Charging by friction

The presence of different atoms in objects provides different objects with different electrical properties. One such property is known as electron affinity. Simply put, the property of electron affinity refers to the relative amount of love that a material has for electrons. If atoms of a material have a high electron affinity, then that material will have a relatively high love for electrons. This property of electron affinity will be of utmost importance as we explore one of the most common methods of charging - charging by friction or rubbing.

Suppose that a rubber rod is rubbed with a sample of animal fur. During the rubbing process, the atoms of the rubber are forced into close proximity with the atoms of the animal fur. The electron clouds of the two types of atoms are pressed together and are brought closer to the nuclei of the other atoms. The protons in the atoms of one material begin to interact with the electrons present on the other material. Amidst the sound of crackling air, you might even be able to hear the atoms saying, "I like your electrons." And of course, the atoms of one material - in this case, the atoms of rubber - are more serious about their claim for electrons. As such, the atoms of rubber begin to take electrons from the atoms of animal fur. When the rubbing has ceased, the two objects have become charged.

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Charging by Induction

Induction charging is a method used to charge an object without actually touching the object to any other charged object. An understanding of charging by induction requires an understanding of the nature of a conductor.

One common demonstration performed in a physics classroom involves the induction charging of two metal spheres. The metal spheres are supported by insulating stands so that any charge acquired by the spheres cannot travel to the ground. The spheres are placed side by side  so as to form a two-sphere system. 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. 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. 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. 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. 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.

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Electroscope

An electroscope is a device that is capable of detecting the presence of a charged object. It is often used in electrostatic experiments and demonstrations in order to test for charge and to deduce the type of charge present on an object. There are all kinds of varieties and brands of electroscope from the gold leaf electroscope to the needle electroscope.

While there are different types of electroscopes, the basic operation of each is the same. The electroscope typically consists of a conducting plate or knob, a conducting base and either a pair of conducting leaves or a conducting needle. Since the operating parts of an electroscope are all conducting, electrons are capable of moving from the plate or knob on the top of the electroscope to the needle or leaves in the bottom of the electroscope. Objects are typically touched to or held nearby the plate or knob, thus inducing the movement of electrons into the needle or the leaves (or from the needle/leaves to the plate/knob). The gold leaves or needle of the electroscope are the only mobile parts. Once an excess of electrons (or a deficiency of electrons) is present in the needle or the gold leaves, there will be a repulsive affect between like charges causing the leaves to repel each other or the needle to be repelled by the base that it rests upon. Whenever this movement of the leaves/needle is observed, one can deduce that an excess of charge - either positive or negative - is present there. It is important to note that the movement of the leaves and needle never directly indicate the type of charge on the electroscope; it only indicates that the electroscope is detecting a charge.

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