Electric Charge

1. Electric Charge Charge Conductors and Insulators Coulomb’s Law Permittivity Constant Spherical Conductors Charge Quantization Conservation of Charge pps by C Gliniewicz

2. Early Greeks first discovered that amber, when rubbed, caused straw to jump onto the amber. This was the first observation of electric charge and electrical forces. One can observe sparks when shoes are dragged across a rug and then another person or a piece of metal is touched. Electric charge is an intrinsic property of matter. There are positive and negative charges. If the charges are equal or balance each other, the material is electrically neutral. Charges with the same electrical sign repel each other. Charges with opposite signs attract each other. The labels for the charges were arbitrarily assigned by Benjamin Franklin. He easily could have exchanged the signs. Photocopying uses electrical charges to transfer the toner to paper. Conductors are materials through which electrical charges can move freely. Nonconductors or insulators are materials through which charges cannot move easily. Semi-conductors are materials between conductors and insulators. Superconductors are materials that are perfect conductors wich allow the charge to move without any hindrances. pps by C Gliniewicz

3. A conductor, since it allows the free movement of charge, will provide a means for charges in another conductor to move to a place with a different charge. The earth is a repository of charges. A conductor can allow charge to move to the earth. Electrons have a negative charge and protons have a positive charge. Only the electrons move since they circle the nucleus which contains the protons. Metals, like copper, have their outermost electrons loosely held allowing them to move to other atoms which are close by. Objects become negatively charged when excess electrons are accumulated. If objects loose electrons the object becomes positively charged because there are more protons than electrons in the object. If one adapts their eyes to darkness for ten or more minutes and then another person bites into a wintergreen LifeSaver, a blue flash can be observed. As the sugar crystals are broken into pieces, different charges end up on each piece and the charge jumps between the pieces causing the blue spark. The screen of a computer or television accumulates a charge and dust and bacteria are attracted to the screen. Touching the screen can transfer bacteria. pps by C Gliniewicz

4. Suppose that two particles have charges q₁ and q₂ and are separated by a distance r. The electrostatic force of attraction or repulsion between them is in which k is a constant. Charles Augustin Coulomb completed experiments in 1785 which led him to his law. The law is very similar to Newton’s Law of Gravitation. A positive value means repulsion and a negative value means attraction. The coulomb is the SI unit of charge. Electric current is the time rate of change of electric charge. A shell of uniform charge attracts or repels a charged particle that is outside the shell as if all the shell’s charge were concentrated at the center of the shell. If a charged particle located inside a shell of uniform charge has no net electrostatic force acting on the particle due to the shell of charge. When first discovered by Franklin, electric charge was thought to be a fluid. We now know that electric charge is quantized. pps by C Gliniewicz

5. The elemental charge on an electron or proton has the same magnitude, but opposite charge. That charge is 1.602×10⁻¹⁹ Coulombs and is designated by the symbol, e. In a system containing electric charges, the total number of charges remains the same. No experiment has yet shown that electric is not conserved. Therefore, one can add conservation of electric charge to conservation of energy, momentum and angular momentum. Nuclear interactions all show this conservation. Examples are shown below. pps by C Gliniewicz