Download Charged material A will repel other charged material A. Charged

16.1 Two types of charge (plus neutral)
Wednesday, July 15, 2015
4:06 PM
Evidence: Hang various materials that have been rubbed with
other materials.
Charged material A will repel other charged material A.
Charged material B will repel other charged material B.
Charged material A will either attract or repel, but never have
no effect, on charged material B.
Similarly charged material A will repel or attract charged
material C. And if A attracts B then A will also attract C but only
if B and C repel. Similarly, if A repels B then A also repels C but
only if B and C repel.
Ben Franklin chose the name positive charge to apply to a glass
rod rubbed with a piece of cloth. A rubbed plastic rod will then
have a negative charge (not that Franklin had any plastic to play
with!)
7/15/2015 4:20 PM - Screen Clipping
Ch. 16 Page 1
16.2 Modern notion of charge
Wednesday, July 15, 2015
4:27 PM
We now associate the positive charge with protons and
negative charge with electrons. As electrons are on the
outside of atoms they can be rubbed off one material
and transferred to another. Ben Franklin's positively
charged glass rods have had some outer electrons
rubbed away.
All atoms are normally electrically neutral but can
temporarily have charge added or taken away. Some
molecules, though electrically neutral over all, can be
polar, which is to say the electrons are not evenly
distributed over the molecule and one side of
molecule will have a positive charge and the other
side a negative charge.
Water molecules are polar, and since they are found
in most air, will transport charges and cause charged
objects to lose their charge over time. (An electron on
a negatively charged object will get attracted to the
positive end of a water molecule which can move
through the air and then give up that loosely held
electron to any positively charged object it is
attracted to.)
Ch. 16 Page 2
16.3 Insulators and Conductors
Wednesday, July 15, 2015
7:29 PM
Some materials (metals) allow charges to be conducted through
them. Such materials are called conductors.
Some materials (wood, stone) do not allow charge to move
through them. These materials are called insulators.
There are materials intermediate to the above two categories
called semi-conductors.
In the modern view the difference between conductors and
insulators is how firmly the orbiting electrons are attached to
their nuclei. The more easily the electrons are dislodged, the
better the material is as a conductor.
Ch. 16 Page 3
16.4 Induced Charge; the electroscope
Wednesday, July 15, 2015
7:33 PM
An electroscope has two pieces of conducting foil hanging at the end of a
metal rod. When a charged object, say one with a negative charge, is
brought into contact with the electroscope the extra electrons that form
the negative charge repel one another, sending some electrons into each
leaf. Thus the leaves repel each other, and the larger the charge the
greater the repelling force. This is charging by contact. When the charged
object, which will still have some negative charge on it, is removed from
the electroscope the charges remain on the two leaves of the
electroscope and the leaves will remain separated.
An electroscope (or any other conductor for that matter) can be have a
charge put on the leaves without actual contact being made between the
charged object and the electroscope. Bring a charged object (say negative
charge) close to, but not touching, the electroscope's ball. Electrons in
the ball will be repelled and move down into the two leaves causing them
to repel each other. If the charged rod is now moved away the surplus
electrons in the leaves will redistribute themselves over the entire
electroscope returning the leaves to the original position. However, when
the charged object is close to the electroscope a conducting wire can be
used to connect the electroscope to the ground. The electrons being
repelled by the charged object now flow along the wire to ground (which
has, essentially, an infinite capacity to absorb electrons). The
electroscope is now missing some of its original electrons (they are in the
ground). If the wire is removed those electrons are trapped in the ground
and the electroscope has a net positive charge. Some of that positive
charge is distributed over each leaf so that the leaves remain separated,
even when the (negatively) charged object is removed from the
electroscope's vicinity. This method of putting (relatively) permanent
charge on an electroscope (or other conductor) is called charging by
induction.
Ch. 16 Page 4
induction.
Electroscopes can indicate the size of a charge because the greater the
leaf separation the greater the charge. Electroscopes can also be used to
determine the sign of a charge on an object if the electroscope has
already been given a known charge. Suppose you put a negative charge
on the electroscope. The leaves will separate. Now, if an object with
unknown charge is brought near the electroscope one of two things will
happen. If the unknown charge is negative that charge will repel the
electrons in the ball of the electroscope, pushing them down towards the
leaves, and causing the leaves to separate even further. On the other
hand if the unknown charge is positive, it will attract electrons into the
ball, leaving fewer on the leaves, and thus causing the leaves to lessen
their separation.
Answer Questions 1,3,5,8 on page 496.
Ch. 16 Page 5
16.5 Coulomb's Law
Thursday, July 16, 2015
7:01 PM
Using a light rod suspended by a twisted fibre (like Cavendish apparatus
for measuring G) Cavendish was able to determine the force equation
relating the size of electric charges and the distance between them.
The left hand table below shows (fictitious) data. A charge is put on one
end of the suspended rod. A variable charge, of the same sign, is then
brought to a set distance from the end of that rod (the variable charge is
moved to keep the same set distance from the revolving rod). The rod
twists until the tension in the fiber equals the repelling electric force.
The right hand table shows (fictitious) data. A charge has been put on the
suspended rod. A given charge is placed at various distances from the
suspended rod. Once again the rod will twist until the repulsive electric
force is balanced by the twisting force of the fiber.
