Kuliah Fisdas 2 Bab 2 Muatan Dan Medan Listrik

8/12/2019 Kuliah Fisdas 2 Bab 2 Muatan Dan Medan Listrik

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Geological Engineering Department

Faculty of Engineering

Physics for Scientists and Engineers

Electric Charge and Electric Field


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Introduction

The ancient Greeks discovered as early as 600B.C. that after they rubbed amber with wool, the

amber could attract other objects.

Today we say that the amber has acquired a net

electric charge, or has become charged.

The word electric is derived from the Greek

word elektron, meaning amber.


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Electric Charge & The Structure of Matter


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Electric Charge

Protonshave positivecharge

Electronshave negativecharge

Opposite signs attract

Similar signs repel


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Importance

Electric Charge is Conserved


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Conductors & Isolators

Materials permit electric charge to move easilyfrom one region of the material to another called

as CONDUCTORS

Materials do not permit electric charge to move

easily from one region of the material to another,

called as ISOLATORS


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Most metals are good conductors, while most nonmetalsare insulators.

Within a solid metal such as copper, one or more outer

electrons in each atom become detached and can movefreely throughout the material, just as the molecules of a

gas can move through the spaces between the grains in

a bucket of sand.

Some materials called semiconductors are

intermediate in their properties between good

conductors and good insulators


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Charging by Induction

We can charge a metal ball using a copper wire and anelectrically charged plastic rod, as in Figure below.

In this process, some of the excess electrons on the rod

are transferred from it to the ball, leaving the rod with asmaller negative charge.

But there is a different technique in which the plastic rod

can give another body a charge of opposite sign withoutlosing any of its own charge. This process is called

charging by induction


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The SI unit of electric charge is called one coulomb (1 C)


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Coulombs Law


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Strictly speaking, Coulombs law as we have stated itshould be used only for point charges in vacuum. If

matter is present in the space between the charges,

the net force acting on each charge is altered because

charges are induced in the molecules of the interveningmaterial.

As a practical matter, though, we can use Coulombs law

unaltered for point charges in air. At normal atmosphericpressure, the presence of air changes the electric force

from its vacuum value by only about one part in 2000.


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Class assignment


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Electric Field

G l i l E i i D


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G l i l E i i D t t


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G l i l E i i D t t


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G l i l E i i D t t


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Example:



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Home Assignment

A point charge q = -8.0 nC is located at theorigin. Find the electric field vector at the field

point x = 1.2 m, y = -1.6 m!



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Example:



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Answer:



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The Superposition of Electric Field



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Kuliah Fisdas 2 Bab 2 Muatan Dan Medan Listrik

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