Lecture 9 Electricity Chapter 5.1  5.11 Outline Electric Charge Coulomb’s Law Conductors and Insulators Superconductivity Ohm’s Law

The Electrical Phenomenon The phenomenon occurs from rubbing of one material (e.g., amber  electron in Greek) with another material (e.g., fur).phenomenon There are 2 types of charge: positive and negative.2 Like charges repel, unlike charges attract. Discovered in ancient Greece more than 2500 years ago It manifests itself by attraction or repellence of ‘charged’ materials.

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.chemical elements 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.particles 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 kg and is positively charged Neutron has a mass kg and is uncharged Electron has a mass 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  19 C. The electron has a charge of   19 C. The quantity of charge is abbreviated e. Electric charge occurs only in multiples of  e. Coulomb’s Law: Q 1 Q 2 F = K R 2 K = N m 2 /C 2

Electricity and Gravity The law of gravity and Coulomb’s law have the same form. It is harder to collect a large electric charge of either sign than a large mass of matter. However, gravitational force is always attractive. 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.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.ions

Superconductivity At low temperatures (~1 K) some substances lose their electric resistance. This phenomenon, called superconductivity, was discovered in In 1980’s superconductivity at higher temperatures was discovered (~200 K). SuperconductivitySuperconductivity 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 property of a conductor that opposes the flow of charge in it is called resistance. The electric current in a wire is proportional to the potential difference. This is known as Ohm’s law. I = current V = voltage R = resistance V I = --- R 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. Most properties of ordinary matter can be traced to electrical forces Electric properties of matter allow us to transmit and store energy.