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Charging by Conduction, The Law of Conservation of Charge, Practice Problems, FAQs

Charging by Conduction, The Law of Conservation of Charge, Practice Problems, FAQs

Arun went to the school physics lab and he was amazed to see a small van de graaff generator. The outer shell was made of metal. He touched the shell and he realized that he got an electric shock! He asked his lab teacher why this was happening. Lab teacher told him that the charge was conducted from the shell to his body and he got an electric shock . Let’s understand more about the charging by conduction process in this article.

Table of content

  • Charging by Conduction
  • The Law of Conservation of Charge
  • Practice Problems
  • FAQs

Charging by Conduction

In case of charging by conduction, it involves the direct contact of a charged object with a neutral object which is to be charged.

Let’s take a positively charged metal object which is touched to a neutral metal object. The neutral object gets charged. If a negatively charged metal object is touched to a neutral object, the neutral object becomes negatively charged as the result of being contacted by the metal object.

So here there is a contact between a charged object and a neutral object. Charging by conduction involves making a physical contact between the charged object and the neutral object. This process of charging is also called charging by contact.

The Law of Conservation of Charge

The law of conservation of charges tells, the charge is neither created nor destroyed during this charging process; it is simply transferred from one object to the other object in the form of electrons.

The law of conservation of charge is maintained in the conduction charging process also like any other process. Let’s take the example of the two spheres system. Before the start of the charging process, the overall charge of the system was +Q (say) as the whole system was not neutral. So, there were more protons than electrons in the two spheres combined. Here one of the spheres is charged +Q C and the other is 0 C. When the positively charged sphere is made to touch the other sphere, electrons from the neutral sphere move to the positively charged sphere. After the contact, individual spheres become charged. The quantity of net positive charge on the previously charged sphere becomes +Q-q C and the net charge on the previously neutral sphere becomes +q C. So, the net charge for the whole system remains the same i.e., +Q-q+q=Q C. Here q1.6×10-19  number of electrons moved from the neutral sphere to the positively charged sphere.

It is thus proved that the overall charge on the system of two objects remains the same after the charging process.

Practice problems

Q. A conducting sphere (say A) charged by +1.6 μC is used to charge another similar sphere (say B) which is neutral. After the contact has been made, what will be the number of electrons transferred and from where to where?

A. The spheres A and B are similar in size so their charge capacity will be the same.

As the spheres are touched with one another, the charge in sphere A gets split into two.

So, the neutral sphere gets +0.8 μC of charge. It also means there is a charge transfer of +1.62=0.8 μC from the sphere A. So, there will be a transfer of 0.8×10-6 C1.6×10-19 C=5×1012 electrons from B to A.

Q. A conducting sphere (say A) charged by +2 μC is used to charge another conducting sphere (say B) which is already charged by -1 μC. Now, contact has been made and then they are separated. The charge on A is now +0.8 μC. What will be the number of electrons transferred and from where to where?

A. In this process of conduction, the total charge of the system will remain the same.

The total charge is +2μC+-1μC=1 μC

A loses charge by 2-0.8 μC=+1.2 μC i.e. electrons infused into A

B gains the same amount of charge i.e., +1.2 μC

It means electrons infused into A from B.

The number of electrons will be 1.2×10-61.6×10-19=75×1011

Q. An object initially charged positively is touched to another object and then they are separated. Then this object is brought nearer to a negatively charged object and found that a repulsive force exists between them. Comment on the change of mass for the object if any.

A. At last when the object is brought nearer to the negatively charged object a repulsive force was in between them. So, it implies that the object was also negatively charged, as the like charges repel each other.

Initially the object was positively charged. Now it is negatively charged. So, there is an influx of electrons into the body. Electron is not a massless particle. It has mass but is very small. So, the mass of the body must have increased.

