Electricity+and+magnetism

Physics unit 2- Electricity and magnetism
 * **Electrostatics**- electricity at rest, involoing electric charges, the forces between them, and their behavior in materials
 * **The atom**- electric forces arise from particles in atoms
 * **Early 1900s**- Ernest Rutherford, and Neils Bohr proposed a model of an atom with a positively charged nucleus surounded by electrons
 * **The atom**- the protons( positive charge) in the nucleus attract the electrons ( negative charge) and hold them in orbit
 * The electrons are attracted to the protons, but are repelled by the other electrons
 * The fundamental electrical property of mutual attraction or repulsion between electrons and protons is called __**CHARGE**__.
 * By general agreement, electrons are negatively charged, and protons are positively charged
 * **Neutrons**- have no charge, are neither attracted nor repelled by charged particles


 * __Facts about atoms__:
 * 1. every atom has a positively charged nucleus, surrounded by negatively charged electrons
 * 2. all electrons are identical; each has the same mass and the same quantity of negative charge as every other electron
 * 3. the nucleus is composed of protons and neutons (the only exception is the common form of hydrogen, which has no neutrons) All protons are identical, all neutrons are identicl. A proton has nearly 2000 times the mas of an electron, but its positive charge is equal in magnitude to the negative charge of an electron. Neutron has slightly greater mass than a proton, but no charge
 * 4. Atoms have as many electrons as protons, so a neutral atom has zero **__NET__** charge


 * __Attraction and repulsion__:
 * The fundemental rule at the base of all electrical phenomena is that __**like**__ charges repel, and **__opposite__** charges attract


 * __Conservation of charge__:
 * In a neutral atom, electrons = protons
 * If an atom loses an electron (negative), then the protons (positive), outnumber the electrons, and the net (everything added up/subtracted) is now a positive chare
 * If any atom gains an electron, then the electrons will outnumber the protons, and the net charge will be negative
 * **Rule**- an object that has unequal numbers of electrons and protons is electrically charged
 * **Coservation of charge**- principal that electrons are neither created nor destroyed, but are simply transferred from one material to another
 * Any object that is electrically charged has an excess or deficiency of some whole number of electons
 * //Electrons cannot be divided into fractions of electrons//
 * This mean that the whole charge of an object is a whole-number multiple of the charge of an electron, cannot be 1.5 or 1000.5
 * SI unit of charge is the **__COULUMB__**
 * While common sense might say that the coulomb is the charge of a single electron, it is not.
 * For historical reasons, the charge if 1 coulomb is the charge of 6.24 billion billion (6.24 X 10(18)) electrons
 * While this might seem like a large number, it represents only the amount of charge that passes through a common 100-W light bulb in one second
 * **Electrical forces in atoms** - because most objects have almost exactly equal numbers of electrons and protons, electrical forces usually balance out
 * Between the Earth and the Moon, there is no measurable electrical force
 * In general, the weak (relative to electrical forces) gravitational force, which only attracts, is the predominant force between astronomical bodies
 * Compared to the standard units, the electrical force is far greater than the gravitational force.
 * Although electrical forces balance out for astronomical and everyday objects, it is not always the case at the atomic level.
 * Often electrons are shared between two or more atoms. The shared electrons spend some time near one atom's positive nucleus, and some time near the other atom's positive nucleus. This is bonding that makes molecules.


 * __Conductors and Insulators__
 * Electrons move more easily in some materials than others
 * Outer electrons in atoms of **__metals__** are not anchored to the nucleus an a particular atom, but are free to move around in the __**metal**__ as a whole
 * Materials though which electrical charge (electrons) can flow are called **__CONDUCTORS__**
 * Metals are good conductors for the motion of electric charges for the same reason they are good conductors of heat: their electrons are loose
 * Electrons in other materials (rubber, glass) are tightly bound, and remain with particular atoms, these materials, known as **__INSULATORS__** are poor conductors of electricity, (for the same reason they are poor conductors of heat).
 * **Rule-** electrons move easily in good conductors and poorly in good inslators
 * All substances can be classified based on their **__CONDUCTIVITY__**
 * The diffrence between those at the top and at the bottom of the list is great
 * The conductivity of a metal can be a million trillion times greater than an insulator such as glass. For this reason, electricity flows through the metal wire in power lines even over greater distances, rather than going through the few cm of insulation between the wire and a supporting tower.
 * Some materials are good inslators in one form, but if even one atom in ten million is changed, the material's conductivity changes tremendously.
 * Materials that can be made to behave sometimes as insulators and sometimes as conductors are called **__SEMICONDUCTORS__**
 * Atoms in a semiconductor tend to hold onto their electrons until given a small energy boost, these are used in photovoltaic cells, transistors
 * __Charging by Friction and Contact__
 * Two ways electric charge can be transferred are by friction and by contact.
 * **Friction**- ex. shuffling shoes across carpet, then touching something metal (light fixture, doorknob, etc.) or stroking a cat's fur and hearing the crackle
 * Electrons can also be transferred by simply touching a charged object to another object. This is called charging by contact
 * If the touched object is
 * a good conductor, the charge will spread to all parts of its surface, as like charges repel each other.
 * If the new surface is an insulator (poor conductor) then the charge stays close to where the object was touched
 * **Charging by induction**- if a charged object is brought near a conducting surface, without touching it, electrons will move in the conducting surface.
 * This process of charging an object without direct contact is **__CHARGING BY INDUCTION__**
 * When we touch a metal surface with a finger, charges that repel each other have a conducting path to a practicaly infinite reservoir for electric charge- the ground.
 * When we allow charges to move off (or onto) a conductor by touching it, it is common to say that we are "grounding" it
 * Charging by induction occurs during thunderstorms.
 * The negatively charged bottoms of clouds induces a positive charge on the surface of the earth below the clouds
 * Most lightning is an electrical discharge between oppositely charged parts of clouds.
 * Lighting we are most familiar with is a discharge between the clouds and the oppositely charged ground below
 * **Charge Polarization-** Charging by induction is not restricted to conductors
 * //Charge polarization can occur in insulators that are// near //a charged object//
 * //W//hen a charged rod is brought near an insulator, there are no free electrons to migrate throughout the insulating material
 * instead of the electrons moving within material, (as in conductors), there is a rearrangement of the //positions// of charges within the atoms and molecules themselves
 * one side of the atom or molecule is induced to be slightly more positive than the opposite side
 * the atom or molecule is said to be __**ELECTRICALLY POLARIZED**__
 * Examples of charged polarization
 * bits of paper attracted to a charged comb
 * a balloon rubbed against a head of hair, then stuck to a wall
 * same balloon placed near a stream of water
 * __Magnetism__
 * electricity and magnetism were thought to be two independent things well into the 19th century
 * Danish professor Hans Christian Oersted was demonstrating electric currents in front of his students
 * when electric current was passed through a wire near a magnetic compass, the compass needle was deflected