INTRODUCTION TO ELECTROSTATICS

What is Electrostatics? The study of charges at rest.

Origin of Electricity: started with the observation of the unusual behavior of amber and lodestone.

Lodestone --- a magnetized iron ore

Amber ---- translucent yellowish-brown fossil resin

*(amber is “elektron” in Greek, hence the word electricity)

Theories on Charges1. One Fluid Theory by Benjamin Franklin

- suggests that all bodies possess a certain amount of “electric fluid” needed to keep them uncharged

2. Two Fluid Theory by Charles Du Fay- suggest that all bodies contain equal amounts of 2 kinds of fluids

3. Dielectric Theory by Maxwell and Faraday- considers charges as a form of strain in the hypothetical ether surrounding a body

4. Electron Theory- explains the existence of charges by the structure of an atom

The Atomic Structure: A Review Neutron ------ uncharged Protons ------- positive charge Electron ------ negative charge

* An atom is uncharged or neutral if it has equal number of protons and electrons.

* An atom becomes negatively charged (ANION) if it gains additional electrons (more electrons than protons),

* An atom becomes positively charged (CATION) if it loses electrons (more protons than electrons).

Electric Charges A fundamental property

of some particles which could either be negative or positive.

COULOMB’s LAW Rule of Charges

LIKE CHARGES REPEL UNLIKE CHARGES ATTRACT

Material Types Based on Electrical ConductivityConductor

-a material which has plenty of free (valence) electrons which readily allow charges to flow

Insulator- a material that resists the flow of charges

Semiconductor- an intermediate group of materials between conductors and insulators which can act either as an insulator or conductor depending on specific orientation and/or conditions

Superconductor- a material that offers practically no resistance to the flow of charges below some critical temperatures

Electrostatic Charging- a process by which an insulator or an insulated

conductor receives a net charge; it involves a neutral body and a charged (positive or negative) body

Methods of Electrostatic Charging1. Charging by Conduction

- charging in which there is actual contact between the neutral body and the charged body

a. charging by friction- done by rubbing a neutral material with cloth or fur, electrons are then transferred making the body charged

b. charging by contact- done by placing the neutral body into contact with a charge body; charges will then be conducted into the neutral body

The Triboelectric Series – a list that ranks various materials according to their tendency to gain or lose electrons.

Most Positive (+)

Air +++

      +

Human Hands, Skin

Rabbit Fur

Glass

Human Hair

Nylon

Wool

Lead

Cat Fur

Silk

Aluminum

Paper

Cotton

Steel

-    

 - - -

Wood

Sealing Wax

Amber

Rubber Balloon

Hard Rubber

Nickel

Copper

Silver

Brass

Synthetic Rubber

Gold, Platinum

Sulfur

Teflon

Most Negative

Methods of Electrostatic Charging2. Charging by Induction

- charging that does not involve contact between the neutral body (body to be charge) and the charge body- this is done by placing both bodies close together; polarization then occurs in the neutral body; by grounding the neutral body (touching it with a third body) charges will flow into the neutral body thereby charging it

Methods of Electrostatic ChargingWhat is Polarization The realignment or separation of charges in

one body when another body, that is charged, is placed close to it.

fin

Quantization of Charge The SI unit of charge is the Coulomb ( C ). The charge of one electron is q = -1.6 x 10-19 C ; the charge of one proton is q = +1.6 x 10-19 C.

. Since a body is charged either by gaining or losing

electrons, it follows that the charge of a body must be integral multiples of the charge of one electron (or proton). And the smallest charge of a particle must the be 1.6 x 10-19 C.

 OPERATIONAL DEFINITION

q = n x (1.6 x 10-19 C) where: q is the total charge of n electrons

n is the number of electrons

BASIC Examples1. An object is to have a charge of 1.0 C. How

many electrons are involved in the process? Are the electrons removed or given to the object?

2. A comb rubbed on the hair several times acquires a net charge of -0.96 C. Did the comb gain or lose electrons? How many electrons were transferred?

Ohm’s Law

And the basics of CIRCUITS

The Concept of Potential Difference POTENTIAL DIFFERENCE

- the work done or energy needed in moving a unit electric charge between two points in an electric field

Sources of Potential Difference1. electric cells – uses chemical energy2. generators – uses mechanical energy3. solar cells (or photovoltaic cells) –

uses light energy

EMF vs.Voltage Electromotive Force (emf)

- the potential difference between the terminals of a cell when no current flows

Terminal Voltage (or simply voltage, V) or J/C- the potential difference between the terminals of a cell when current flows

  Since a cell has an internal resistance that

requires energy (or potential difference) to overcome, then the terminal voltage is always less than the electromotive force.

( V < emf )

Current and Resistance ELECTRIC CURRENT ( I )

- amount of charge passing through any point in a conductor per unit time I = q / t unit: C/s or ampere (A); 1C/s = 1A

RESISTANCE ( R )- the opposition a material offers to the flow of charges

through it- the SI unit of resistance is ohm (Ω) or J.s / C2

- named after George Simon Ohm

OHM’s LAW it states that in an electric circuit; the current (I) passing through a

conductor varies directly as the potential difference (V) applied at its ends and inversely as the resistance (R) of the conductor; this maybe applied to the entire circuit or to a particular part of a circuit

OPERATIONAL DEFINITIONfor the whole circuit:IT = VT / RT

for a portion of the circuit:I = V / R

Units: I --- amperes (A); 1A = 1C/s V – volt (V) R – ohm (Ω)

Example (Ohm’s Law)

A typical value of the resistance of a person from hand to hand through the body is 2000

Ω. If a person with this resistance accidentally touches a 220V live wire, what current passes

through the person?

Simple Circuits SERIES CIRCUIT a simple circuit that

contains more than one piece of electrical apparatus (or resistors) connected one after the other in a single line

in this circuit, the current flows in a single path and is the same in all parts; and the current stops flowing whenever a part of the circuit fails

Simple Circuits PARALLEL CIRCUIT a simple circuit where two or more pieces of electrical

apparatus (or resistors) are connected side by side so that the current is divided between them

in this circuit, each apparatus operates independently of the others so even if one piece fails, current still flows through the others