Chapter 17:

Static Electricity:

Static Electricity:

Electrostatic Phenomena

Have you ever experience:

You walk across the rug, reach for the doorknob and..........ZAP!!! You get a shock. WHY?

Static Electricity:

Electrostatic PhenomenaOR have you ever experience:

You come inside from the cold, pull off your hat and......BOING!!! All your hair stands on end. What is going on here? And why does it only happen in the winter?

Static Electricity:

Electrostatic Phenomena

Static Electricity:

Electrostatic Phenomena

Static Electricity:

Electrostatic Phenomena

Static Electricity:

Electrostatic Phenomena

Summary

Static Electricity:

Structure of an atom

Static Electricity:

Insulators & Conductors

Static Electricity:

Neutralising Charged Conductors

Static Electricity:

Neutralising Charged Conductors

Static Electricity:

Charging by Induction

Static Electricity:

Charging by Induction

Static Electricity:

Charging a single conductor

Static Electricity:

Charging a single conductor

Electric Field:

Unit of Charge – The Coulomb (C)The Coulomb is the quantity of electric

charge that passes through a given section in a circuit when a steady

current of one ampere flows for one second.

ItQt

QI

Electric Field:

Unit of Charge – The Coulomb (C)

Unlike Charges Like Charges

Electric Field:

Coulomb’s Law

The electrostatic force of interaction between two charged particles is proportional to the product of their charges and inversely proportional to the square of their separation, and its direction is along the line joining the t

wo charges.

Where є=8.85 x 10-12 C2 N-1 m-

2

221

E rQQ

4π1

F

Electric Field:

An Electric Field exists in a region of space where a small positive charge experiences an electric force.

The direction of the field is defined as the direction of the force on a small positive charge.

Electric Field:

An Electric Field exists in a region of space where a small positive charge experiences an electric force.

The direction of the field is defined as the direction of the force on a small positive charge.

Electric Field:

An electric dipole

Practical Application of

Electrostatics:

1. Electrostatic Precipitator

Practical Application of

Electrostatics:

2. Spray Paint

Practical Application of

Electrostatics:

3a. Photocopier

Drum coated with Selenium, a photoconductor

White parts of paper reflect lot of light to some part of drum, making these areas conducting and loses charges

Practical Application of

Electrostatics:

3b. Photocopier

Fine particles of carbon powder (toner) are attracted to charged area of the drum

Potential Hazards with Electrostatics1a. Lightning

• Friction between clouds and wind causes the cloud to be charged

NEGATIVELY• Highly Charged clouds ionise the air around it

• Air becomes a conductor to these charges

• Charges flow through the air to the earth at very high speeds, giving off light

• Charges take the shortest path

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Potential Hazards with Electrostatics1b. Lightning

• Lightning conductors are fitted on top of buildings

• When thunderclouds are nearby, positive charges are induced on

lightning conductors

• Induced positive charges remove electrons from the nearby air and ionise surrounding air

• Positive charged air flow upwards to neutralise the thundercloud

Potential Hazards with Electrostatics1c. Lightning Rod

• Charge flows readily to or from sharp metal points

• Point of the rod collects electrons from the air, preventing large buildup of positive charge on the building by induction.

• Continual “leakage” of charge prevents a charge buildup that might lead to sudden discharge between the cloud and buildingPurpose of lightning rod:

To prevent a lightning discharge. Hence, to prevent a fire caused by lightning!

Potential Hazards with Electrostatics1c. Lightning Rod

• Lightning rod collects the electrons from the bottom of the cloud and discharges

them, preventing a large discharge in the form of lightning.

Should a discharge occur, the sharp

point causes a concentration of positive charge at the

tip, attracting the lightning and

conducts the electrons to the ground, preventing damage to the building

Potential Hazards with Electrostatics2. Fires and

explosions• Tyres of moving truck acquires

negatively charges by friction from the road

• Metal body near tyres become positively charged by induction

• Sparks may be produced

• Metal chain at the rear end conducts electrons from ground to neutralise positive charges on the metal body of truck

• It also allows the truck to discharge excess charge

Van De Graaff

Generator

• Charges on surface of dome cause the air nearby to be ionised, making the air partially conducting

• A pathway for charges on the dome to leak off into the air, causing “Lightning”

Electrostatic – Shock!• When your shoes scuff against the rug, your shoes are actually picking up electrons from the rug.

• The electrons move over your body, giving you a negative charge. 

• If you touch a metal doorknob, the electrons on your body will leap into the metal, attracted by the protons there.

• The transfer of electrons is actually a small

electrical current, and produces the tiny electric shock you feel. 

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Electrostatic