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Electric charges and fields class 12 notes, electrostatic induction, charging by induction

Electric charges and fields class 12 Notes

All the substances are made of small particles called atoms, which are made of three elementary particles namely electrons, protons, and neutrons. Because of their masses, these particles exert gravitational forces on each other. However, in addition to gravitational force protons and electrons exert an exerted force is called electric force. Therefore, apart from their masses protons and electrons must possess some additional intrinsic property.

The additional property of protons and electrons which gives rise to electric force between them is called electric charge.
Electric charge is a scalar quantity. A proton possesses a positive charge +e while an electron possesses an equal negative charge -e where

e= 1.6* 1/10 coulomb

Electrostatic Induction Class 12 Notes

We know that a body can be charged by rubbing it against some other suitable body. If another body is brought in contact with such a charged body, it also gets charged. It acquires the charge of the same sign as that of the charged body. If a positively charged body is brought in contact then electrons flow from it to the body to be electrified. Such a process is called the charging of a body by conduction.
However, there is another method by which a body may be charging. If a charged body say a glass rod rubbed with silk is brought near an isolated conductor it is found that a charge of an opposite kind is developed on the near end of the conductor and a charge of the same kind on the far end. However, if the charged body is moved away from the conductor the charges developed on its two ends disappear.
This temporary electrification of a conductor, When a charged body is brought near it is called electrostatic induction.
In contrast to conduction, there is no transfer of electrons between the two bodies during charging by induction. It is because no physical contact takes place between the charging body and the conductor.

Explanation. When a positively charged glass rod is brought near the conductor AB, it attracts the free electrons present in the conductor towards it. As a result, the electrons accumulate at near end A and therefore, this end becomes negatively charged. As far end B becomes deficient in electrons, it acquires a positive charge. The opposite kind of charge appears merely due to the redistribution of electrons in the conductor. This redistribution of electrons takes place instantaneously when the charged glass rod is brought near the conductor.

If instead of a positively charged glass rod, a negatively charged ebonite rod is brought near the conductor, the electrons in the conductor will get repelled. Likewise, the near-end A becomes positively charged and the far-end B becomes negatively charged.

The two kinds of charges that appear on the two ends of the conductor due to electrostatic induction are called induced charges. The charge present on the charging body (used to produce electrostatic induction) is called the inducing charge.

It is found that either kind of induced charge is equal to the inducing charge. If the charging body is removed, the charges induced on the conductor disappear. Thus, induced charges stay on the conductor only so long as the charging body is kept held near the conductor.

Charging By Induction class 12 notes

Electric charges and fields class 12 notes, electrostatic induction, charging by induction

A conductor may be charged permanently by induction in the following four steps:

Step 1. To charge a conductor negatively by induction, a positively charged glass rod is brought close to it. The near-end A of the conductor becomes negatively charged, while its far-end B becomes positively charged as shown in Fig. 1.02(a).

Step II. Keeping the glass rod in its position, the conductor is connected to the earth (by touching it with a finger) as shown in Fig. 1.02(b). The positive induced charge will disappear. Earlier, it was thought that the positive induced charge flows to the earth. In fact, electrons flow from the earth to neutralize the positive charge on the far end of the conductor. The negative induced charge on the near end of the conductor remains bound to it due to the attractive force exerted by the positive glass rod.

Step III. The conductor is disconnected from the earth (by removing the finger), keeping the glass rod still in its position as shown in Fig. 1.02(c). The near end of the conductor continues to hold the negative induced charge. Step IV. Finally, when the glass rod is removed, the negative induced charge on the near end spreads uniformly over the whole conductor as shown in Fig. 1.02(d). The conductor can be positively charged (by using a negatively charged ebonite rod) by following the above four steps in the same order.


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