When I was a primary
student,my teacher told
me to rub my plastic ruler
by using a piece of cloth.
As a result, the ruler
attracted pieces of small
paper automatically.
•Matter is made up of atoms. An atom contains positive charges
and negative charges. The negative charges are due to the electrons.
•The atom as a whole is neutral; the quantity of positive charge and
negative charge are equal.
There is empty space.
The electrons are
easily to lose away.
•Electrons can be transferred from one atom to another.
•When an electron is removed from a molecule, the molecule
contains more positive charges than negative charges.
In other words, it contains net positive charges .
It becomes a positive ion (離子).
+
Neutral Atom
Positive ion
electron
•When an electron is added to an atom, the atom contains more
negative charges than positive charges.
•It has a net negative charge. We say the atom is negatively charged;
it becomes a negative ion.
+
Neutral Atom
Electron
Negative ion
•The quantity of electric charge is expressed in the unit
coulomb (C).
•An electron carries a charge of
-19
e = -1.6 x 10 -19 C
The negative sign means the charge is negative.
Charging by friction
Rub two acetate strips with a dry woolen cloth and hold them side
by side. The strips repel each other. Repeat with two rubbed
polythene strips. The strips also repel each other.(Fig 1.4)
Repeat with an acetate strip and polythene strip. The strips attract
each other.
Before an acetate strips is rubbed, it contains the same quantity of
positive and negative charges. The strip is neutral because there is
no net charge. The same is true for the woolen cloth.
When the acetate strip is rubbed, some electrons in the strip are
transferred to the cloth . As a result, the strip contains more positive
charges than negative charges. It has a net positive charge. On the
other hand, the cloth is negatively charged because it contains more
negative charges than positive charges.
When a polythene strip is rubbed, it is negatively charged while the
cloth is positively charged. Some electrons in the cloth have been
transferred to the strip.
Same material strips
repel each other.
Different material strips
attract each other.
*Fig 1.5
There are forces called electric forces (電力), which cause the
strips repel or attract each other.
Like charges repel; unlike charges attract.
In all cases, no new charges are produced in the rubbing
process. The charges are merely separated. They are
transferred from one object to the other through direct
contact of the two objects. The quantity of charge
obtained by one object is equal to the quantity of
opposite charge obtained by the other. The net charge of
the two objects taken together remains zero.
Charge cannot be created or destroyed.
Charging by pushing
Connect two aluminium strips
to the positive terminal of the
extra high tension (EHT)
power supply as shown below.
The strips repel each other.
*Fig 1.6
Connect one strip to the
positive terminal and the other
to the negative terminal (Fig
1.7). The strips attract each
other.
*Fig 1.7
When both strips are connected to the positive terminal, the EHT
power supply draws electrons from the strips, which then become
positively charged. For simplicity, we may say that the EHT
power supply pushes positive charges onto the strips. When the
strips are connected to different terminals, the power supply
pushes positive charges onto one strip through the positive
terminal and negative charges onto the other strip through the
negative terminal. The charge are transferred by conduction.
Charging by sharing
*Fig 1.7
The Van de Graff generator generates charges.
The Hairs share the same type of charge with the
generator by conduction.
Fig 1.8 shows a charged metal ball electrically connected to the
ground. This can be done by connecting the metal ball and the
ground with a wire, or simply by touching the ball with the finger.
Since the earth is a huge object, it practically obtains all the charges
from the ball, which then becomes neutral.
In fact, any charged metal will be completely discharged as result
of earthing or grounding.
+
+
+
+
ground
*Fig 1.8
*Fig 1.9 Symbol for earth
*Fig 1.10 An uncharged sphere becomes positive after touching positively charged
sphere
*Fig 1.11 An uncharged sphere becomes negative after touching negatively charged
sphere
Air contains positive ions and free electrons. When a positively
charged rods is left in the air, the rod gradually attracts free
electrons into it. The positive charge is then neutralized by the
electrons.
When a negatively charged rod is left in the air, the excess
electrons are gradually lost into the air and neutralize the positive
ions.
In both cases, the rod is said to discharge through air.
Negative rod
*Fig 1.12
+ +
+++++
----Positive rod
+
+ +
Positive ions and free electrons in air
A charged object discharges easily when the air contains more ions.
This can be done by heating the air with a flame(Fig l.12) or hot
body, or irradiating it with a radioactive source. In these processes,
many ions and free electrons are produced.
*Fig 1.12
A charged object also discharges more easily when the air contains
a lot of moisture. It discharges less easily when the air is dry.
Objects are classified as electrical conductors (導體) or
insulators (絕緣體). In a conductor, the charges can move easily
from one place to another
Electrons are free to move in conductor.
*Fig 2.1
On the hand, in an insulator, the charges cannot move easily.
Conductors and insulators are not two distinct class of material.
Good insulators are bad conductors and vice versa. Even the best
insulator can allow a very small flow of charge. Similarly, charges
are not completely free in a conductor. Metals are the most
common conductors while non-metals are usually insulators. Some
common conductors and insulators are listed below.
Conductors
Insulator
Silver
copper
aluminium
graphite
water
Vacuum
plastics
wax
dry air
wood
Earth, human body
In decending order of goodness
For a conducting sphere, the excess charges settle evenly on the
surface (Fig 2.2). For a conductor of irregular shape, the charge
density is higher at sharp points.
*Fig 2.2 It is the same of positive ions
The distribution of charges on a conductor is also affected by
charges outside. Fig 2.3 shows a charged rod (which may be a
conductor or an insulator) brought near a neutral conductor. The
positive charges in the rod attract unlike charges in the conductor to
the side closer to the rod and push the like charges to the far side.
The conductor is charged locally. But on the whole it still remains
neutral.
rod
++++
- -- + + ++
conductor
*Fig 2.3
This phenomenon is called electric
induction (電感應). The charges
separated in the conductor are called
induced charges. The more charges the
cod contains and closer is the rod to the
conductor, the more charges are induced
Charging by induction:
*Fig 2.4
Alternatively,
*Fig 2.5
Negative charges are induced on the side closer to the rod and positive
charges on the far side. The attractive force between the positively charged
rod and the negative charges on the aluminium piece is larger than the
repulsive force between the rod and the positive charges. As a result, there is
a net attractive force between the rod and the aluminium piece.
*Fig 2.6 The pieces are attracted towards the rod
For the same reason, tiny paper
strips are attracted towards a
charged object.
I understand now.
Thank you!
*Fig 2.7
I know the same phenomenon can
be observed when a charged rod is
brought near a stream of running
water.
Electrostatic precipitation
*Fig 3.1
Photocopying
*Fig 3.2
Static charges accumulate easily on insulators due to friction.
This causes a nuisance in many respects.
~ When you comb your hair on a dry
day. You can feel the pull on you hair
by the static charges produced.
No need to
mention this
point!!
#Flying aeroplanes and moving vehicles accumulate charges too.
Aeroplanes are fitted with conductive tyres so that the charges are
carried to the earth upon landing. If not, a spark may occur during
refuel and may cause fires and even explosions.
Oil trucks prevent such accidents by carrying a conductive belt or
metal chain touching the ground.(Fig 3.4)
#Charges accumulated in clouds are
carried to the earth through lightning
during a thunderstorm.
To prevent the lightning from hitting
tall buildings, lightning conductors
(避雷針) are installed on the rooftop.
Help!