inherently non-conductive, but heating the glass can also become a conductor. The principle of heating is equivalent to those mischievous friends who encourage students to skip class. The electrons in the glass that were originally bound by the nucleus get rid of the nucleus and become free electric charges.