3. Two kinds of charges
• After being rubbed, a
plastic ruler can attract
paper scraps.
Ruler carries electric charge.
It exerts electric force on paper.
The interaction between static electric charges is called
electrostatics.
This charging method is called charging by friction.
4. Where do charges come from?
Matter is made up of atoms.
+
+
+
–
––
+
–
Proton (positive charge)
neutron (neutral)
electron (negative charge)
atom nucleus
5. If electrons = protons ⇒neutral
Where do charges come from?
If electrons > protons ⇒ gaining electrons, negative
charge
If electrons < protons ⇒ losing electrons, positive
charge
6. Electro-negativity
Relative
electro-negativity
ranking for some
common materials
from electron donating
materials (+, glass) to
electron accepting
materials (-, teflon)
• Glass
• Human Hair
• Nylon
• Silk
• Fur
• Aluminum
• Paper
• Cotton
• Copper
• Rubber
• PVC
• Teflon
+ + + + +
+ + + +
+ + +
+ +
+
-
- -
- - -
- - - -
- - - - -
7. Rubbing materials does NOT create
electric charges. It just transfers
electrons from one material to the
other.
Where do charges come from?
8. When a balloon rubs a piece of wool...
Where do charges come from?
– electrons are pulled from the
wool to the balloon.
The balloon has more electrons than
usual.
+
+
+
+
+
–
–
–
–
–
The balloon: – charged,
The wool: +charged
wool
9. Insulators and conductors
Insulators: materials that do NOT
allow electrons to flow through them
easily.
Insulators can be easily charged by friction as the extra
electrons gained CANNOT easily escape.
10. 4 Insulators and conductors
Conductors: materials that allow electrons to flow
through them easily.
Conductors CANNOT be easily charged by friction as
the extra electrons gained can easily escape.
11. Induction: The production of a charge in an
uncharged body by bringing a charged
object close to it
When negatively charged rod is put near a metal can...
electrons of the can are pushed
away from the rod.
⇒ top of the can: positive
& attraction > repulsion
+
+ + +
+
metal
can
- -
-
-
-
----- --
buttom of the can: negative
induced
charges attraction
repulsion
12. Attraction of uncharged objects
Similarly, when charged rod is close to paper
scrap...
----- --
molecules of paper align.
⇒ attraction between the
rod and + charge >
repulsion between the rod
and - charge.paper +
–
+
–
+
–
+
–
+
–
+
–
+
–
+
–
attraction
repulsion
13.
14. Static Discharge
Human body can not feel less than
2,000 volts of static discharge
Static charge built up by scuffing
shoes on a carpet can exceed 20,000
volts?
15.
16. Gas Station Fires
• Carol said a static gas pump fire is blamed for
burning her daughter so badly she needed skin
grafts on her legs.
• Carol had put the gas pump nozzle on automatic
and re-entered her car to write a check. When her
then-12-year-old daughter, wearing a sweater and
jacket that may have created static electricity,
reached for the nozzle, flames suddenly ignited her
clothing.
17. 1 If the balloon can attract some paper scraps, which of the
following cannot be the charge of paper scraps?
A Neutral B Positive
C Negative
A balloon has a negative charge
when rubbed by a woollen cloth.
18. 2 During rubbing, what have been transferred between the
woollen cloth and the balloon?
A Electrons B Protons
C Neutrons
A balloon has a negative charge
when rubbed by woollen cloth.
19. When a + charged rod is put near neutral object,
______________ is induced on the side of the object
near the rod and _____________ is induced on the side
away from the rod. The rod can attract the netural object
because _________ between rod and – induced charge >
the ________ between rod and + induced charge.
How does a positively charged rod attract a
neutral object?
negative charge
positive charge
attraction
repulsion
20. Grounding
An object is grounded when it is connected
to the earth through a connecting wire.
What is grounding?
If a charged conductor is grounded, it will
become neutral.
21. Grounding
b How does grounding occur?
+
+
++
+
When we touch a metal ball of
positive charge...
electrons flow from the
earth to the metal ball to
neutralize the metal ball.
–
Metal ball becomes neutral.
22. Similarly, if the metal ball is of
negative charge...
Grounding
How does grounding occur?
–
–
––
–
–
extra electrons flow
from the metal ball to
the earth and the ball
becomes neutral.
23. Why do gasoline tankers usually have metal
chains at the back?
When cars run, their tires and bodies are usually
charged by _______. For gasoline tankers, if the
accumulated charge is large enough, _______can be
produced and _________ will occur if gasoline vapor is
ignited. Those metal chains conduct the charge on the
bodies of tankers to the _______ and avoid the danger.
friction
sparks
explosion
ground
24. Lightning
• kills more
than 60
people and
• injures more
than 400
people a year
in the US
• one mile every
five seconds
• about 20,000 C
• Voltage of up to
1.2x108
volts
•
