Transcript chapter7-Section2

Vern J. Ostdiek
Donald J. Bord
Chapter 7
Electricity
(Section 2)
7.2 Electric Force and Coulomb’s Law
• The original amber effect illustrates that electric
charges can exert forces.
• You may have noticed hair being pulled toward a
charged comb or “static cling” between items of
clothes removed from a dryer.
•
These are among the most common situations
illustrating this effect—two objects with opposite
charges attracting each other.
7.2 Electric Force and Coulomb’s Law
• The negatively charged comb exerts an attractive
force on the positively charged hair.
•
In addition, two objects with the same kind of
charge (both positive or both negative) repel each
other.
• When two similarly charged combs are suspended
from threads, they push each other apart.
•
Just remember this simple rule: like charges repel,
unlike charges attract.
7.2 Electric Force and Coulomb’s Law
• This force between charged objects is extremely
important in the physical world, particularly at the
atomic level.
• It is this interaction that holds atoms together and
makes it possible for them to exist.
•
In each atom, the positively charged protons in the
nucleus exert attractive forces on the negatively
charged electrons.
7.2 Electric Force and Coulomb’s Law
• The electric force on each electron provides the
centripetal force that keeps it in its orbit, much as
the gravitational force exerted by the Sun keeps
Earth in its orbit.
•
The forces between the atoms in many compounds
arise because opposite charges attract.
7.2 Electric Force and Coulomb’s Law
• For example, when salt is formed from the
elements sodium and chlorine, each sodium atom
gives up an electron to a chlorine atom.
•
The resulting ions exert attractive forces on one
another because they are oppositely charged.
• Matter as we know and experience it would not
exist without the electrical force.
7.2 Electric Force and Coulomb’s Law
• Recall Newton’s law of universal gravitation, which
gives the size of the force acting between two
masses.
•
The corresponding law for the electrical force,
Coulomb’s law, is very similar.
Coulomb’s Law: The force acting on each of two
charged objects is directly proportional to the net
charges on the objects and inversely proportional
to the square of the distance between them:
q1q2
Fµ 2
d
7.2 Electric Force and Coulomb’s Law
• The constant of proportionality in SI units is:
9 × 109 N-m2/C2
• Therefore, in SI units, with F in newtons, q1 and q2
in coulombs, and d in meters.
9 ´10 ) q q
(
F=
9
1 2
d
2
7.2 Electric Force and Coulomb’s Law
• The force on q1 is equal and opposite to the force
on q2, by Newton’s third law of motion.
•
•
Note that if both objects have the same kind of
charge (both positive or both negative), then the
force F is positive.
This indicates a repulsive force.
• If one charge is negative and the other positive,
then the force F is negative, indicating an
attractive force.
7.2 Electric Force and Coulomb’s Law
• If the distance between two charged objects is
doubled, then the forces are reduced to one-fourth
their original values.
7.2 Electric Force and Coulomb’s Law
• Perhaps it is not a surprise that Coulomb’s law has
the same form as Newton’s law of universal
gravitation.
•
After all, mass and charge are both fundamental
properties of the particles that comprise matter.
• We must remember, however, that the
(gravitational) force between two bodies because
of their masses is always an attractive force,
whereas the (electrostatic) force between two
bodies from their electric charges can be attractive
or repulsive, depending on whether or not they
have opposite charges.
7.2 Electric Force and Coulomb’s Law
• Also, all matter has mass and so experiences and
exerts gravitational forces, whereas the
electrostatic force normally acts between objects
only when there is a net charge on one or both of
them.
• Generally, when two objects have electric
charges, the electrostatic force between them is
much stronger than the gravitational force.
•
For example, the electrostatic force between an
electron and a proton is about 1039 times as large
as the gravitational force between them.
7.2 Electric Force and Coulomb’s Law
• It is possible for a charged object to exert a force
of attraction on a second object that has no net
charge.
• This is what happens when a charged comb is
used to pick up bits of paper or thread.
•
Here the negatively charged comb attracts the
nuclei of the atoms and repels the electrons.
7.2 Electric Force and Coulomb’s Law
• The orbits of the electrons are distorted so that the
electrons are, on the average, farther away from
the charged comb than the nuclei.
7.2 Electric Force and Coulomb’s Law
• This results in a net attractive force because the
repulsive force on the slightly more distant,
negatively charged electrons is smaller than the
attractive force on the closer, positively charged
protons.
• The process of inducing a small charge separation
(or displacement) between the nucleus of an atom
and its electrons is called polarization.