Q. Can a complete charge transfer be possible for two finite bodies, one is neutral?

A. When charge transfers from a body to another body, electrons transfer takes place till the moment when the voltage becomes equal for the bodies. When two bodies are isolated and one of them is neutral, then a complete charge transfer is not possible. As long as a body has a non-zero voltage, it can’t have zero charge.

FAQs:

Q. Explain the charging by conduction using a negatively charged object.
A.
Let a negatively charged metal object be used to charge a neutral object. We know that, like charges repel each other and unlike charges attract each other. Like charges because of the repulsive forces between them they like to spread out as far as possible. A negatively charged object has an excess of electrons. These electrons always like to distance themselves from each other as far as possible.

These excess electrons on the metal object will tend to flow to the neutral object if touched. As soon as the contact between the charged and uncharged object is made, a number of excess electrons from the negatively charged object will flow to the neutral object. Generally, the object offering the maximum space for the similarly charged objects will be the object who will accommodate the most of the excess electrons. As soon as the contact is made, the transfer of electrons is achieved. The previously charged metal sphere will still be negatively charged. This time it has less negative charge i.e., electrons in excess than it had before the conduction charging process.

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Q. Discuss conduction charging requires a conductor.
A.
The charging by conduction process involves the touching of two conductors.

The excess of charge, be it positive or negative, is given a larger area to spread in order to reduce the repulsive forces between them. In this process electrons move and redistribute themselves. Insulators hinder such movement of electrons between the objects. This is observed in the case of charging by induction when the charged objects are prevented from being grounded with the help of insulators. Though the two surfaces are in contact, transfer of charge doesn't occur.

Q. Can an insulator conduct a charge to another object upon touching? Can an insulator be charged by conduction?
A.
We all have used a charged insulator (like plastic, cork) to charge a neutral electroscope or some other object just by making contact. In these cases, the charging occurs by some process which may seem like a conduction process but it is not. There was no sharing of charge between the insulators and the metal parts of the electroscope. Whenever excess charge is acquired by the electroscope, it distributes itself on the surface of the electroscope. But that redistribution doesn’t exactly happen with the insulators.

The charging of an object by contact with another charged insulating object can be described as charging by lightning. In lightning the electrons burst through the space which is air (not vacuum) between the objects. The presence of a charged insulator is quite capable of ionizing the surrounding medium and it allows the excess electrons to travel through the air to the other object. This transfer of charge can happen with or without touching. Similar thing happens on a dry winter day. Dry air is more easily ionized. Thus, a greater quantity of electrons become capable of bursting through the medium between the two objects. A crackling sound is often heard with a flash of light (spark) is seen. This phenomenon of lightning may occur even from several centimetres away. Thus, this process certainly does not fit into the description of contact charging or conduction.

Q. Explain the charging by conduction using a positively charged object.
A.
Let a positively charged metal object be used to charge a neutral object. The fundamental principle of electrostatics is like charges repel each other and unlike charges attract each other. A positively charged object has a deficiency of electrons. Because of this, electrons from other objects with either negatively charged objects or neutral objects will always like to travel to the electron deficient body.

These deficiency of electrons on the metal object will tend to flow the electrons from the neutral object when touched to it. The previously charged metal sphere will still be positively charged. This time it has less positive charge i.e., less electrons deficiency than it had before the conduction charging process.

A positively charged metal object is touched to a neutral metal object. A positively charged metal plate has excess protons. A positively charged metal plate has a shortage of electrons. Proton is tightly bound to the nucleus of an atom. It therefore can’t leave an atom in search of an electron. It attracts mobile electrons towards itself in order to become neutral and stable. When a conducting pathway is provided between a pool of excess electrons and a pool of excess protons, certainly electrons will move. This is why only the electrons move from the neutral object to the positively charged objects when the positively charged metal object is touched with the neutral metal object. After losing electrons to the positively charged metal object, there creates a shortage of electrons on the previously neutral metal object. And the already charged (positively) metal object is still positively charged. Now, it has less positive charge in excess than it had before the contact took place.

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