Download Physics Chapter 17 Notes Electric forces and fields

Physics Chapter 17 Notes:
Electric forces and fields
Dryness is important for
most electrical circuits
because excessive moisture
can provide a pathway for
charge to leak off a charged
object.
One of the most incredible
creations is the human
1
body, which is electrical.
In the human body,
electrical structures operate
freely in a moist
environment.
Usually, this spells disaster.
We know that like charges
repel and opposite charges
attract.
2
Typically, we describe
movements of electrons
when we track charges,
simply because protons and
neutrons are trapped in
nuclei and are not as free to
move about.
In the early 1900’s, it was
Millikan that quantified or
measured the electron with
an oil-drop experiment.
3
An electron has a charge
(C) of
-19
q = –1.60 x 10 C and a
-31
mass in kg of 9.109 x 10
kg.
A proton has a charge of
-19
q = +1.60 x 10 C and a
-27
mass of 1.673 x 10 kg.
4
A neutron has a charge of
0 C and a mass of
-27
1.675 x 10 kg.
Materials in which electric
charges move freely, such
as copper and aluminum
are called conductors.
Materials in which electric
charges do not move freely,
such as glass, rubber, silk
5
and plastic are called
insulators.
When a balloon is rubbed
against something, a static
charge results. This
process of transferring
charge directly from one
object to another is called
charging by contact.
6
When a conductor is
connected to the earth by
means of a conducting wire
or copper pipe, the
conductor is said to be
grounded. The earth can
be considered to be an
infinite reservoir for
electrons because it can
accept or supply an
unlimited number of
electrons.
7
Induction is the process of
charging a conductor by
bringing it near another
charged object and
grounding the conductor.
Coulomb’s Law is used to
calculate how small or
large and electric force can
be.
8
Felectric=kC(q1q2)
2
r
The value of the Coulomb
constant is:
8.99 x 10 N  m
2
C
The Coulomb force is the
second example we have
studied of a force that is
exerted by one object on
another even though there
8
2
9
is no physical contact
between the two objects
(field force). The other
example of field force is
gravitational attraction.
Both forces are inversely
proportional to the square
of the distance of
separation.
10
Because electrical field
strength is a ratio of force
to charge, the SI units of E
are Newtons per Coulomb
(N/C).
The formula for
determining electric field
strength from a point
charge is:
E = kC q
2
r
11
Electric field lines are
lines that represent both the
magnitude and direction of
the electric field.
When one end of a
conductor is sharper than
the other, excess charge
tends to accumulate at the
sharper end, resulting in a
larger charge per unit area
and therefore a larger
12
repulsive electric force
between charges at this end.
In a Van de Graaf
generator, charge is
transferred to the dome by a
rotating belt that is kept in
motion by a pulley. This is
the device that makes your
hair stand on end when you
touch it.
13