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Elementary Electrostatics. Rubbing a balloon on a wool jumper makes the balloon attract your hair The balloon is said to be “charged” or to have an “electric charge” Similarly glass rubbed with silk/fur will become “charged” Charged glass will attract a charged balloon
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Elementary Electrostatics • Rubbing a balloon on a wool jumper makes the balloon attract your hair • The balloon is said to be “charged” or to have an “electric charge” • Similarly glass rubbed with silk/fur will become “charged” • Charged glass will attract a charged balloon • Two charged balloons will repel each other
Quantum Mechanical Model Bohr Model of Atom http://csep10.phys.utk.edu/astr162/lect/light/bohr.html http://www.physics.ucla.edu/~dauger/orbitals/ Materials with equal numbers of neg. and pos. charge are said to be electrically neutral. Types and Sources of Electric Charge Two kinds of electric charge – positive negative protons electrons
Electric Charge • All ordinary matter contains both positive and negative charge. • You do not usually notice the charge because most matter contains the exact same number of positive and negative charges. • An object is electrically neutral when it has equal amounts of both types of charge.
Properties of Electric Charge • Charge is quantized. • Positive and Negative charges have same magnitude • e = p = 1.6 x 10-19 C • SI unit of charge is the Coulomb • Charge is conserved.
Behaviour of Charges • ‘Like’ charges repel • ‘Unike’ or opposite charges attract • Q = + ne where n is a whole number
Seeing the effects of charge: the electroscope • the electroscope is a simple device for observing the presence of electric charge • it consists of a small piece of metal foil (gold if possible) suspended from a rod with a metal ball at its top • If a negatively charged rod is placed near the ball, • the electrons move away because of the repulsion. • The two sides of the metal foil then separate.
Different elements will have different types of bonds – different strengths. Insulators Conductors Electrons are free to move about the material Electrons are bound to the atoms - can’t move (conduct). Atoms and Materials Classification of Materials – Conductors, Insulators, Semiconductors
Different Methods of Charging • Friction e.g. rubbing a balloon with wool • Conduction e.g. touching an electroscope • Induction e.g. balloon sticking to a wall
How do we charge an object? Charging by rubbing • When two neutral objects are rubbed together, charge is transferred from one to the other and the objects become oppositely charged. • This is called charging by friction. • Objects charged by this method will attract each other.
On non-symmetric objects, charge collects at sharp points. On symmetric objects, charge distributes uniformly Insulators – charge cannot move, so it remains localized around the contact region. How do we charge an object? Charging by rubbing Conductors – charge will distribute around the surface to try until equilibrium is achieved. - charge only resides on surface, not inside.
Charging by induction (two conductors) Charging by induction (1 conductor and ground) How do we charge an object? B. Induction – Inducing a charge without touching. Conductors Charge is physically moved in/on the materials.
http://www.tutorvista.com/content/physics/physics-iv/electric-charges/charging-induction-animation.phphttp://www.tutorvista.com/content/physics/physics-iv/electric-charges/charging-induction-animation.php
Free Standing Atoms are randomly oriented Charge does not actually move In/on the insulator – redistributes. Polarizes the material. Induction (contd) Insulators
Fields and forces • You can think of the field as the way forces are transmitted between objects. • Charge creates an electric field that creates forces on other charges.
Lightning- outdoor spark • causes 80 million dollars in damage each year in the US • On average, kills 85 people a year in the US • over in a thousandth of a second • carries up to 200,000 A • causes the thunder!
development of a lightning bolt charge separation stepped leader leader & streamer leader meets streamer lightning bolt
applications of electrostatics • Xerox copiers use electrostatic attraction to put the ink droplets on the paper • electrostatic precipitators use the attraction of charged dust to remove dust particles from smoke. • can be used to hold balloons on your head
Removing soot particles Positive cylinder Chimney stack Charging units spray electrons on the soot particles soot