Transcript NASC 1110
Lecture 9 Electricity Chapter 5.1 5.11 Outline • • • • • Electric Charge Coulomb’s Law Conductors and Insulators Superconductivity Ohm’s Law The Electrical Phenomenon Discovered in ancient Greece more than 2500 years ago The phenomenon occurs from rubbing of one material (e.g., amber electron in Greek) with another material (e.g., fur). It manifests itself by attraction or repellence of ‘charged’ materials. There are 2 types of charge: positive and negative. Like charges repel, unlike charges attract. The Nature of Charge Charge is not produced by rubbing. Uncharged objects contain equal amounts of negative and positive charge. They are electrically neutral. The nature of charge can be traced to the composition of matter, to atoms. Atomic Structure 92 chemical elements have been identified in the Universe. Nearly 20 more have been created artificially. Each chemical element is made from a different type of atom. Atoms are made from particles called protons, neutrons, and electrons. Protons and neutrons form the nucleus in the center of the atom. Electrons surround the nucleus. Atomic Structure Positively charged protons are hold together by the strong force, which overcomes electrical repulsion. Negatively charged electrons are attracted to the nucleus. Proton has a mass 1.673 10-27 kg and is positively charged Neutron has a mass 1.675 10-27 kg and is uncharged Electron has a mass 9.11 10-31 kg and is negatively charged Proton and neutron are ~2000 times heavier than electron The Coulomb The unit of electric charge is the coulomb (C). The proton has a charge of + 1.6 1019 C. The electron has a charge of 1.6 1019 C. The quantity of charge is abbreviated e. Electric charge occurs only in multiples of e. Coulomb’s Law: Q1 Q2 F = K -------R2 K = 9 109 N m2/C2 Electricity and Gravity The law of gravity and Coulomb’s law have the same form. However, gravitational force is always attractive. It is harder to collect a large electric charge of either sign than a large mass of matter. Thus, gravity is more significant on a large scale (cosmic size), while electric forces are more significant on a small scale (atomic size) Conductors and Insulators A conductor is a substance through which electric charge flows readily. Example, metals. In an insulator charge flows with great difficulty. The difference between the 2 types of substances is in the strength of connection between outer electrons and nuclei. Conductivity in fluids and gases involves ions. Superconductivity At low temperatures (~1 K) some substances lose their electric resistance. This phenomenon, called superconductivity, was discovered in 1911. In 1980’s superconductivity at higher temperatures was discovered (~200 K). Superconductivity saves a lot of energy, which is otherwise wasted as heat. The Ampere A buttery turns chemical energy into electric energy. Electrons flow in a wire from negative to positive terminal. The moving electrons are the electric current. The unit of electric current is called the ampere. 1 ampere = 1 Coulomb/1 second 1A = 1C/s Potential Difference Consider a coulomb of negative charge on the negative terminal. It has potential energy (PE). While flowing from to +, it produces work due to its PE. The decrease of PE is called the potential difference between the 2 terminals. The potential difference between 2 points is equal to the corresponding energy difference per coulomb. It is measured in volts (1 volt = 1J/C) and called voltage. Ohm’s Law The electric current in a wire is proportional to the potential difference. This is known as Ohm’s law. The property of a conductor that opposes the flow of charge in it is called resistance. I = current V I = --R V = voltage R = resistance 1 ohm = 1 volt/ampere 1 = 1 V/A Electric Power Electric energy is useful, because it is: • carried by wires • easily converted into other kinds of energy The rate at which a current is doing work is called the power of the current. Power P = I V (current x voltage) is measured in Watts. 1 Watt = 1 ampere x 1 volt Summary • Electrical forces are different from gravity. They are more influential on small scales. • Electric properties of matter allow us to transmit and store energy. • Most properties of ordinary matter can be traced to electrical forces