Induction : A charged object is brought near, but does not touch, a neutral object. The neutral object becomes polarized. In this process, the opposite charge is attracted to the rod and moves closer, while the similar charge is repelled and moves farther away. If the neutral object is grounded, the similar charge will leave the sphere, which results in the sphere becoming oppositely charged in comparison to the original charged object. Nothing leaves the neutral object Once the charged object leaves, the neutral object is still neutral Induction Permanent One is neutral, one is charged None With grounding wire, like charges leave the neutral object Once the charged object leaves, the neutral object is charged opposite to the original charged object.
Barun is correct. This is charging by induction. The positive rod coming close to the electroscope will cause the positive charge in the electroscope to be repelled, traveling down to the leaves which makes the leaves separate. Was this guide helpful? Create a free account to bookmark content and compete in trivia.
Browse Study Guides By Unit. Instant access to every resource you need to get a 5. Ready for College? Talk to a trained counselor for free. Electrons move from the object with weaker hold, to the object with stronger hold.
No grounding wire, like charges move away from the charged object, opposite charges move towards the object. Nothing leaves the neutral object. Once the charged object leaves, the neutral object is charged opposite to the original charged object. The metal sphere is still charged negatively, only it has less excess negative charge than it had prior to the conduction charging process.
The previous example of charging by conduction involved touching a negatively charged object to a neutral object. Upon contact, electrons moved from the negatively charged object onto the neutral object.
When finished, both objects were negatively charged. But what happens if a positively charged object is touched to a neutral object? To investigate this question, we consider the case of a positively charged aluminum plate being used to charge a neutral metal sphere by the process of conduction. The diagram below depicts the use of a positively charged aluminum plate being touched to a neutral metal sphere.
A positively charged aluminum plate has an excess of protons. When looked at from an electron perspective, a positively charged aluminum plate has a shortage of electrons. In human terms, we could say that each excess proton is rather discontented. It is not satisfied until it has found a negatively charged electron with which to co-habitate. However, since a proton is tightly bound in the nucleus of an atom, it is incapable of leaving an atom in search of that longed-for electron.
It can however attract a mobile electron towards itself. And if a conducting pathway is made between a collection of electrons and an excess proton, one can be certain that there is likely an electron that would be willing to take the pathway.
So when the positively charged aluminum plate is touched to the neutral metal sphere, countless electrons on the metal sphere migrate towards the aluminum plate. There is a mass migration of electrons until the positive charge on the aluminum plate-metal sphere system becomes redistributed.
Having lost electrons to the positively charged aluminum plate, there is a shortage of electrons on the sphere and an overall positive charge. The aluminum plate is still charged positively; only it now has less excess positive charge than it had before the charging process began. The above explanation might raise a rather difficult question: Why would an electron on the previously neutral metal sphere desire to move off the metal sphere in the first place?
The metal sphere is neutral; every electron on it must be satisfied since there is a corresponding proton present.
What would possibly induce an electron to go through the effort of migrating to a different territory in order to have what it already has? The best means of answering this question requires an understanding of the concept of electric potential. But since that concept does not arise until the next unit of The Physics Classroom , a different approach to an answer will be taken. It ends up that electrons and protons are not as independent and individualized as we might think.
From a human perspective, electrons and protons can't be thought of as independent citizens in a free enterprise system of government. Electrons and protons don't actually do what is best for themselves, but must be more social-minded. They must act like citizens of a state where the rule of law is to behave in a manner such that the overall repulsive affects within the society at large are reduced and the overall attractive affects are maximized.
Electrons and protons will be motivated not by what is good for them, but rather by what is good for the country. And in this sense, a country's boundary extends to the perimeter of the conductor material that an excess electron is within. And in this case, an electron in the metal sphere is part of a country that extends beyond the sphere itself and includes the entire aluminum plate. So by moving from the metal sphere to the aluminum plate, an electron is able to reduce the total amount of repulsive affects within that country.
It serves to spread the excess positive charge over a greater surface area, thus reducing the total amount of repulsive forces between excess protons. In each of the other methods of charging discussed in Lesson 2 - charging by friction and charging by induction - the law of conservation of charge was illustrated. The law of conservation of charge states that charge is always conserved. When all objects involved are considered prior to and after a given process, we notice that the total amount of charge among the objects is the same before the process starts as it is after the process ends.
The same conservation law is observed during the charging by conduction process. If a negatively charged metal sphere is used to charge a neutral electroscope, the overall charge before the process begins is the same as the overall charge when the process ends.
So if before the charging process begins, the metal sphere has units of negative charge and the electroscope is neutral, the overall charge of the two objects in the system is units. Perhaps during the charging process, units of negative charge moved from the metal sphere to the electroscope. When the process is complete, the electroscope would have units of negative charge and the metal sphere would have units of negative charge the original units minus the units it transferred to the electroscope.
The overall charge of the two objects in the system is still units. The overall charge before the process began is the same as the overall charge when the process is completed. Charge is neither created nor destroyed; it is simply transferred from one object to another object in the form of electrons. In all the above examples, the charging by conduction process involved the touching of two conductors. Does contact charging have to occur through the contact of two conductors?
Can an insulator conduct a charge to another object upon touching? And can an insulator be charged by conduction? A complete discussion of these questions can get messy and quite often leads to a splitting of hairs over the definition of conduction and the distinction between conductors and insulators. The belief is taken here that only a conductor can conduct charge to another conductor.
The process of noticeably charging an object by contact involves the two contacting objects momentarily sharing the net excess charge. The excess charge is simply given a larger area over which to spread in order to reduce the total amount of repulsive forces between them.
This process demands that the objects be conductors in order for electrons to move about and redistribute themselves. An insulator hinders such a movement of electrons between touching objects and about the surfaces of the objects. This is observed if an aluminum pie plate is placed upon a charged foam plate. When the neutral aluminum plate is placed upon the charged foam plate, the foam plate does not conduct its charge to the aluminum.
Despite the fact that the two surfaces were in contact, charging by contact or conduction did not occur. Or at least whatever charge transfer might have occurred was not noticeable by the customary means of using an electroscope, using a charge testing bulb or testing for its repulsion with a like-charged object. Many might quickly suggest that they have used a charged insulator to charge a neutral electroscope or some other object by contact. In fact, a negatively charged plastic golf tube can used to charge an electroscope.
The plastic tube is touched to the top plate of the electroscope. On most occasions, the plastic tube is even rubbed or rolled across the plate of the electroscope?
Wouldn't this be regarded as charging by conduction? Not really. In this case, it is more than likely that the charging occurred by some process other than conduction. There was not a sharing of charge between the plastic tube and the metal parts of the electroscope. Of course, once some excess charge is acquired by the electroscope, that excess charge distributes itself about the surface of the electroscope. Yet the charge is not uniformly shared between the two objects.
The protons and electrons within both the plastic golf tube and the electroscope are not acting together to share excess charge and reduce the total amount of repulsive forces. The charging of an electroscope by contact with a negatively charged golf tube or any charged insulating object would best be described as charging by lightning. Rather than being a process in which the two objects act together to share the excess charge, the process could best be described as the successful effort of electrons to burst through the space air between objects.
The presence of a negatively charged plastic tube is capable of ionizing the air surrounding the tube and allowing excess electrons on the plastic tube to be conducted through the air to the electroscope. This transfer of charge can happen with or without touching. In fact, on a dry winter day the process of charging the metal electroscope with the charged insulator often occurs while the insulator is some distance away. The dry air is more easily ionized and a greater quantity of electrons is capable of bursting through the space between the two objects.
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