7.2 Electric Force and Coulomb’s Law
• Some molecules are naturally polarized—that is,
they have a net negative charge displaced to one
side of the net positive nuclear charge.
• They are called polar molecules.
•
•
Water molecules have this property.
If a polar molecule is free to rotate—as in a
liquid—it will be attracted to a charged object.
•
Its side with the charge opposite that on the object
will turn toward the object, and the attractive force
on that side will be stronger than the repulsive force
on the other side.
7.2 Electric Force and Coulomb’s Law
• The electrostatic force is another example of
“action at a distance.”
• As with gravitation, the concept of a field is useful.
In the space around any charged object, there is
an electric field.
• This field is the “agent” of the electrostatic force:
•
it will cause any charged object to experience a
force.
7.2 Electric Force and Coulomb’s Law
• The electric field around a charged
particle is represented by lines that
indicate the direction of the force
that the field would exert on a
positive charge.
•
The electric field lines around a
positively charged particle point
radially outward, and the field lines
around a negatively charged
particle point radially inward.
7.2 Electric Force and Coulomb’s Law
• The strength of an electric field at a point in space
is equal to the size of the force that it would cause
on a given charged object placed at that point,
divided by the size of the charge on the object.
force on a charged object
electric field strength =
charge on the object
7.2 Electric Force and Coulomb’s Law
• Where the field is strong, a charged object will
experience a large force.
• The strength of the electric field is indicated by the
spacing or density of the field lines:
•
where the lines are close together, the field is
strong.
• The electric field around a charged particle is
clearly weaker at greater distances from it.
7.2 Electric Force and Coulomb’s Law
• Any time a positive charge is in an electric field, it
experiences a force in the same direction as the
field lines.
•
A negative charge in an electric field feels a force in
the opposite direction of the field lines.
7.2 Electric Force and Coulomb’s Law
• Perhaps you have had the experience of walking
across a carpeted floor and receiving a shock
when you touched a metal doorknob.
•
This is more likely to happen in winter than in
summer because the relative humidity is usually
lower then, and electrostatic charging takes place
more readily.
7.2 Electric Force and Coulomb’s Law
• The shock results from charges flowing between
you and the doorknob, and it may be accompanied
by a visible spark.
•
•
Air normally does not allow charges to flow through
it.
A spark occurs when there is an electric field strong
enough to ionize atoms in the air.
• Freed electrons accelerate in a direction opposite
to the direction of the electric field, and positive
ions accelerate in the same direction as the field.
•
The electrons and ions pick up speed and collide
with other atoms and molecules, ionizing them or
causing them to emit light.
7.2 Electric Force and Coulomb’s Law
• Lightning is produced in this same way on a much
larger scale as Benjamin Franklin demonstrated
using kites, keys, and metal rods in the middle of
the 18th century.
7.2 Electric Force and Coulomb’s Law
• Although most of the electrical devices we rely on
make use of electric currents, some depend
primarily on electrostatics.
•
One important example of the latter is the
electrostatic precipitator, an air-pollution control
device.
7.2 Electric Force and Coulomb’s Law
• Tiny particles of soot, ash, and dust are major
components of the airborne emissions from power
plants that burn fossil fuels and from many
industrial processing plants.
• Electrostatic precipitators can remove nearly all of
these particles from the emissions.
•
The flue gas containing the particles is passed
between a series of positively charged metal plates
and negatively charged wires.
7.2 Electric Force and Coulomb’s Law
7.2 Electric Force and Coulomb’s Law
• The strong electric field around the wires creates
negative ions in the particles.
•
These negatively charged particles are attracted by
the positively charged plates and collect on them.
• Periodically, the plates are shaken so the collected
soot, ash, and dust slide down into a collection
hopper.
•
This “fly ash” must then be disposed of, but
sometimes it has its own uses—for example, as a
filler in concrete.
7.2 Electric Force and Coulomb’s Law
• Electronic signs that behave like electronically
erasable paper make use of electric fields to form
letters and other images.
•
One type, called SmartPaper, consists of millions of
tiny beads between two thin plastic sheets.
7.2 Electric Force and Coulomb’s Law
• One side of each bead is a particular color and
negatively charged, and the other side is a
contrasting color and positively charged.
•
•
An electric field exerts opposite forces on the two
sides, causing the beads to rotate until they are
aligned with the field.
Letters are formed on the electronic paper by
selectively applying upward and downward electric
fields at different places so that parts of the display
are one color and the rest are the other color.
Concept Map 7.1