Electric Charge Origin
It is known that all matter is made up of atoms and/or molecules, the basic unit being an atom; we also know that every atom consists of a central core called the atomic nucleus, around which negatively charged electrons revolve in circular orbits. Every atom is electrically neutral, containing as many electrons as the number of protons in the nucleus. Thus even though normally, the materials are electrically neutral, they do contain charges, but their charges are exactly balanced.
The vast amount of charge in an object is usually hidden as the object contains equal amounts of positive charge and negative charge. With such an equality or balance of charge, the object is said tube electrically neutral, it contains no net charge.
If the positive and negative charges are not in balance, then there is a net charge. Thus an object is charged if it has a charge imbalance or some net charge. Hence, to electrify or charge a neutral body we need to ass or remove one kind of charge. Then we say that a body is charged, we always refer to excess charge or deficit charge.
In solids some of the electrons are less tightly bound in the atom. These are the charges which are transferred from one body to the other.
When we rub two insulating substances against each other, we provide energy to overcome friction between them. This energy is used in removing electrons from one substance and transferring them to the other. The transfer takes place from the material in which electrons are he3ld less tightly to the material in which electrons are held more tightly electrons are transferred from the material whose work function is lower to the material whose work function is higher. Consequently the material which loses electrons acquires a positive charge and materialwhcih gains electrons acquires an equal negative charge. For example, when we rub a glass rod with silk electrons acquires an equal negative charge. For example when we rub a glass rod with silk electrons are transferred from glass rod to silk. The electrons are transferred from glass rod to silk. The glass rod becomes positively charged and silk acquires an equal negative charge. Thus charging by rubbing is due to actual transfer of electrons.
The cause of charging is actual transfer of electrons broom one material to the other. The insulating material with lower work function loses electrons and becomes positively charged and vice-versa.
Further, as an electron has a mass, howsoever small it may be therefore there does occur some change in mass on charging. A positively charged body has lost some elections and hence its mass reduces slightly. On the other hand, a negatively charged body has gained some electrons and hence, its mass increases slightly.
Note in rubbing the number of electrons that are transferred, is a very small fraction of the total number of electrons in the material body. Hence the charge acquired by frication is a very small fraction of the total positive and negative charge content of the body.
Further, as only the less tightly bound electrons in a material body can be transferred from it to another by rubbing, only under suitable conditions; we have to stick to cretin pairs of materials to observe charging on rubbing the bodies.
In table , we have listed the pairs of objects which get charged on rubbing against each other. They have been divided into two classes, one acquiring positive charge and the other acquiring negative charge o rubbing.
Table: objects acquiring two kinds of charges on rubbing
Positive charge
|
Negative charge |
| Glass rod |
Silk cloth |
| Fur or woolen cloth |
Ebonite amber rubber rod |
| Woolen coat |
Plastic seat |
| Woolen carpet |
Rubber shoes |
| Nylon or acetate |
Cloth |
| Dry hair |
Comb |
Obviously any tow charged objects, in the same column repel each other and any tow charged objects from different columns attract each other.
Charging by induction
We know that a body cane charged by putting it in contact with another charged body either directly or by means of a conductor. For example, when a charged glass rod is in contact with a pith ball or connected to it by a copper wire, it transfers some of its charge to the pith ball. This is charging by conduction.
In charging by induction a charged body a imparts to another body B, some charge of opposite sign without any actual contact between A and B. obviously, body A shall not lose any charge as it is not in contact with B.
The steps involved in charging a metallic sphere by induction
(a) To begin with a metallic sphere on an insulating stand is uncharged.
(b) When a charged glass rod is brought near the uncharged metallic sphere, free electrons of the sphere are attracted and start piling up at the near end. This end therefore, becomes negatively charged and the farther end of the sphere becomes positively charged due to deficit of electrons. The redistribution of charge is almost instantaneous and stops as soon as net force on free electrons in the metallic sphere becomes zero.
(c) When the sphere is grounded it is connected to earth by a conducting wire, electrons flow from the ground to the sphere and neutralizes the positive charge on the farther end of the sphere. The negative charge at the near end of the sphere remains bound there due to attractive force of glass rod.
(d) When the sphere is disconnected from the ground, the negative charge continues to be held on the near end.
(e) When the glass rod is remolded the negative charge spreads uniformly over the sphere. Similar steps are involved when a negatively charged rod is used for charging the sphere positively by induction.
Let us Noe understand how we charge tow spheres by induction fig. shows tow metal spheres A and B supported oninsulatig stands, held in contact with each other,.
Let a positively charged glass rod be brought meat the sphere A free electrons in both the spheres are attracted towards the rod therefore left surface of left sphere A has an excess of negative charge and right surface of right sphere B has an excess of positive charge. Note that all the electrons in the spheres have not accumulated on the left surface of sphere A. this is because as negative charge starts building up at the left face of A, further electrons are repelled by these. Equilibrium is reached almost instantly under the act in of force of attraction of the rod and the force of repulsion due to the accumulated negative charges shows this equilibrium situation.
Further the accumulated charges would remain on the surfaces as shown till the glass rod is held near the sphere A. if the rod were removed the charges would return to their original neutral state in the absence of any outside force.
Separate the spheres A and B by a small distance while the glass rod is still held near the sphere A the two sphere carry opposite charges as shown in they attract each other.
Remove the glass rod.
This is how two metal spheres get oppositely charged by induction. Note that in this process of electric induction, the positively charged glass rod does not lose any charge. This is contrary to charging by conduction charging by actual contact where the charged glass rod loses some charge.
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