Electrostatics Herron Physics, Unit 7

(greek for indivisible) Democritus, 460 BC He pounded up materials in his pestle and mortar to reduce them to smaller and smaller particles which he called atoma ATOMA (greek for indivisible)

Atoma—Ground up particles; dust

John Dalton, 1808 Suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them atoms. ATOM

Atom—Tiny spheres

JospePh John (J.J.) Thompson, 1904 Found that atoms could sometimes eject a far small negative particle which he called an electron. Thompson developed the idea that an atom was made up of electrons scattered unevenly within a sphere surrounded by a soup of positive charge to balance the electron's charge ELECTRON PLUM PUDDING MODEL

Electron/Plum Pudding

CENTRAL NUCLEUS PROTON Ernest Rutherford, 1910 Fired Helium nuclei (very small atoms) at a piece of gold foil which was only a few atoms thick. They found that most of them passed through. A small number were deflected and, to their surprise, some helium nuclei bounced straight back. Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus. He suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction CENTRAL NUCLEUS PROTON

CENTRAL NUCLEUS PROTON Ernest Rutherford CENTRAL NUCLEUS PROTON

Central Nucleus/Protons

ELECTRON ORBITS Niels Bohr, 1913 Bohr refined defined the location of the electrons. He said that the electrons were in set orbits, rather like planets orbiting the sun. Each orbit is only able to contain a set number of electrons. ELECTRON ORBITS

Bohr’s Atom Niels Bohr, 1913 nucleus ELECTRON ORBITS

Electron Orbits—Electrons move in set orbits around the nucleus

NEUTRON James Chadwick, 1932 Discovered the neutron. He found it to measure slightly heavier than the proton with a mass of 1840 electrons and with no charge (neutral). NEUTRON

Introduction of the Neutron

QUARK Murray Gell-Mann, 1964 Proposed the existence of a new level of elementary particles and called them "quarks.“ These quarks made up the protons and neutrons of the nucleus. He thought there existed at least three types of quarks. They have the names, "up," "down," and "strange." From 1974 thru 1984 the theory predicted three more quarks called "charm," "bottom" (or beauty), and "top" (or truth). QUARK

Quark—Smallest known particles; groups of 3 make up protons and neutrons

atom Structure + - proton electron neutron Orbital/shell HELIUM

Particle Symbol Charge Mass atom Structure Particle Symbol Charge Mass Proton p+ + 1 Neutron n0 Electron e— — Very small (use 0) Neutral atom: p+= e—

Periodic Table

Periodic Table Atomic Number Element Symbol p+ e— Element Name (Only in a Neutral Atom!) Element Name Mass Number Average Atomic Mass **Round this to the nearest whole number to estimate mass number p+ n0 #n0 = mass # – #p+

electrons Electrons arranged in Shells around the nucleus of an atom. 1st shell maximum of 2 electrons 2nd shell maximum of 8 electrons 3rd shell maximum of 18 electrons 7p+ 7n0

Charged Objects Positive Objects: Lose e— Negative Objects: Gain e — Objects cannot gain or lose p+

Electric Force and Charge CHECK YOUR NEIGHBOR When you brush your hair and scrape electrons from your hair, the charge of your hair is A. positive. negative. both A and B. neither A nor B. A. positive.

Electric Force and Charge CHECK YOUR ANSWER When you brush your hair and scrape electrons from your hair, the charge of your hair is A. positive. negative. both A and B. neither A nor B. Your hair loses electrons to the brush, making it positive. A. positive.

Electric Force and Charge CHECK YOUR NEIGHBOR When you brush your hair and scrape electrons from your hair, the charge of your brush is A. positive. negative. both A and B. neither A nor B. A. positive.

Electric Force and Charge CHECK YOUR ANSWER When you brush your hair and scrape electrons from your hair, the charge of your hair is A. positive. negative. both A and B. neither A nor B. Your brush gains electrons from your hair, making it negative. A. positive.

Electric Force and Charge Fundamental rule for electricity: Like charges repel; opposite charges attract.

ELECTRIC FORCE Forces occur between two or more charged objects The stronger either charge is, the more the force The more distance between the objects, the less force

Shocking! If you reach for a metal doorknob after walking across a carpet, you might see a spark. The spark is caused by extra electrons moving from your hand to the doorknob.

Conductor! A material in which electrons are able to move easily is a conductor. The best electrical conductors are metals. Conductors are used in electric wires Copper Silver

Examples of Conductors silver copper gold aluminum iron steel brass bronze mercury graphite dirty water concrete the human body

Not a Conductor! A material in which electrons are not able to move easily is an insulator. Most plastics are insulators. Insulators can protect you from shocks Coating on electrical wires Wearing rubber boots in hazardous areas near electricity

Examples of Insulators glass rubber oil asphalt fiberglass porcelain ceramic quartz (dry) cotton (dry) paper (dry) wood plastic air diamond pure water

Conductors and Insulators Semiconductors are materials that can be adjusted to be good conductors or good insulators Superconductors are certain metals that become perfect conductors at temperatures near absolute zero (really, really cold!).