Ch 18: Electric
Force & Electric
Fields
• The electrical nature of matter comes
from atomic structure
Electrons
• Orbit the nucleus
in a “cloud”
• Relatively massless
• me=9.11x10-31kg
Nucleus –
• made up of protons
and neutrons
• Most of the mass of
the atom is
contained in the
nucleus
• mp=1.673x10-27kg
• mn=1.675x10-27kg
The Origin of Electricity
• The electrical nature of matter comes
from atomic structure
Electrons
• Have a negative
charge
• -e = -1.60x10-19C
Protons –
• Have a positive
charge
• +e = +1.60x10-19C
Neutrons –
• No net electrical
charge
The Origin of Electricity
• The electrical nature of matter comes from
atomic structure
• The magnitude of the charge on a proton =
the magnitude of the charge on the
electron. The magnitude is e=1.6x10-19 C.
• e is an elementary charge (it is indivisible)
• The SI unit for electrical charge is the
coulomb (C).
The Origin of Electricity
• The SI unit for electrical charge is the
coulomb (C).
• One coulomb is the charge which would
flow through a 120-watt light bulb in one
second.
• Two charges of 1 C each separated by a
meter would repel each other with a force
of about 1,000,000 tons!
• Usually, we measure charge in nanoCoulombs (x10-9 C)
The Coulomb
• In nature, atoms are usually found
with equal numbers of protons and
electrons => electrically neutral
• A net charge (q) can be built up on an
object by adding or removing
electrons
• q=Ne , where N is an integer
Electrical Charge
• How many electrons are there in one
coulomb of negative charge?
• q=Ne
• N=q/e = 1.00C/1.60x10-19C
• N=6.25x1018 electrons
Example 1
• Electrons can be transferred from one
object to another
• The object that gains electrons
acquires a negative charge
• The object that loses electrons has a
positive charge
Charged objects & electrical force
• When a plastic rod is rubbed with fur, some
of the electrons from the fur are transferred
to the rod.
• No electrons or protons are created or
destroyed
• The net charge of the fur/rod system
remains constant (Law of conservation of
electrical charge)
Charged objects & electrical force
• Two electrically charged objects exert a
force on each other.
• This electrostatic force can alter the
motion of an object. (Newton’s 2nd Law)
Charged objects & electrical force
• Object A and Object B are each
electrically neutral. 2 million electrons
are removed from A and placed on B.
What is the resulting charge on A and B?
• q=Ne
• q=(2,000,000)(1.60x10-19C) = 3.2x10-13 C
• A = +3.2x10-13 C
• B = - 3.2x10-13 C
Example 2
• Object A has a charge of -1.6x10-13C, and Object
B is electrically neutral. 2 million electrons are
removed from A and placed on B. What is the
resulting charge on A and B?
• q=Ne
• q=(2,000,000)(1.60x10-19C) = 3.2x10-13 C
• A = -1.6x10-13 C – (-3.2x10-13C) = +1.6x10-13C
• B = 0C + (- 3.2x10-13 C) = - 3.2x10-13 C
Example 3
• Charge can exist on an object, and also move
through an object.
• Substances that readily conduct electric charge
are called electrical conductors. (ex: metals)
• Materials that conduct electric charge poorly
are called electrical insulators. (example:
rubber, plastic, wood)
Conductors and Insulators
• Recall that a metallic bond has a “sea of
electrons”
• The valence electrons are not “attached” to the
parent atom – they can wander freely
throughout the material
• The ready movement of valence electrons is
what makes a good conductor.
Remember chemistry???
• In an insulator, there are very few electrons
free to move throughout the material.
• Insulators tend to have a more full valence
shell and higher ionization energies.
• The valence electrons are being held tightly by
the nucleus of its parent atom.
• Without the free electrons, there is very little
flow of charge electrical insulator
Remember chemistry???
• When a negatively charged rod is touched to a
metal sphere, some of the excess electrons
from the rod are transferred to the sphere.
• Once the rod is removed, the electrons repel
each other and spread out as far away from
each other as possible.
Charging by contact
• Identical conducting spheres equalize their
charge upon touching.
+5q
-3q
+5q
+q
Total charge is
+5q -3q = 2q
equally
distributed over
both spheres
-3q
+q
Charging by contact
• A negatively charged rod that is brought close
to (but not touching) a metal sphere induces
the electrons in the sphere to move away from
the negative rod causing a separation of
charge.
Charging by induction
• Grounding the sphere causes some of the
electrons to leave the sphere and redistribute
themselves over the earth (remember, they
want to be as far away from each other as
possible. The earth is a good conductor.)
Charging by induction
• If the grounding wire is removed, then the
negative rod is removed, the sphere is left with
a positive net charge.
Charging by induction
• If the sphere were made of an insulating
material (like plastic), you would not be able to
induce a net charge on it.
• However, you would still be able to separate
the positive and negative charges slightly and
induce a slight positive charge at the surface.
Charging by induction
• A rod made from insulating material carries a net
charge (which may be positive or negative),
whereas a copper sphere is electrically neutral. The
rod is held close to the sphere but does not touch it.
Which statement is true?
• A) the forces are always attractive
• B) the forces are always repulsive
• C) the forces are attractive when the rod is negative
and repulsive when positive
• D) the forces are repulsive when the rod is negative
and attractive when positive
• E) there are no forces
Example 4
• Focus p. 563 #1,2, 4
• Problems #1-6
Assignment