Since the rod is positively charged, the conduction electrons which themselves are negatively charged are attracted, flowing toward the insulator to the near side of the conductor Figure. Now, the conductor is still overall electrically neutral; the conduction electrons have changed position, but they are still in the conducting material.
However, the conductor now has a charge distribution ; the near end the portion of the conductor closest to the insulator now has more negative charge than positive charge, and the reverse is true of the end farthest from the insulator.
The relocation of negative charges to the near side of the conductor results in an overall positive charge in the part of the conductor farthest from the insulator. We have thus created an electric charge distribution where one did not exist before. This process is referred to as inducing polarization —in this case, polarizing the conductor.
The resulting separation of positive and negative charge is called polarization , and a material, or even a molecule, that exhibits polarization is said to be polarized.
A similar situation occurs with a negatively charged insulator, but the resulting polarization is in the opposite direction. Neutral objects can be attracted to any charged object. The pieces of straw attracted to polished amber are neutral, for example. If you run a plastic comb through your hair, the charged comb can pick up neutral pieces of paper. Figure shows how the polarization of atoms and molecules in neutral objects results in their attraction to a charged object. When a charged rod is brought near a neutral substance, an insulator in this case, the distribution of charge in atoms and molecules is shifted slightly.
Opposite charge is attracted nearer the external charged rod, while like charge is repelled. Since the electrostatic force decreases with distance, the repulsion of like charges is weaker than the attraction of unlike charges, and so there is a net attraction.
Thus, a positively charged glass rod attracts neutral pieces of paper, as will a negatively charged rubber rod. Some molecules, like water, are polar molecules. Polar molecules have a natural or inherent separation of charge, although they are neutral overall. Polar molecules are particularly affected by other charged objects and show greater polarization effects than molecules with naturally uniform charge distributions.
When the two ends of a dipole can be separated, this method of charging by induction may be used to create charged objects without transferring charge. In Figure , we see two neutral metal spheres in contact with one another but insulated from the rest of the world. A positively charged rod is brought near one of them, attracting negative charge to that side, leaving the other sphere positively charged.
Another method of charging by induction is shown in Figure. The neutral metal sphere is polarized when a charged rod is brought near it. The sphere is then grounded, meaning that a conducting wire is run from the sphere to the ground. Since Earth is large and most of the ground is a good conductor, it can supply or accept excess charge easily. We will notify you when Our expert answers your question. To View your Question Click Here. View courses by askIITians.
Click Here Know More. Full Name. Email ID. Phone no. Being made of metal a conductor , electrons are free to move between the spheres - from sphere A to sphere B and vice versa. If a rubber balloon is charged negatively perhaps by rubbing it with animal fur and brought near the spheres, electrons within the two-sphere system will be induced to move away from the balloon.
This is simply the principle that like charges repel. Being charged negatively, the electrons are repelled by the negatively charged balloon. And being present in a conductor, they are free to move about the surface of the conductor. Subsequently, there is a mass migration of electrons from sphere A to sphere B.
This electron migration causes the two-sphere system to be polarized see diagram ii. Overall, the two-sphere system is electrically neutral. Yet the movement of electrons out of sphere A and into sphere B separates the negative charge from the positive charge. Looking at the spheres individually, it would be accurate to say that sphere A has an overall positive charge and sphere B has an overall negative charge.
Once the two-sphere system is polarized, sphere B is physically separated from sphere A using the insulating stand.
Having been pulled further from the balloon, the negative charge likely redistributes itself uniformly about sphere B see diagram iii. Meanwhile, the excess positive charge on sphere A remains located near the negatively charged balloon, consistent with the principle that opposite charges attract.
As the balloon is pulled away, there is a uniform distribution of charge about the surface of both spheres see diagram iv. This distribution occurs as the remaining electrons in sphere A move across the surface of the sphere until the excess positive charge is uniformly distributed. This distribution of positive charge on a conductor was discussed in detail earlier in Lesson 1. The law of conservation of charge is easily observed in the induction charging process.
Considering the example above, one can look at the two spheres as a system. Prior to the charging process, the overall charge of the system was zero. There were equal numbers of protons and electrons within the two spheres. In diagram ii. At this point, the individual spheres become charged. The quantity of positive charge on sphere A equals the quantity of negative charge on sphere B.
If sphere A has units of positive charge, then sphere B has units of negative charge. Determining the overall charge of the system is easy arithmetic; it is simply the sum of the charges on the individual spheres. The overall charge on the system of two objects is the same after the charging process as it was before the charging process.
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 above examples show how a negatively charged balloon is used to polarize a two-sphere system and ultimately charge the spheres by induction. But what would happen to sphere A and sphere B if a positively charged object was used to first polarize the two-sphere system? How would the outcome be different and how would the electron movement be altered? Consider the graphic below in which a positively charged balloon is brought near Sphere A.
The presence of the positive charge induces a mass migration of electrons from sphere B towards and into sphere A. This movement is induced by the simple principle that opposites attract. Negatively charged electrons throughout the two-sphere system are attracted to the positively charged balloon. This movement of electrons from sphere B to sphere A leaves sphere B with an overall positive charge and sphere A with an overall negative charge.
The two-sphere system has been polarized. With the positively charged balloon still held nearby, sphere B is physically separated from sphere A. The excess positive charge is uniformly distributed across the surface of sphere B. The excess negative charge on sphere A remains crowded towards the left side of the sphere, positioning itself close to the balloon.
Once the balloon is removed, electrons redistribute themselves about sphere A until the excess negative charge is evenly distributed across the surface. In the end, sphere A becomes charged negatively and sphere B becomes charged positively. This induction charging process can be used to charge a pair of pop cans. It is a simple enough experiment to be repeated at home. Two pop cans are mounted on Styrofoam cups using scotch tape. The cans are placed side-by-side and a negatively charged rubber balloon having been rubbed with animal fur is brought near to one of the cans.
The presence of the negative charge near a can induces electron movement from Can A to Can B see diagram. Once the cans are separated, the cans are charged.
The type of charge on the cans can be tested by seeing if they attract the negatively charged balloon or repel the negatively charged balloon. Of course, we would expect that Can A being positively charged would attract the negatively charged balloon and Can B being negatively charged should repel the negatively charged balloon.
During the process of induction charging, the role of the balloon is to simply induce a movement of electrons from one can to the other can. It is used to polarize the two-can system.
The balloon never does supply electrons to can A unless your hear a spark, indicating a lightning discharge from the balloon to the can. In the charging by induction cases discussed above, the ultimate charge on the object is never the result of electron movement from the charged object to the originally neutral objects.
The balloon never transfers electrons to or receive electrons from the spheres; nor does the glass rod transfer electrons to or receive electrons from the spheres.
The movement of electrons leaves an imbalance of charge on opposite sides of the neutral conductor. While the overall object is neutral i. Once the charge has been separated within the object, a ground is brought near and touched to one of the sides. The touching of the ground to the object permits a flow of electrons between the object and the ground. The flow of electrons results in a permanent charge being left upon the object. When an object is charged by induction, the charge received by the object is opposite the charge of the object which was used to charge it.
The animation below depicts the induction process.
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