Electric Charge

The Universe is not possible without electric charge. If you consider the smallest entity of matter, you end up with something called an atom. First we detect the kinds of charges and how they react with each other. Later we discuss the structure of an atom and how an atom is electrically charged. We also focus on the conservation of electric charge.

Kinds of Charges

We can detect that there are two kinds of charges with simple experiments such as rubbing something over something. One example is which you can also do with a pen and small pieces of paper that you can gently rub your pen in your hair for some time may be for 5 to 10 seconds, put the pen near the pieces of paper without touching them (the tip of the pen rubbed should not touch anything) and you'll see that the pen attracts the pieces of paper. But you can not say that only the pen was attracting the pieces of paper, the pieces of paper were also attracting the pen; it means there was mutual attraction between the pieces of paper and the tip of the pen rubbed.

If you have the same kind of two pens and rub both of them separately and bring the tips of both pens near without touching, the pens now repel each other. You should have taken a bath recently to get the best effect - your hair should be clean. Similar kinds of experiment reveal the fact that there are exactly two kinds of charges. One type of charge is called negative charge and other type is called positive charge.

The experiments tell us that the same charges repel each other and opposite charges attract each other. The same charges are also called like charges and opposite charges are also called unlike charges. It means there is attraction between negative and positive charges (unlike charges) and repulsion between negative charges or positive charges (like charges). Note that it's not necessary to rub your pen in your hair, you can rub it with anything but take something soft for experimental purposes and also it's not necessary to use your pen but instead you can take a rod of plastic or glass. Rubbing pen in hair may seem awkward but do not hurt your hair!

Structure of an Atom

Now we discuss what an atom has in its microscopic level and we discuss the structure of an atom. Mainly an atom is made up of three sub-atomic particles which are electron, proton and neutron. The Figure 1 shows a lithium atom which consists of three protons, two electrons and four neutrons. Proton is positively charged sub-atomic particle, electron is negatively charged and neutron is neutral (that is, having no charge).

Proton and neutron make a core called nucleus where almost all of the atom's mass is concentrated (99.9% of the atom's mass) while the electrons move in space in definite orbits surrounding the nucleus. The nucleus is a dense core which is surrounded by the electrons which has a diameter of the order of about ${10^{-15}}{\rm{m}}$ and the electrons extend to the distances of about ${10^{-10}}{\rm{m}}$.

The electrons surrounding the nucleus are held by the attractive electric force between protons and electrons in the nucleus. The repulsion between the protons is overcome by the short-ranged (not having effect beyond nucleus) strong nuclear force which binds protons and neutrons in the nucleus.

Figure 1 Structure of an atom.

Any matter in the world is made up of atoms. When you rub one object with another, some electrons of one object move to another object and the one which loses its electrons has greater number of protons in the nucleus and becomes positively charged. And the one which gains electrons has greater number of electrons than protons and becomes negatively charged. Now if you bring these objects closer after rubbing, there will be electrostatic attraction between unlike charges.

Note that electron charge is negative charge and proton charge is positive charge while the neutron has no charge. If a body has no net charge the body is said to be electrically neutral. The magnitude of the charge on an electron and a proton is the same.

Charging by Induction

In electromagnetism the types of matter are considered as conductor, insulator and semiconductor. An object is said to be a conductor if it permits the charge to move from one point to another within the material of the object. A conductor has considerable amount of free electrons which move when electric field is applied to it. For example, copper wire is a good conductor of electric charge.

The metals are always good conductor of electric charge. Now what's the case for an insulator? An object is said to be an insulator if it doesn't allow the movement of electric charge from one point to another within the material of the object. An insulator does not have free electrons or has very low free electrons. You can not find considerable amount of free electrons in an insulator. An example of insulator is a nylon thread.

And there is another type of material called semiconductor which has a property between the conductor and the insulator. This means a semiconductor has some free charges and allow the charges to move within the material. But a semiconductor does not allow the flow of charges as a conductor and does not stop the flow of charges as an insulator.

Now let's focus on charging by induction. First we consider a metal ball $A$ which is initially positively charged hanging on a nylon thread. You can see Figure 2 for illustrative purposes. When you bring the charged metal ball near an uncharged metal ball $B$ resting on an insulating stand as in Figure 2(a), negative charge is induced on the ball $B$ on the side near the ball $A$.

Figure 2(a) Negative charge is induced on the near side of ball $B$ from ball $A$ while positive charge is induced on the far side.
Figure 2(b) Free electrons from the ground flow towards the ball $B$ and the induced positive charge on its surface gets those free electrons. Hence there is no induced positive charge after connecting the ball to the ground.
Figure 2 Charging by induction

This happens because there is electrostatic attraction between opposite charges and repulsion between the same charges. The positively charged metal ball attracts electors towards it (because there are already free electrons in the uncharged metal ball) and pushes the positive charge away form it. Therefore, positive charge is induced on the far side of ball $B$ until an equilibrium condition is reached; that is the attractive force between positive charge on ball $A$ and negative induced charge on ball $B$ is balanced by the attractive force between negative and positive induced charges on the ball $B$.

The ball $B$ is now connected to the ground by a conducting wire (see Figure 2(b)). Note that the Earth is a conductor and already has free electors and some of those free electrons flow through the wire to the ball $B$ where the induced positive charge get those electrons and no positive charge remains on the ball $B$ (but there is still negative induced charge until the ball $A$ is placed near ball $B$).

When we first remove or disconnect the conducting wire from the ball $B$ and later remove the ball $A$, only a net negative charge remains on the ball $B$. But if you remove the ball $A$ first, the excess negative charge on the ball $B$ (the charge that flowed to the ball from the ground) flows back to the ground through the conducting wire and the ball becomes electrically neutral again. In this way you can charge a metal ball by induction.

Conservation of Electric Charge

No proof until now has been found against the principle of conservation of electric charge. It states that the net charge inside a closed system is always conserved. Similar to the conservation of energy electric charge can not be created or destroyed. And another important thing is that electric charge is quantized.

The smallest possible charge is the charge on an electron or proton. The magnitude of charge of a proton is the same as the magnitude of charge of an electron. The basic unit of charge is therefore the charge on a proton or an electron. The electron charge is negative charge and the proton charge is positive charge, and if we add all the positive and negative charges in a body we finally get a net charge. If this net charge is zero, the body is electrically neutral or the body has no charge. It means that the net electric charge in a body is the algebraic sum of all the charges (either positive or negative) in the body.

A net charge in a body is the integer multiple of the basic unit of charge, for example if a body has a net negative charge we can represent the net charge as $ne$ where $n$ is an integer and $e$ is the charge on an electron. If there was a net positive charge we can put negative sign as $-ne$ because an electron has negative charge.

Electromagnetism

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