Transcript Phy 103: Chapter 22

Phy 103: Fundamentals of
Physics
Chapter 22: Electrostatics
Lecture Notes
Electric charge
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The fundamental electrical property to which the
mutual attractions or repulsions between electrons or
protons (in the atom) is attributed.
Units of charge are called coulombs (C)
Two types (called polarity)
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Like charges repel
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Negative-negative
Positive-positive
Opposite charges attract
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Positive (e.g. protons)
Negative (e.g. electrons)
Positive-negative
The attraction-repulsion between charges is called
electric force
Electric Charge Tidbits
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Electric charge is “quantized”
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Electric charge has a smallest value that occurs in nature 
a single “packet” of charge
The smallest quantity of electric charge is called a quantum
of charge
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an electron has a quantum of negative electric charge (-e)
a proton has a quantum of positive electric charge (+e)
1 quantum of electric charge, e, has a value of 1.6 x 10-19C
Conservation of Electric Charge
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Electric charge is neither created nor destroyed
The total electric charge before an interaction equals the
total electric charge after the interaction
Electricity
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Electricity is the movement (or flow) of electric
charge
Electricity is due to the presence of electric forces
acting on charges (causing them to move)
Electricity can be the movement of:
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electrons (in metals)
ions (in fluids such as water or air)
The rate of flow of electric charge is called electric
current
The SI units for electric current are amperes (A)
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1 A is the flow of 1 coulomb of charge per second
How many electrons is this (per second)?
Newton’s Laws (quick review)
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What are Forces?
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The pushes or pulls on things that cause changes in motion
SI Units of force are called newtons (or N)
Types of forces:
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Contact (involves direct touching/contact)
Non-contact (occurs without direct contact, across space)
Newton’s 1st Law: when no net force acts on
something, its state of motion will remain unchanged
Newton’s 2st Law: a net force acts on something, its
state of motion will change: Fnet = ma
Newton’s 3rd Law: forces always act in actionreaction pairs between 2 objects
Coulomb’s Law & Electric Force
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Describes the magnitude of electric force between 2
“point” charges
Magnitude of electric force depends on
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Magnitude of the charges involved (q1 & q2)
Distance between the charges, 1/d2 (actually the distance squared!)
To calculate electric force:
Fk
q 1q 2
d
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2
The direction of the electric force depends on the
polarity of the charges
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Opposite charges (attractive)
Like charges (repel)
Charles Augustin de Coulomb
(1736-1806)
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Engineer by education
Won prize for his work on the
subject of friction
Won prize for using calculus of
variations to solve engineering
problems
Published 7 important papers on
electricity & magnetism (between
1785-1791), including:
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The law of attraction and repulsion
The electric point charges
Magnetic poles
The distribution of electricity on the surface of charged bodies
The Electric Field
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The ability of a charge to influence other charges in its
vicinity its electric field
The SI units for electric field are N/C
The electric field is a vector property
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E field lines originate at + charges & terminate at - charges
The direction of an electric field vector (at a point in
space) is the direction of electric force that would be
exerted by on a positive charge at that location
+
-
The Electric Field (cont.)
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The force (FE) exerted by an electric field on a charge
(qo) placed in the field (E) is
FE = qoE
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The direction of the electric force is:
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The same direction as the field (for positive charge)
The opposite direction as the field (for negative charge)
Formal definition of electric field:
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the electric force per unit charge that acts on a test charge at
a point in space or
E = FE/qo
Conductors & Insulators
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Conductance is a physical property of matter
Conductance describes the ability of a substance to
conduct electricity (allow charge to flow through it)
Electrically speaking, matter can be classified as
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Conductors: allow electrons/charge to flow through them
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Insulators: do not readily conduct electric charge
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non-metals, most ceramics, plastics, pure water
Semiconductors: under certain conditions conduct eleetric
charge but not under other conditions
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metals, dissolved salts in water
metalloids, some ceramics
Superconductors: allow electric charge to flow without any
material resistance
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superconduction occurs at very low temperatures (<100 K)
certain ceramics and metal oxides
Charging
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Charging is the process of
generating excess electric charge
 localizing regions of charge within a substance
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There are 2 types of Charging:
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Charging by friction and contact
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Electrons can be transferred from one material to
another by simply touching.
Charging by induction
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Redistribution of electric charges in and on objects
caused by the electrical influence of a charged object
close by but not in contact.
Work & Energy (review)
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Work is a measure of mechanical output (movement)
due to some effort (a force)
Work = (Force Applied)*(Distance Traveled)
Energy is the capacity to do work
Energy comes in many forms:
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Kinetic energy (energy of motion)
Heat (energy that flows from hot to cold)
Potential energy (energy due to position)
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Gravitational
Electrical
Nuclear
Chemical
The SI Units of work & energy are Joules (J)
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Other units include calories & kilowatt-hours (kW.hr)
Electric Potential
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A charge placed in an electric field has an associated
electric potential energy (EPE)
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EPE is essentially the work required to place the charge at
that location
EPE is the energy a charged object possesses by
virtue of its location in an electric field
The electric potential energy reflects the amount of
work the electric field can perform on the charge if it
is free to move it
Electric potential is a charged object’s EPE divided
by its charge or
Electric potential = EPE/charge
or
V = EPE/q
Units of electric potential are J/C (or volts)
Alessandro Volta (1745-1827)
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Italian physicist & inventor
First person to isolate methane
Fascinated with electricity at an
early age
Pioneered the field of
electrochemistry
Constructed the first battery to produce
electricity (called a voltaic pile)
Capacitors
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an electrical device that stores electric charge and
electrical potential energy
in its simplest form a pair of parallel conducting
plates separated by a small distance
+q
+
-
+
-
+
-
+
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E
-q
-
Electric charge of opposite sign accumulates on face
of each plate, establishing an electric field between
the plates
The electric field between the plates has a constant
value (magnitude & direction)