Electrostatics

ElectrostaticsTextbook Chp 16

What happened here?

Topics

Electric ChargesElectric FieldsElectrostatic ChargingHazards of Electrostatic ChargingApplication of Electrostatic Charging –

Photocopying Machine

Electric Charges

Electric Charge is a physical quantityUnits of charge is Coulomb (C)It is NOT an object (e.g. an electron)There are two types of electric charge –

positive charge & negative chargeNote: charge is a scalar quantity, even

though there is positive and negative!

Electric Charges

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

[not in syllabus]Particles which carry charges are called

charge carriers.The electron is the most common charge

carrierAqueous ions are also charge carriersThe charge of an electron is -1.60 x 10-19 CThis number is represented by the symbol

“e”

Electric Force

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 Fields

Recall Gravitational Field - a region in which a mass experiences a force due to gravitational attraction

Electric Field is a region in which a charge experiences a force

Electric Field Worksheet

Electrostatic Charging

You need to be familiar with two methods of electrostatic charging

1) by rubbing2) by induction

Charging by Rubbing

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

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 Induction

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 conductorsb) Unlike charges attract, like charges repelc) The Earth has an unlimited supply of

charges

Charging by Induction

Charging by Induction Worksheet

Hazards and Applications

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.

Lightning (Simplified version)

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)

That’s why:sometimes during a thunderstorm, your

house circuit breaker goes offBuildings have lightning conductors on

them to protect them from lightningIn a thunderstorm, do NOT seek shelter

under a tree!

Sparking

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

Applications of Electrostatics Charging

Not in Syllabus, but in your MC textbook:Electrostatic PrecipitatorsSpray Painting / Crop SprayingVan de Graff generator

How do we explain these?

Photocopier

key component of the photocopier: electrostatic drum:

Photocopier

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

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

Summary

Electric ChargesElectric FieldsCharging by RubbingCharging by InductionHazards of Electrostatic ChargingApplication of Electrostatic Charging –

Photocopying Machine

Quiz