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The laws of Physics Topics are used to explain everything from the smallest subatomic particles to the largest galaxies.
Explain the Process of Electrostatic Induction? What do you mean by Bound Charge and Free Charge?
When a charged body is brought near, but not in contact with an insulator or insulated uncharged conductor, opposite charge is produced at the near end and similar charge at the far end of the insulator or conductor. The charges so produced disappear as soon as the charged body is removed. This phenomenon is called electrostatic induction which is defined as the temporary charging of a body by the influence of nearby charges.
Experiment: A positively charged glass rod A is brought near the end B of an insulated uncharged conductor BC [Fig.], Now the following operations are performed.
i) The disc of a proof plane is made to touch the end B. Next the proof plane is brought near an uncharged gold-leaf electroscope without actually touching it. It is found that the leaves diverge. Hence the end B of the conductor must be charged. The proof plane is now discharged by touching with hand. If the other end C of the conductor be examined, it will be found that this end is also charged.
But the middle of the conductor BC will be found to have practically no charge. So it may be concluded that induction produces charges only at both ends of a conductor.
ii) The nature of the charges induced, can he tested with a charged gold-leaf electroscope. Suppose we take a negatively charged electroscope. Keeping the glass rod A near the end B, a proof plane is made to touch the end B [Fig.]. Now ir is taken near the negatively charged gold-leaf electroscope. It is observed that the divergence of the leaves increases which shows that the end B of the conductor is negatively charged.
The gold-leaf electroscope is now charged positively. The proof plane is discharged by touching with hand. The end C of the conductor is touched with the proof plane. It is brought near the gold-leaf electroscope and it is found that the leaves diverge further. So the end C is positively charged, i.e., it has a charge similar to that of the glass rod.
In this experiment if, instead of the glass rod A, an ebonite rod rubbed with flannel be taken, it will be found that the end B has positive charge and the end C has negative
charge. In the middle of BC there is no charge [Fig.].
iii) Now if the glass rod or the ebonite rod be removed from the vicinity of the conductor BC. it will be found that no charge exists either at the end B or at the end C. So if the inducing body is removed, the charges of the conductor disappear. It is proved that under the influence of a charged body an uncharged conductor
- becomes temporarily charged,
- develops unlike charge at the near end and like charge at the far end, and O the induced charges disappear as soon as the charged body is moved away from the conductor.
Explanation of Electrostatic Induction from Electronic Theory
Electrostatic induction In conductor: Electrostatic induction in a conductor can be easily explained by the electronic theory. Every conductor has a large number of free electrons which can flow from one atom to another within the conductor. In the first experiment [Fig.], due to the presence of the positive charge in rod A, some free electrons in the conductor BC are attracted to the end B and cause an abundance of electrons at that end.
Hence the end B becomes negatively charged. On the other hand, a deficit of electrons by the same number has occured at the end C and so it becomes equally positively charged.
In the second experiment [Fig.] the negative charges of the ebonite rod repel the free electrons from the end B to the end C causing an excess of electrons at C and a deficiency of electrons at the end B. Thus the end C becomes negatively charged and the end B positively charged.
It may be noted that induction only changes the arrangement of the free electrons in the conductor. The total number of electrons in the conductor remains the same. So when the charged body is removed from the vicinity of the conductor, the electrons are redistributed uniformly and the conductor becomes uncharged.
Electrostalic induction in insulator: Insulators have no free electrons. Electrostatic induction takes place in the insulators due to polarisation of atoms placed in an electric field. If a positively charged body is placed near an insulator, the closest atom of the insulator finds itself in the field of the positive charge.
This attracts the electrons of the atom and repels the nucleus. So a deformation takes place in the structure of the atom. As a result, the nucleus of the atom is no longer symmetrical with respect to the electrons Fig. A slight relative displacement occurs between the positive and the negative charges inside the atom. This happens for all the atoms of the insulator [Fig.]. This is called polarisation of atoms. When a positively charged body Is brought near an insulator, the negative charge of each atom is
pulled towards the body while the positive charge is repelled in the opposite direction. Since both types of charges exist in equal amounts inside an insulator, they neutralise each other. Hence, the inside of an insulator is electrically neutral. Just the opposite phenomenon will happen if a negatively charged body is brought near an insulator.
Inducing Charge and Induced Charge : (Free Charge and Bound Charge)
Inducing and Induced charge: The charge responsible for creating induction, is called inducing charge and the charge created due to induction is called induced charge. In section 1.8, the positive charge on the glass rod A [Fig.] or the negative charge on the ebonite rod [Fig.] is inducing charge and the charge developed in the conductor BC is induced charge.
Free and bound charges: The induced charge at the end of a conductor near the inducing charge is opposite in nature to that of the inducing charge. Hence the charges induced at the near end of the conductor remain immobile due to the electrostatic force of attraction and cannot escape by con-duction. These fixed charges at the near end of the induced con-ductor are called bound charges.
The charges induced at the far end of the conductor are of the same kind as the inducing charge and hence a force of repulsion exists between these two. So these charges can immediately flow to the earth by conduction if the conductor is touched by hand. Hence these charges at the far end of the induced conductor are called free charges.
In section 1.8, charges developed due to induction at the end B of the conductor BC are called bound charges and those devel-oped at the end C are called free charges.
A few facts about induction: From the discussions in the previous sections regarding electrostatic induction the following conclusions can be made:
- Two kinds of charges, positive and negative are produced simultaneously due to induction.
- Unlike charge is induced at the near end of the conductor and like charge at the far end.
- Positive and negative charges are induced in equal amounts. 050 it is a temporary phenomenon. The induced opposite charges neutralise each other as soon as the inducing body is removed.
Induction Precedes Attraction
When a charged body is brought near an uncharged body, the uncharged body is attracted towards the charged body. This is due to electrostatic induction. When a charged body is brought near an uncharged body, unlike charges are induced at the near end and like charges at the far end of the uncharged body. The force of attraction or repulsion between two charges varies inversely as the square of the distance between them.
As the unlike induced charge is nearer to the inducing charge than the like induced charge, the attraction between the unlike charges predominates over the repulsion between the like charges. So a resultant force of attraction acts on the uncharged body and the body as a whole is attracted towards the charged body. So induction precedes attraction. Effectively, induction is the cause and attraction is its effect.