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. In this case, electrons are attracted to the sphere through a wire called the ground wire, because it supplies a conducting path to the ground. The ground connection is broken before the charged rod is removed, leaving the sphere with an excess charge opposite to that of the rod. Again, an opposite charge is achieved when charging by induction, and the charged rod loses none of its excess charge.
Summary A conductor is a substance that allows charge to flow freely through its atomic structure. An insulator holds charge fixed in place. Polarization is the separation of positive and negative charges in a neutral object. Polarized objects have their positive and negative charges concentrated in different areas, giving them a charge distribution.
Conceptual Questions An eccentric inventor attempts to levitate a cork ball by wrapping it with foil and placing a large negative charge on the ball and then putting a large positive charge on the ceiling of his workshop. Instead, while attempting to place a large negative charge on the ball, the foil flies off. When a glass rod is rubbed with silk, it becomes positive and the silk becomes negative—yet both attract dust. Does the dust have a third type of charge that is attracted to both positive and negative?
No, the dust is attracted to both because the dust particle molecules become polarized in the direction of the silk. Why does a car always attract dust right after it is polished?
Note that car wax and car tires are insulators. Yes, polarization charge is induced on the conductor so that the positive charge is nearest the charged rod, causing an attractive force. While walking on a rug, a person frequently becomes charged because of the rubbing between his shoes and the rug. This charge then causes a spark and a slight shock when the person gets close to a metal object.
Why are these shocks so much more common on a dry day? Charging by conduction is charging by contact where charge is transferred to the object. Charging by induction first involves producing a polarization charge in the object and then connecting a wire to ground to allow some of the charge to leave the object, leaving the object charged. Small pieces of tissue are attracted to a charged comb. Soon after sticking to the comb, the pieces of tissue are repelled from it.
Trucks that carry gasoline often have chains dangling from their undercarriages and brushing the ground. This is so that any excess charge is transferred to the ground, keeping the gasoline receptacles neutral. If there is excess charge on the gasoline receptacle, a spark could ignite it.
Why do some clothes cling together after being removed from the clothes dryer? The dryer charges the clothes. If they are damp, the presence of water molecules suppresses the charge. Suppose someone tells you that rubbing quartz with cotton cloth produces a third kind of charge on the quartz. This is simply the electrostatic principle that says unlike charges repel.
Since the conducting spheres are charged negatively, the electrons are repelled by the negatively charged balloon. Due to the presence of electrons in a conductor, they are free to move about the surface of the conductor. Being charged negatively, the electrons are repelled by the negatively charged balloon. Now, there is a mass transition of electrons from sphere A to sphere B.
This electron transition or the migration causes the two-sphere system to be polarized. Ultimately, the two-sphere system is electrically neutral. Yet the migration of electrons out of sphere A and into sphere B separates the charge from the charge. By watching the spheres individually, it might be appropriate to mention that sphere A has overall positive charges and sphere B has overall negative charges.
After the two-sphere system is polarized, sphere B is physically separated from sphere A using the insulating stand. Having been pulled beyond the balloon, the negative charges likely redistribute themselves uniformly about sphere B. At the same time, the excess positive charges on sphere A remain located near the negatively charged balloon, consistent with the electrostatic principle that opposite charges attract.
As the balloon is moved away, there is a uniform distribution of charge about the surface of both spheres As demonstrated in the figure.
This distribution of charges occurs as the remaining electrons in sphere A transit across the surface of the sphere until the excess positive charge is uniformly distributed. Thus, when a negatively charged balloon is brought near the two-sphere system, the electrons in the sphere will be forced to move away due to repulsion. The migration of electrons will cause sphere A to become completely positive and sphere B to become negative.
Similarly, we study the transfer of charge through the method of induction employing a charged object. The charged object is only used to induce electron movement. Electrons, being negatively charged, move about the conducting sphere as they are repelled by the negatively charged tube. Once the ground is touched to the sphere, the electrons can even move further from the negatively charged tube by leaving the sphere and moving through the ground.
It is at this point that the sphere acquires an unbalance of charge. Since there was a loss of negative charge in the form of electrons, the overall charge on the sphere is positive.
Additional information on physical descriptions of electrostatic phenomenon is available at The Physics Classroom Tutorial. Detailed information is available there on the following topics:. Physics Tutorial.
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