Examples of Conservation of Charge
To understand charge conservation in more detail, let us consider a few examples which involve the charging of a material while the net charge of the system remains conserved. There are three different charging methods: charging by friction, charging by induction, and charging by conduction. In this part, we will learn how the net electric charge is conserved in each charging method.
Charging by Friction
Imagine we rub a glass rod with a silk cloth. The electron affinity of the silk cloth is high. Thus, the electrons are transferred from the glass rod to the silk cloth. As a result, the glass rod becomes positively charged, and the silk rod becomes negatively charged.
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Fig. 3 – Charging of silk cloth and glass rod is carried out by the method of charging by friction with the transfer of electrons from the glass rod to silk cloth.
Initially, the glass rod and the silk cloth do not have any net charge, meaning the initial charge is zero, and both objects are said to be neutrally charged.
From Fig 3, we can see that after charging by friction, the electric charge on the glass rod is (+6e) (with each plus symbol representing an amount of charge equal to the elementary charge), and the electric charge on the silk cloth is (-6e) (and the negative signs indicating an amount of charge equal to the elementary charge but with the opposite polarity). The net charge on the system in the final state becomes (-6e+6e=0).
In conclusion, the net electric charge in the initial state equals the net electric charge in the final state. This demonstrates the law of conservation of electric charge.
Charging by Induction
Imagine we place the initially positively charged glass rod near the uncharged metal sphere. Due to the electrostatic force of attraction, the negative charge will collect on one side of the sphere facing the rod. However, due to the electrostatic repulsion force, the positive charge will gather on the other side of the sphere.
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Fig. 4 – Polarization of metal sphere due to the presence of charged glass rod in the proximity of the metal sphere.
In the initial state, the electric charge on the glass rod is (+6e), and the electric charge on the metal sphere is zero. Therefore, the net electric charge in the initial state is (+6e+0=+6e).
After the charging by induction, the electric charge on the metal sphere is (-6e+6e=0), and the electric charge on the glass rod is (+6e). So, the net electric charge in the final state is (0+6e=+6e). In conclusion, the net electric charge in the initial state equals the net electric charge in the final state. This example once again demonstrates the law of conservation of electric charge.
Charging by Conduction
Imagine we place a charged metal sphere in contact with an uncharged metal sphere. Then the charge transfer takes place from charged sphere to the uncharged sphere till the net charge on each sphere becomes the same.
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