Download Chapter 21. Electric Charge

Chapter 21. Electric Charge
21.1. What is Physics?
21.2. Electric Charge
21.3. Conductors and Insulators
21.4. Coulomb's Law
21.5. Charge is Quantized
21.6. Charge is Conserved
What is Physics?
What is Electric Charge?
• An intrinsic property of protons
and electrons, which make up
all matter, is electric charge.
• A proton has a positive charge,
and an electron has a negative
charge.
Properties of electric charge
•
•
•
Two types of electric charge, positive and
negative; a proton has a positive charge,
and an electron has a negative charge.
The SI unit for measuring the magnitude
of electric charge is the coulomb (C).
The electric charge is said to be
quantized. The smallest amount of free
charge is e=1.6×10-19 C. Any electric
charge, q, occurs as integer multiples of
the elementary charge e
Example 1 A Lot of Electrons
How many electrons are there in one
coulomb of negative charge?
Solution The number N of electrons is
Continue on Properties of
electric charge
• Two electrically charged
objects exert a force on
one another, called as
electrostatic force : like
charges repel and unlike
charges attract each
other.
Continue on Properties of
electric charge
•
It is possible to transfer electric charge from one object to
another. Usually electrons are transferred, and the body
that gains electrons acquires an excess of negative
charge. The body that loses electrons has an excess of
positive charge.
•
During any process, the net electric charge of an entire
isolated system remains constant (is conserved). This is
referred to as the law of conservation of electric
charge.
Conductors and Insulators
Not only can electric charge exist on an object,
but it can also move through an object.
• Substances that readily conduct electric charge
are called electrical conductors.
• Materials that conduct electric charge poorly are
known as electrical insulators.
• Semiconductors are materials that are
intermediate between conductors and insulators
Charging by Contact and by Induction
• By Contact:
• By Induction :
Example: Bacterial Contamination and
Electrostatics
Checkpoint
The figure shows five pairs of plates: A, B, and D are
charged plastic plates and C is an electrically neutral
copper plate. The electrostatic forces between the pairs
of plates are shown for three of the pairs. For the
remaining two pairs, do the plates repel or attract each
other?
Coulomb's Law
• The electrostatic force is directed
along the line joining the charges,
and it is attractive if the charges
have unlike signs and repulsive if
the charges have like signs.
• The magnitude F of the
electrostatic force exerted by one
point charge q1 on another point
charge q2 is directly proportional to
the magnitudes |q1| and |q2| of the
charges and inversely proportional
to the square of the distance r
between them.
Coulomb's Law
The quantity ε0 , called the
permittivity constant,
sometimes appears separately
in equations and is
Example 3 A Model of the Hydrogen Atom
In the Bohr model of the hydrogen atom,
the electron (–e) is in orbit about the
nuclear proton (+e) at a radius of
r=5.29×10–11 m, as Figure shows.
Determine the speed of the electron,
assuming the orbit to be circular.
THE FORCE ON A POINT CHARGE DUE TO
TWO OR MORE OTHER POINT CHARGES
There are three charges q1, q2 and q3.What would
be the net force on q1 due to both q2 and q3?
• First, find the magnitude and direction of the force
exerted on q1 by q2 (ignoring q3).
• Then, determine the force exerted on q1 by q3
(ignoring q2).
• The net force on q1 is the vector sum of these
forces.
THE FORCE ON A POINT CHARGE DUE
Many POINT CHARGES
If we have n charged particles, they
interact independently in pairs, and the
force on any one of them, let us say
particle 1, is given by the vector sum
shell theorem
• A shell of uniform charge attracts or repels
a charged particle that is outside the shell
as if all the shell's charge were
concentrated at its center.
• If a charged particle is located inside a
shell of uniform charge, there is no net
electrostatic force on the particle from the
shell.
Spherical Conductors
• If excess charge is placed on a spherical
shell that is made of conducting material,
the excess charge spreads uniformly over
the (external) surface.
• If we remove negative charge from a
spherical metal shell, the resulting positive
charge of the shell is also spread uniformly
over the surface of the shell.
Example Three Charges in a Plane
Figure shows three point charges that lie in the x, y plane in
a vacuum. Find the magnitude and direction of the net
electrostatic force on q1.
Sample Problem
In Fig. 21-11a, two identical, electrically isolated conducting spheres
A and B are separated by a (center-to-center) distance a that is large
compared to the spheres. Sphere A has a positive charge of , and
sphere B is electrically neutral. Initially, there is no electrostatic force
between the spheres. (Assume that there is no induced charge on
the spheres because of their large separation.)
• (a) Suppose the spheres are connected for a moment by a
conducting wire. The wire is thin enough so that any net charge on it
is negligible. What is the electrostatic force between the spheres
after the wire is removed?
• (b) Next, suppose sphere A is grounded momentarily, and then the
ground connection is removed. What now is the electrostatic force
between the spheres?
Conceptual Questions
1.
In Figure the grounding wire is removed first,
followed by the rod, and the sphere is left with a
positive charge. If the rod were removed first,
followed by the grounding wire, would the sphere be
left with a charge? Account for your answer.
2. A metallic object is given a positive charge by the
process of induction, as illustrated in Figure 18.8.
(a) Does the mass of the object increase, decrease,
or remain the same? Why? (b) What happens to the
mass of the object if it is given a negative charge by
induction? Explain.
3.Blow up a balloon and rub it against your shirt a number of times.
In so doing you give the balloon a net electric charge. Now touch
the balloon to the ceiling. On being released, the balloon will
remain stuck to the ceiling. Why?
4. A particle is attached to a spring and is pushed so that the
spring is compressed more and more. As a result, the spring
exerts a greater and greater force on the particle. Similarly, a
charged particle experiences a greater and greater force when
pushed closer and closer to another particle that is fixed in
position and has a charge of the same polarity. In spite of the
similarity, the charged particle will not exhibit simple harmonic
motion on being released, as will the particle on the spring.
Explain why not.