Class 12 Chapter 1 – Electric Charges and Fields
Electric Charges and Its Properties
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Electric Charge
Study about static charges or charges at rest is called electrostatics.
The name electricity is coined from the Greek word electron meaning amber. When a pair of suitable materials (like glass rod and silk / amber and wool) rubbed together, they show properties of attraction and repulsion.
There are two kinds of an entity called electric charges Positive and Negative. i) like charges repel each other and ii) unlike charges attract each other. The property which differentiates the two kinds of charges is called the polarity of charge.
By convention, the charge on glass rod is called positive charge and charge on silk is termed as negative. That is loosely bound electrons in glass rod are transferred to silk. Charge is a basic property associated with the elementary particles.
This electricity produced by rubbing two suitable bodies and transfer of electrons from one body to other is called frictional electricity.
SI Unit of Charge
S.I. unit of charge is coulomb and is denoted by C. It is defined in terms the unit of current. So, 1 coulomb is the charge flowing through a wire in 1 s if the electric current in it is 1 A.
Methods of Charging
1. Charging by Friction
When a pair of suitable materials (like glass rod and silk / amber and wool) rubbed together, loosely bound electrons from one material will move to the other material makes one positively charged and the other negatively charged. This is Mostly observed with insulators.
2. Charging by Contact
In this method, a charged body is placed in contact with another body, the later body will get charged. This is due to the transfer of electrons from one body to other. That results in decrease of charge in charged body. Whether they will equally distributed, that depends on material, size and shape of the body.
3. Charging by Induction
Another method is called induction. The method is as follows:
Take two metal spheres A and B, supported on insulating stands.
(i) Bring the metal spheres, A and B, in contact. Fig (i)
(ii) Bring a positively charged rod near one of the spheres, say A. Fig (ii)
The positive rod attracts free electrons in the spheres towards it. This makes the left surface of sphere A negatively charged and right side of sphere B positively charged.
(iii) Separate the spheres by a small distance while the glass rod is still held near sphere A. The two spheres are found to be oppositely charged and attract each other. Fig (iii)
(iv) Remove the rod. Now, separate the spheres quite apart. The charges on them get uniformly distributed over them, as shown in Fig (iv)
In this process, the metal spheres will each be equal and oppositely charged. This is charging by induction. The positively charged glass rod does not lose any of its charge, contrary to the process of charging by contact.
Note:
Why electrified rods attract light objects when brought near to it? When electrified rode brought near light objects, the rods induce opposite charges on the near surfaces of the objects and similar charges move to the farther side of the object. The centers of the two types of charges are slightly separated. Since the magnitude of force depends on the distance between the charges, therefore the force of attraction is more than force of repulsion. So light objects are pulled towards the rods.
Conductors, Insulators and Semiconductors
Substances which allow electricity to pass through them easily are called conductors. They have electric charges (electrons) that are comparatively free to move inside the material. Example: Metals, human and animal bodies, earth etc.
Substances which offer high resistance to the passage of electricity through are called insulators. Example: glass, plastic, nylon, wood etc.
The number of free electrons in a semiconductor is much lesser than the conductor, so the behavior is in between a conductor and an insulator. It behaves as an insulator at 0 K temperature. But at higher temperatures, some electrons free themselves and they respond to applied electric field thus causes conduction. A freed electron in a semiconductor causes a vacancy in its bound position and this vacancy also helps in conduction. Example: Silicon
Note:
When a charge is transferred to a conductor, it readily gets distributed over the entire surface of the conductor. That is the reason why when metals rubbed, they are not get charged. The charges on metal leak through our body to the ground as both are conductors of electricity.
If some charge is put on an insulator, it stays at the same place. This is the reason when materials like glass, plastics rubbed, get charged.
Earthing
When a charged body is in contact with the earth, all the excess charge on the body disappears by causing a momentary current to pass to the ground through the connecting conductor. This process of sharing the charges with the earth is called grounding or earthing.