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Electrostatics

Electrostatics. Textbook Chp 16. What happened here?. Electric Charges Electric Fields Electrostatic Charging Hazards of Electrostatic Charging Application of Electrostatic Charging – Photocopying Machine. Topics. Electric Charge is a physical quantity Units of charge is Coulomb (C)

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Electrostatics

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  1. Electrostatics Textbook Chp 16

  2. What happened here?

  3. Electric Charges • Electric Fields • Electrostatic Charging • Hazards of Electrostatic Charging • Application of Electrostatic Charging – Photocopying Machine Topics

  4. Electric Charge is a physical quantity • Units of charge is Coulomb (C) • It is NOT an object (e.g. an electron) • There are two types of electric charge – positive charge & negative charge • Note: charge is a scalar quantity, even though there is positive and negative! Electric Charges

  5. An object is said to be neutral when it has equal number of positive and negative charges (i.e. no net charge) • An object is said to be positively charged when it has more positive charges than negative charges • An object is said to be negatively charged when it has more negative charges than positive charges Electric Charges

  6. [not in syllabus] • Particles which carry charges are called charge carriers. • The electron is the most common charge carrier • Aqueous ions are also charge carriers • The charge of an electron is -1.60 x 10-19 C • This number is represented by the symbol “e” Electric Charges

  7. When two objects which have net charge are brought close together: • They attract each other if their charges are opposite • They repel each other if their charges are alike • E.g. electrons and protons attract each other; electrons repel each other, protons repel each other • This force of attraction / repulsion is called an Electric Force Electric Force

  8. Recall Gravitational Field - a region in which a massexperiences a force due to gravitational attraction • Electric Field is a region in which a charge experiences a force • Electric Field Worksheet Electric Fields

  9. You need to be familiar with two methods of electrostatic charging • 1) by rubbing • 2) by induction Electrostatic Charging

  10. Electrostatic charging by rubbing involves two non-conductors • When charging by rubbing occurs, ONLY electrons are transferred from one object to the other • The object receiving electrons becomes negatively charged • The object donating electrons becomes positively charged Charging by Rubbing

  11. When wool is used to rub against a perspex rod, wool becomes negatively charged • When wool is used to rub against a polythene rod, wool becomes positively charged • In both cases ONLY electrons are transferred, NOT protons. Charging by Rubbing

  12. Unlike insulators, you cannot charge conductors by rubbing • Conductors can be charged by another process called induction • 3 Principles of Charging by Induction: • a) Charges are free to move about within a conductor, or across conductors • b) Unlike charges attract, like charges repel • c) The Earth has an unlimited supply of charges Charging by Induction

  13. Charging by Induction Worksheet Charging by Induction

  14. pg 314 of your textbook describes the formation of lightning • Warning: this is a simplified model. Nobody really knows for sure how lightning works….yet. Hazards and Applications

  15. During a thunderstorm, bottom layer of clouds are negatively charged • This induces the ground to be positively charged • Charges continue to build up on both sides until air molecules are forced to split and become ions • A current of electricity flows and this is the lightning bolt Lightning (Simplified version)

  16. That’s why: • sometimes during a thunderstorm, your house circuit breaker goes off • Buildings have lightning conductors on them to protect them from lightning • In a thunderstorm, do NOT seek shelter under a tree! Lightning (Simplified version)

  17. On a smaller scale, electrostatic discharge also occur as sparks • Most of the time sparks are harmless, but they can be dangerous, e.g. when near flammable fluids (ref: Ironman 3) • Old oil tanker trucks dangle chains at the back of the truck to prevent such sparks from occurring Sparking

  18. Not in Syllabus, but in your MC textbook: • Electrostatic Precipitators • Spray Painting / Crop Spraying • Van de Graff generator Applications of Electrostatics Charging

  19. How do we explain these?

  20. key component of the photocopier: electrostatic drum: Photocopier

  21. Step 1) The entire drum is positively charged • Step 2) light shines on the original and image is reflected unto the drum (recall lenses) • [in laser printers, lasers are used to discharge the drum] • Step 3) light areas are discharged, while dark areas remain charged Photocopier

  22. Step 4) The positively charged portions of the drum attract toner (black powder) while the discharged portions do not • Step 5) Toner powder (in the shape of the image) is transferred over to positively charged paper • Step 6) Paper is heated, causing toner to fuse with the paper Photocopier

  23. Electric Charges • Electric Fields • Charging by Rubbing • Charging by Induction • Hazards of Electrostatic Charging • Application of Electrostatic Charging – Photocopying Machine Summary

  24. Quiz

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