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Charge Field Forces Electricity, like gravity and magnetism, is a field force. Field forces can act at a distance and through a vacuum. Masses attract each other by gravitational force Charges attract and repel by electric force. And as we shall see later, moving charges become magnetic and attract and repel one another with the magnetic force. We will see many similarities between electricity and gravity. As a result, electricity will be compared to gravity. © RHJansen Particles of Charge Proton: positively charged particle. Electron: negatively charged particle. Neutron: a neutral particle that has no electrical properties. Variables for Charge e used for the smallest charged particle, the electron 1 e = 1.6 × 10−19 Coulombs (the units of charge) All charge in the universe is a multiple of this charge. This is also the charge of a proton. q a small amount of charge, often a point charge or a spherical object containing charge. Q a large amount of charge, often the charge of parallel plates. © RHJansen Charge Unit of charge: Coulombs (C) Charge on an electron: 1 e = 1.6 × 10−19 C Charge on a proton: 1 e = 1.6 × 10−19 C Charge on a neutron: zero Number of charges in 1 Coulomb: 6.25 × 1018 charged particles Not given, but easy to find: It is the reciprocal of 1.6 × 10−19 Attraction and Repulsion Like charges repel Opposite charges attract Neutrons have no effect © RHJansen Conserved Charge cannot be created or destroyed The number of protons, electrons, and neutrons will not change (until we get to Modern Physics) The questions in this chapter will be where are the charges and what are they doing. Quantized Charge comes in exact quantities (packets). Every charge in the universe is made of protons and electrons. Since there are no ½ electrons or ½ protons all charge in the universe will be a multiple of 1.6 × 10−19 . © RHJansen Neutral Charge ??? If an object has neutral charge, does it mean that it is missing electrons and protons. No Neutral charge means it has the same number of protons and electrons, and the positively charged protons are canceling the negatively charged electrons. This means we really don’t know how much charge is actually on a neutral object. © RHJansen 1 C of charge ??? In fact even when an object has its charge given, we will not know the actual amount of charges on the object. When a problem gives you the charge of an object, it is only telling you how much extra charge is on the object. An object that has a + 1 C charge could consist of: 256 C of protons and 255 C of electrons, or 78 C of protons and 77 C of electrons. We can’t really be sure. However, it does not matter, because only the excess charge is important. The excess charge gives an object its electrical properties. Polarized Objects A polarized object has a region of positive charge at one end and negative charge at the other (opposite poles) These objects may have an overall neutral as shown in the example below. One of the most famous objects is the water molecules, where the oxygen atom with its higher electronegativity pulls the electrons closer to it making the oxygen end of the molecule more negative, while the hydrogen ends become more positive. © RHJansen Conductors A conductor is a material that allows electrons to move through it with ease. Metals are excellent conductors Metals act like one gigantic molecule that shares the outer (valence) electrons with all the atoms within the metal. Excess charges pile up on the outside surface of conductors. Insulator An insulator is a material that holds its electrons tightly. Insulators do not allow charges to move through them. Plastic is used to coat metal wires so that the charges cannot leave the wire. Excess charges can be placed on an insulator, but where they are placed initially is where they remain until removed. © RHJansen Electroscope An electroscope is a device that shows the presence of charge. The simplest version consists of frame with a metal plate on top and two pieces of foil hang next to each other inside the frame. Once again I am showing all the charges, even though they cancel and the electroscope is neutral. I just want to show how the charges are moving. A charge rod is brought near the metal plate of the electroscope. Metal Plate Foil leaves Electroscope The diagram at the right shows the result. The negative rod repels the electrons from the metal plate to the foil leaves. The leaves pick up a net negative charge and repel each other. The force of electricity is many time stronger than gravity, and the foil leaves rise indicating the presence of charge in the rod. Charging by Friction Rubbing two substances together can transfer outer (valence) electrons from one to the other. Example walking on a carpet on a dry day builds excess charge on you which is released in a static shock when you touch a metal conductor. Charging by Conduction This process involves touching a charged object to an uncharged object. Like charges repel each other and the excess charges on the charged object want to get away from one another. The uncharged object represents a lot of empty space where these charges can move into and decrease the force of repulsion on each other. Before After Charging by Induction This process involves charging an object without transferring charge by touching it to another charged object. Neutral objects do contain charges. The protons and electrons are equal in number and cancel. Normally we only look at excess charge, but this time look at all the charges. Start with 2 neutral spheres that are touching each other. A negatively charged rod is brought near (but not touching) the two neutral spheres. What will happen ? Charging by Induction This process involves charging an object without transferring charge by touching it to another charged object. The negative charges in the spheres are repelled by the negative charges in the rod. (Note: only electrons can move, as they are on the outside of the atom. Protons are trapped deep in the nucleus.) The left sphere gains a net positive charge, while the right sphere gains a net negative charge. The spheres are technically still one object. They are now Polarized (one positive pole and one negative pole). Charging by Induction This process involves charging an object without transferring charge by touching it to another charged object. Now the spheres are separated. The result is two separate spheres that are charged. This process occurred without either sphere being touched by the rod.