Chapter 16
Electric
Charge and
Electric
Field
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•
The earliest studies in electricity date back to
ancient times
•
Electricity comes from the Greek word elektron
•
Electrostatics is the study of electric charges
that are at rest or can be collected in one place.
•
The effects of electrostatics are observable over
a vast scale, from huge displays of lightning to
the submicroscopic world of atoms and
molecules.
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16.1 Static Electricity; Electric Charge
and Its Conservation
An object can be charged by rubbing, and now
contains a net electric charge
What happened?
Outer electrons were removed from atoms.
The freed electrons remain unattached or
become attached to other atoms
From a microscopic viewpoint, acquiring
charge is a process of transferring
electrons.
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Charge comes in two types;
positive and negative
•like charges repel and
opposite charges attract
•Charge, like mass and
energy, is not created or
destroyed but conserved
•The law of conservation
of electric charge states
that the net amount of
electric charge produced in
any process is zero
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16.2 Electric Charge in the Atom
Atom is electrically
neutral.
Nucleus (small,
massive, positive
charge)
Electron cloud (large,
very low density,
negative charge)
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16.2 Electric Charge in the Atom
Polar molecule: neutral overall, but charge not
evenly distributed
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Conductors
Insulators
Semiconductors
Electrons not tightly
bound to atom
Electrons bound
tightly to atom
Vary
Charge flows freely
Almost no charge
flows
In between
conductors &
insulators
Metals
Most other materials silicon, germanium.
(wood, rubber, etc)
Used extensively in
computers &
technology
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16.4 Induced Charge; the Electroscope
•Metal objects can be charged by conduction,
when two objects come into contact and end up
with the same sign of charge
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16.4 Induced Charge; the Electroscope
•They can also be charged by induction, where charges
have been separated, or induced
•Objects are brought close to each other but do not touch
•Electrons move to the end of the material but do not leave
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16.4 Induced Charge; the Electroscope
Nonconductors won’t become charged by
conduction or induction, but will experience charge
separation
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16.4 Induced Charge; the Electroscope
The electroscope can be used for detecting
charge
Can be charged either by induction(a) or by
conduction (b).
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16.5 Coulomb’s Law
•1783 French physicist Charles de Coulomb
•Describes the force between two charged particles
•Equation gives the magnitude of the force.
•F is the electrostatic force in Newtons
•k is Coulomb’s constant :
•Q1 is the magnitude of the charge on particle 1
•Q2 is the magnitude of the charge on particle 2 in coulombs (C)
• r is the radius between the particles
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The force is directed along the line connecting the
charges, and is attractive if the charges are opposite,
and repulsive if they are the same.
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Coulomb’s Law
Law of Universal
Gravitation
Electric charge
Gravity
Attractive or repulsive
Attractive
Can be + or -
Can only +
Acts over a distance
Acts over a distance
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Elementary Charge
•Objects that are positively charged have a
deficit of electrons
•Objects that are negatively charged have an
excess of electrons
•Charge on an electron is negative and is
•Electric charge is quantized in units of the
electron charge.
•Exists only in discrete amounts (1e, 2e, 3e,
etc. Can’t have ½ or ¾!)
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•
Applies only to point charges
• Objects whose size is much smaller than the
distance between them

Applies to the force on a charge due to
only one other charge
◦ If several charges are present the net force on
any one of them will be the vector sum of the
forces
◦ This is the principle of superposition
Coulomb’s Law Considerations
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
•
•
•
1791-1867 Michael Faraday developed the
idea of an electric field, which extends out
from a charge (Q1) into space
Fields from charges can interact
The electric field is the force on a small
charge, divided by the charge
E is the electric field in N/C, F is the force
exerted in N and q is the magnitude of the
charge in C
16-7 Electric Field
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• If the field is due to more than one charge the
individual fields can be added vectorially
• This is the superposition principle for electric fields
superposition of
the electric field
radiated from two
antennas on the
top of a building
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www.awe-communications.com/Network/EMC/index.html
16.8 Field Lines
•The electric field can be represented by field lines or
lines of force
•Lines start on a positive charge and end on a negative
charge
•Drawn in 2 dimensions but actually radiate out in all
directions
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•The number of field lines on a charge is
proportional to the magnitude of the charge
•The electric field is stronger where the field
lines are closer together
•Lines never cross
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www.physicsclassroom.com/class/estatics/u8l4c.cfm
16.8 Field Lines
• When charges are equal and opposite an electric
dipole exists
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• When charges are equal and the same the
following occurs
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demo.webassign.net/.../read/main/c18x18_7.htm
Zitewitz. Physics: Principles and Problems. 2004
 Giancoli, Douglas. Physics: Principles with
Applications 6th Edition. 2009.
 http://www.chemistryland.com/CHM130W/Final/OralE
xam/LightningOverCity.jpg
 http://superphysics.netfirms.com/000688e6.jpg


www.awe-communications.com/Network/EMC/index.html

www.wikipremed.com/01physicscards.php?card=628
www.physicsclassroom.com/class/estatics/u8l4c.cfm
mcat-review.org/electrostatics-electromagneti...
www.tutorvista.com/topic/electric-field-lines...
demo.webassign.net/.../read/main/c18x18_7.htm
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References
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