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Elementary Electrostatics

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

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  1. 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

  2. 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

  3. 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.

  4. 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.

  5. Behaviour of Charges • ‘Like’ charges repel • ‘Unike’ or opposite charges attract • Q = + ne where n is a whole number

  6. 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.

  7. 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

  8. 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

  9. 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.

  10. 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.

  11. Charge by Cnduction (contact)

  12. Charge by Induction (no contact)

  13. 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.

  14. 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

  15. Free Standing Atoms are randomly oriented Charge does not actually move In/on the insulator – redistributes. Polarizes the material. Induction (contd) Insulators

  16. 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.

  17. http://qbx6.ltu.edu/s_schneider/physlets/main/efield.shtml

  18. Drawing the electric field

  19. 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!

  20. development of a lightning bolt charge separation stepped leader leader & streamer leader meets streamer lightning bolt

  21. 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

  22. Removing soot particles Positive cylinder Chimney stack Charging units spray electrons on the soot particles soot

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