Electrostatics Concept Page - 2

Definition

Basic properties of electric charges

1. Additivity of electric charges: The total charge of the system can be obtained by algebraically adding the charges.
2. Conservation of charge: The total charge of an isolated system is always conserved.
3. Quantisation of charge: Electric charge is always an integral multiple of e where e=1.602192×10−19C$e=1.602192\times {10}^{-19}C$ 

Definition

Comparision Between Electron, Proton and Neutron

Definition

Electrons, Protons and Neutrons

The atom is made of 3 types of particles. They are:
(i) electrons (ii) protons (iii) neutrons. The electron is negatively charged, the proton is positively charged and the neutron has no charge, it is neutral.

Definition

Quantum nature of charge

The smallest charge that can exist is the charge of an electron. Therefore electric charge always exist as an integral multiple of electronic charge (e=1.6×10−19 C$e=1.6\times {10}^{-19}C$). This is also known as the quantum nature of the charge.
⟹q=±ne$⟹q=\pm ne$
where n = 0, 1, 2, 3 ...................

Law

Law of conservation of charge

According to Law of conservation of charge, charge can neither be created nor destroyed. The net quantity of electric charge in the universe, is always conserved.

Definition

Additive nature of charge

Electric charges can be added algebraically. If a system contains n charges q1,q2,q3,.......,qn,$q_1,q_2,q_3,.......,q_n,$ then the total charge of the system is q1+q2+q3+.......+qn$q_1+q_2+q_3+.......+q_n$ .Proper signs have to be used while adding the charges in a system.

Definition

Electroscope

An electroscope has a metal detector knob on top which is connected to a pair of metal leaves hanging from the bottom of the connecting rod.

When no charge is present the metal leaves hang loosely downward.

However, if an object with a charge is brought near the electroscope, one of two things can happen.

If the charge is positive, electrons in the metal of the electroscope are attracted to the charge and move upward out of the leaves. This causes the leaves to have a temporary positive charge and because like charges repel, the leaves separate. When the charge is removed, the electrons return to their original positions and the leaves relax.

Likewise, if the charge is negative, the electrons in the metal of the electroscope are repelled and move toward the leaves on the bottom.  This causes the leaves to have a temporary negative charge and because like charges repel, the leaves again separate. Then when the charge is removed, the electrons return to their original position and the leaves relax.

So an electroscope reacts to the presence of a charge through the movement of electrons either into, or away from, the leaves. In either case the leaves separate.  But the electroscope cannot tell if the charged object has a positive charge or a negative charge - it is only responding to the presence of an electrical charge.

Example

Pith ball electroscope

A pith ball electroscope can be used to determine, if a body is charged or not and the nature of charge on the body. If the ball moves on bringing the body close to it, it is charged and if it does not move, it is uncharged. To test the nature of charge, the ball is first charged by a known body(say positively charged) by conduction. Then the body under test is brought close the ball. If the ball moves away, it has the same charge(positive) and if it attracts, it has opposite charge(negative)

Definition

Applications of Gold Leaf Electroscope

These are some of the applications of Gold Leaf Electroscope :
1.Detect charge :
Body under test is touched with the metal cap. If the leaves diverge, the body is charged and if there is no effect on leaves, then the body is uncharged.
2.To identify the nature of charge :
The electroscope is charged by a known body(say positively charged body) and then the body is removed. Next, the body under test is brought in contact with the metal cap. If the leaves diverge further, the body has same charge(positive) and if the leaves come closer to each other, the body has opposite charge(negative).
3.Identify a body as conductor or insulator :
Take two electroscopes. Charge one of the electroscopes so that its leaves will diverge. Then, connect the two electroscopes by the object under test. If the leaves of other electroscope diverge, the body is a conductor and if there is no effect on the electroscopes, the body is an insulator.

Definition

Construction and Working of Gold Leaf Electroscope

Gold Leaf Electroscope :
It is a simple device to detect the presence of charge on any body.
Construction : It consists of a metal rod which is fitted in an insulating box. Metal rod has a metal knob at its top. Two gold leaves are also attached at the bottom end of the rod.
Working :Since electroscope is used to detect the presence of charge. So through it we can find whether a body is charged or uncharged.
Therefore the body to be detected is brought close enough to the metal knob. When a charged object touches the knob at the top of the rod, charge flows through the rod on to the leaves.
Both the gold leaves will have same charge and hence as a result they will repel and diverge.
The degree of divergence is an indicator of the amount of charge i.e., more the charge, more will be the divergence.

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