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Lecture 1
Introduction. Electrical Forces.
Chapter 15.1  15.3
Outline
• Electric Charge
• Conductors and Insulators
• Coulomb’s Law
The Electrical Phenomenon
Discovered in ancient Greece more than 2500 years ago
The phenomenon occurs from rubbing of one material
(e.g., amber  electron in Greek) with another
material (e.g., fur).
It manifests itself by attraction or repellence of
‘charged’ materials.
There are 2 types of charge: positive and negative.
Like charges repel, unlike charges attract.
The Nature of Charge
Charge is not produced by rubbing.
Uncharged objects contain equal amounts of negative
and positive charge. They are electrically neutral.
Electric charge is always conserved.
In the process of rubbing, charge is not created.
Negative charge is transferred from one object to the other.
The nature of charge can be traced to the composition of
matter, to atoms.
Atomic Structure
92 chemical elements have been identified in the Universe.
Nearly 20 more have been created artificially.
Each chemical element is made from a different type of
atom.
Atoms are made from particles called protons, neutrons,
and electrons.
Protons and neutrons form the nucleus in the center of the
atom.
Electrons surround the nucleus.
Electrons are responsible for the charge transfer.
Atomic Structure
Positively charged protons are held together by the strong
force, which overcomes electrical repulsion.
Negatively charged electrons are attracted to the nucleus.
Proton has a mass 1.673 1027 kg and is positively charged
Neutron has a mass 1.675 1027 kg and is uncharged
Electron has a mass 9.11 1031 kg and is negatively charged
Proton and neutron are ~2000 times heavier than electron
Conduction and Induction
Charge can be transferred onto an object by:
Conduction  objects are in physical contact
Result: the object being charged gets the same sign charge
as the object doing the charging.
Induction  no contact between the objects
Result: the object doing the charging is left with the same
charge; the object being charged get an opposite charge.
Polarization: charging an insulator by induction.
The Coulomb
The unit of electric charge is the coulomb (C).
The proton has a charge of + 1.6 1019 C.
The electron has a charge of  1.6 1019 C.
The quantity of charge is abbreviated e.
Electric charge occurs only in multiples of e.
It is said to be quantized.
Coulomb’s Law:
|q1| |q2|
F = ke ---------r2
ke = 8.99 109 N m2/C2
Applying Coulomb’s Law
Object A has a charge of +5mC, and object B has
a charge of 10mC. Which statement is true:
a) FAB = 2 FBA
b) FAB = FBA
c) FAB= FBA
d) FAB = 2 FBA
Calculate the magnitude of the Coulomb force
between the 2 charges of 1mC each separated by 1m.
F=8.99 109Nm2/C2 (103 C)(103 C)/(1m)2 = 8990 N
This is equal to the gravitational attraction force
between the 2 masses of 10Mkg separated by 1m.
Electricity and Gravity
The law of gravity and Coulomb’s law have the same form.
Gravitational force is always attractive.
Electric force can be either attractive or repulsive.
Forces have both magnitude and direction.
It is harder to collect a large electric charge of either sign
than a large mass of matter.
Thus, gravity is more significant on a large scale (cosmic
size), while electric forces are more significant on a small
scale (atomic size)
Conductors and Insulators
A conductor is a substance through which electric charge
flows readily. Example, metals.
In an insulator charge flows with great difficulty.
The difference between the 2 types of substances is in the
strength of connection between outer electrons and
nuclei.
Conductivity in fluids and gases involves ions.
Summary
• Electrical forces are different from gravity.
They are more influential on small scales.
• Electric properties of matter allow us to transmit and
store energy.
• Most properties of ordinary matter can be traced to
electrical forces