Electricity

Chapter 19

Introduction One of the oldest problem in physics phenomena related to electric charge deals with

-interactions between charges

-movement of charges -->

(electric currents)

-production of charges

-detection of charges

-measurement of charges

-properties of electric currents

-electric apparatus

Electric charges: a reality Experiments showing the existence of electric charges

- combing the hair

- rubbing a balloon with a piece of wool

- glass rod rubbed with silk

- amber rubbed with wool

- rubber rod rubbed with fur– Two different kind of charges: positive and negative

(one kind on the wool and the other type of the balloon)– Like charges repel; unlike charges attract

- Materials that are able to transfer charges are known as conductors- Materials that cannot transfer charges are known as insulators- Grounding --> giving the charges to the Earth

Conservation of charges When creating a charge with a given sign we have to

create the same amount of opposite charge “The amount of electric fluid should remain the same” -->

Benjamin Franklin Objects are electrically neutral not because they

contain no charges! --> they contain the same

amount of positive and negative charges– The law of charge conservation:

In an isolated system the total charge is conserved

total charge: algebraic sum of the charges

Induced Attractions Attraction is more common than repulsion Charged objects can attract uncharged ones

A charged rod attracts a neutral metal ball Explaining attraction between charged and

uncharged objects --> induced charges separation of charges in the uncharged object (it can be conductor or isolator)

Measuring the amount of charge: the electroscope The electroscope can be used to detect charges

- Working principle: same type of charges repel each other

- Construction and working principle

- The electroscope does not indicate

the sign of the charges!

Experiments with the electroscope:

The electric force Calculating the magnitude of the attractive or

repulsive forces between the charges Variation with distance and amount of charges

221

r

QQkF

Coulomb’s law

-Charles Coulomb 1785 Q1 and Q2 : the two charges (measured in coulomb: C) r: distance between the charges k: 9 x 109 N m2/C2

Coulomb’s constant

Difference and Similarities between Electricity and Gravity

Mathematical form of the Coulomb law and law of gravitation very similar

- gravitation is always attractive (no

negative mass)

- electrical force can be both

attractive or repulsive

- Electric force is dominant in the

atomic world

- Gravitational forces dominates on the

macroscopic scale: people, planets, galaxies

- Electric forces are more stronger !!!

221

r

QQkFe

221

r

mmGFg

2

29109C

Nmk

2

211107.6kg

NmG

Lightning - During the storm clouds develop a separation of

electric charge

- Tops of the clouds positively, bottoms

negatively charged

- The ground positively charged due to the

repulsion of the electrons in the ground

-downward moving electrons

- upward flowing positive charges

The Electric Field Electric charges modify the space around them--> concept of electric

field This electric field is manifested by the force acting on charges Concept of electric field is analogous with the concept of

gravitational field Definition: The value of the electric field (E) at any point in space is

equal to the force experienced by one unit of positive charge at that point

electric field is a vector: q

FE

2r

QkE

- electric field can be represented by a set of arrows around the object (size of the arrows representing the magnitude of the field)- electric field can be represented by a set of continuous lines starting from positive charges and ending at negative ones (density of the lines ~ intensity of the field)

Units: N/C

Electric potential Characterizes energetically the electric field Similar quantity to the gravitational potential Definition of electric potential energy:

For each point, the electric potential energy is equal to the work done against the field in bringing the charged object from some zero reference location to that point (if we do the work EPE> 0, if the field does the work EPE <0)

- It’s value does not depend on the path but on where we choose the zero reference location at a distance r from Q

- Measured in : Joules (J) zero level at infinite Definition of the electric potential: electric potential energy of unit

charge (EPE/charge) at a distance r from Q

- Units: J/C= Volt (V) By knowing the electric potential difference between two points we

can calculate the work done by the field when moving a given charge between the two points

r

QqkEPE

r

QkV

Summary Objects become electrically charged or uncharged by transferring charges Two different charges exist: positive and negative In Isolated systems the total charge is conserved Like charges repel, unlike charges attract The electric force between two charges is given by the Coulomb law Electric charges are surrounded by an electric field equal to the force

experienced by a unit charge electric potential characterizes energetically the electric field The electric potential energy gives the work made against the electric field,

when we bring the given charge from a zero reference location to the given point

The electric potential equals the electric potential energy divided by the objects charge

Home-work Assignment:Part I: 492/3-8,11,15-27; 493/28-31; 495/2-12

Part II: 493/33-35, 37-42; 494/43-48,50-58; 495/59,60,13-18,23,24