Charge Twist
(deg)
1
50
1/2
27
1/4
12
1/8
7
1/16
3
Distance (cm) Twist (deg)
0.1
60
0.2
16
0.4
6.5
0.6
4
0.8
2.5
That is, the force decreases with the square of the distance and
increases with the product of the charges. The equation is
Coulomb's Law
The value of the proportionality constant, k, is needed to make
the equation useful for calculation. In modern units the value of
k is 9 x 109 N-m2/C2. [A coulomb (C) is the amount of charge
which, if placed on two point objects 1 m apart, will result in
each object exerting a force of 9 x 109 N on the other. This is a
huge force .]
Note: Coulomb's Law describes the force between charges at
rest. This force is called the electrostatic force or the Coulomb
force. Moving charges have additional forces on them. The
study of charges at rest, and forces upon them, is the area of
physics called electrostatics.
Note: The form of the equation is similar to the equation
Ch. 16 Page 6
εεε
Note: The form of the equation is similar to the equation
governing gravitation, but the electric force can be either
attractive or repulsive.
In some situations, which we will deal with in the future, the
constant of proportionality k, is written in terms of a different
constant ε (more commonly written as ɛ0)
The constant Ɛ is called the permitivity of free space. The
permitivity of a material (or free space) is related to how well
the material resists the flow of charge from one place to
another.
Study the examples in section 16.6, then do problems Page 497,
# 1,3-5,7-11,13-15,19
There will be a short quiz on sections 16.1 - 16.6 in the very
near future.
Ch. 16 Page 7
16.7 The Electric Field
Saturday, July 18, 2015
8:22 PM
Rather than deal with a mysterious force at a distance,
physicists have developed the idea of a field. Surrounding, or
perhaps emanating from every charge, is an electric field. At
each point in space the field has a magnitude and a direction
(the field is a vector field). When one charge sits in the electric
field of a second charge the first charge feels the field (size and
direction) of the second charge that exists at that point. The
interaction is local.
The electric field is defined for all the points surrounding a
charge as the force exerted on a small positive test charge
placed at each point. That is, the electric field vector at each
point is
Note that we can easily calculate the electric field strength
around an isolated point charge Q. Using Coulomb's Law
If there is an array of point charges we can calculate the electric
Ch. 16 Page 8
If there is an array of point charges we can calculate the electric
force on any single charge by calculating the vector sum of the
electric forces of each of the other charges on that single
charge.
Ch. 16 Page 9
16-8 Electric Field Lines
Sunday, July 19, 2015
8:07 PM
The electric field is a field of vector quantities. We can
represent the field by drawing arrows at all points in the field:
the arrow will point in the direction of the field at each point,
and have a length proportional to the field strength at each
point. Drawing the many arrows required to show the
size/shape of the electric field can be messy so we often draw
"field lines" instead.
Electric field lines are analogous to the magnetic field lines.
Magnetic field lines can be demonstrated by sprinkling iron
filings over a paper which is placed on top of a magnet. Electric
field lines can be demonstrated putting a large number of
compasses around an electric charges. The compasses will
orient as if there were continuous lines of force from one
charge (+) to another (-).
Field lines (the lines of electric force) are drawn with these
conventions:
Field line connect positive and negative charges (though
sometimes charges are not drawn but are understood to exist
somewhere off the diagram).
At each point the tangent to a field line is the direction of the
electric field at that point.
The size of the electric field is proportional to the number of
field lines in that area (volume). [More accurately the electric
field strength is proportional to the number of lines crossing a
unit area, perpendicular to the lines.]
The direction of the field lines at a given point is the direction
that a positive test charge would move if placed at that point in
the field. In particular, field lines originate on positive charges
and terminate on negative charges -- a positive test charge
Ch. 16 Page 10
and terminate on negative charges -- a positive test charge
moves away from positive charges and towards negative
charges.
Do problems: Page 498, # 21-25,27,29,31,33,35
Ch. 16 Page 11
16-9 Electric Fields and Conductors
Monday, July 20, 2015
1:20 PM
If an electric charge (i.e. a surplus of electrons) is placed on a
conducting object the electrons will repel one another and
distribute themselves until the net force on each electron is
zero. Once stability is reached the electric field at every point
must be perpendicular to the surface (otherwise the electrons
would continue to migrate).
Suppose a net positive charge is inside a metallic spherical
shell. Electrons in the shell will move to the inner surface of
the shell (lines of force originating on the central positive
charge must terminate on electrons). But that means there is a
net positive charge on the outer surface of the shell and this in
turn means there will be lines of force originating on those
charges and terminating on electrons somewhere outside the
shell. That is there is an electric field in the central hollow part
of the shell, and there is an electric field outside the shell, but
no electric field in the material of the shell itself.
If a hollow metal box is placed in an electric field (i.e placed in a
place where lines of force are directed from positive charges to
negative charges) the field inside the box will be zero. The
positive charges on one side of the box will attract the free
electrons in the metal of the box, pulling them to one side, but
thus producing a less than normal number of electrons on the
other side of the box. Hence there will be lines of force from
those positive charges in the box to the negative charges that
were responsible for the original field. But there is no charge
inside the box.
Because free electrons are not available to move around in
Ch. 16 Page 12
Because free electrons are not available to move around in
non-conductors, electric fields can and do exist in nonconducting materials. As well, the electric field at the surface of
non-conductors is not necessarily perpendicular to the surface.
Ch. 16 Page 13