general physics 2 presentation about electric potential

2. elektron

3. William Gilbert

4. electrostatic force
Ability to attract all substances

5. What Is Static
Electricity?
• A stationary
electrical charge
that is built up on the
surface of a material

6. 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.

7. Where do charges come from?
Matter is made up of atoms.
+
+
+
–
–
–
+
–
Proton (positive charge)
neutron (neutral)
electron (negative charge)
atom nucleus

8. If electrons = protons neutral
Where do charges come from?
If electrons > protons  gaining electrons, negative
charge
If electrons < protons  losing electrons, positive
charge

10. 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
+ + + + +
+ + + +
+ + +
+ +
+
-
- -
- - -
- - - -
- - - - -

11. Rubbing materials does NOT create
electric charges. It just transfers
electrons from one material to the other.
Where do charges come from?

12. 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

13. How Can You Charge Objects?
• There are 3 ways objects can be charged:
1. Friction
2. Conduction
3. Induction
**In each of these, only the electrons move. The protons stay in the
nucleus**

14. Friction
• Charging by friction occurs when electrons are “wiped”
from one object onto another.
Ex.
If you use a cloth to rub a plastic ruler, electrons move
from the cloth to the ruler.
The ruler gains electrons and the cloth loses electrons.

15. Conduction
• Charging by conduction happens when electrons move
from one object to another through direct contact
(touching).
Ex. Suppose you touch an uncharged piece of metal with a
positively charged glass rod. Electrons from the metal
will move to the glass rod. The metal loses electrons
and becomes positively charged.

16. Induction
• Charging by induction happens when charges in an
uncharged object are rearranged without direct contact with
a charged object.
Ex.
If you charge up a balloon through friction and place the
balloon near pieces of paper, the charges of the paper will be
rearranged and the paper will be attracted to the balloon.

17. Conservation of Charge
• When you charge something by any method, no
charges are created or destroyed.
• The numbers of electrons and protons stay the
same. Electrons simply move from one atom to
another, which makes areas that have different
charges.

18. Conductors and
Insulators
• An electrical conductor is a material in which charges
can move easily.
• Most metals are good conductors because some of their
electrons are free to move.
• Conductors are used to make wires. For example, a
lamp cord has metal wire and metal prongs.
• Copper, aluminum, and mercury are good conductors.

19. Conductors and
Insulators
• An electrical insulator is a material in which charges
cannot move easily.
• Insulators do not conduct charges very well because
their electrons cannot flow freely. The electrons are
tightly held in the atoms of the insulator.
• The insulating material in a lamp cord stops charges
from leaving the wire and protects you from electric
shock.
• Plastic, rubber, glass, wood, and air are good insulators.

20. 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.

22. 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.

24. 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

25. 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

27. Static Electricity
• Static electricity is the
electric charge at rest
on an object.
• When something is
static, it is not moving.
• The charges of static
electricity do not move
away from the object
that they are in. So, the
object keeps its charge.
• Ex. Clothes taken out
of a dryer

28. Electric Discharge
• The loss of static electricity as charges move off an
object is called electric discharge.
Sometimes,
electric
discharge
happens
slowly.
Ex: static on
clothes
Sometimes,
electric discharge
happens quickly.
Ex. wearing
rubber-soled
shoes on carpet,
lightning

29. How Lightning Forms

30. Lightning
• Lightning usually strikes the highest point in a
charged area because that point provides the
shortest path for the charges to reach the ground.
• Anything that sticks up or out in an area can
provide a path for lightning.
• A lightning rod is a pointed rod connected to the
ground by a wire.
• Objects, such as a lightning rod, that are joined to Earth by a conductor, such as a wire,
are “grounded.” Any object that is grounded provides a path for electric charges to
move to Earth.
• Because Earth is so large, it can give up or absorb charges without being damaged.
• When lightning strikes a lightning rod, the electric charges are carried safely to Earth
through the rod’s wire. By directing the charge to Earth, the rods prevent lightning from
damaging buildings.

31. 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.

32. 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.

33. 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

34. 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.

35. 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.

36. 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.

37. 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

38. Do Now
Write a paragraph
about the cartoon. Tell
what he is trying to do.
Will it work and why?
What do you think will
happen? Use the
following words, static
electricity, lightning,
electrons, positive and
negative charges.

39. Electric Charge and Static Electricity
Law of Charges- like charges repel, opposite
charges attract
Symbol for charge is “q”
Charge is measured in COULOMB’s
1 proton = 1.602 x 10-19 C
1 electron = -1.602 x 10-19C

40. Electric Charge and Static Electricity
Coulomb’s Law – describes the electric force between
any two charges, separated by a distance “r”
The force of electrical attraction between two charges is
directly proportional to the product of the two charges
and inversely proportional to the square of the distance
between them.
k = 8.9875 x 109 Nm2
C2
Many charges are expressed in micro-coulombs (C);
1 x 106 C = 1 C. Easiest solution, whenever you see C, just add x
10-6 C to the number.
Ex. 5 C = 5 x 10-6 C; 28 C = 28 x 10-6 C

41. Coulomb’s Law Example
• Two electrostatic point charges of 60 C and 50 C exert a repulsive
force on each other of 175 N. What is the distance between the two
charges?
• Q1 = 60 x 10-6 C; Q2 = 50 x 10-6 C.
m
0.3925
d
N
175
C
10
x
50
C
10
x
60
C
m
N
10
x
8.9875
d
F
Q
Q
k
d
F
Q
Q
k
d
Q
Q
k
d
F
d
Q
Q
k
F
6
6
2
2
9
2
1
2
1
2
2
1
2
2
2
1





















42. Answer this
1. What is the magnitude of the force a 1.5 x
10−6
C charge exerts on a 3.2 x 10−4
C
charge located 1.5 m away?
2. Two equal charges of magnitude 1.1 x
10−7
C experience an electrostatic force of
4.2 x 10−4
N. How far apart are the
centers of the two charges?
3. Calculate the force between charges of 5.0
x 10−8
C and 1.0 x 10−7
C if they are 5.0
cm apart.

45. The End

46. 1. Electric Potential
2. Capacitance and Capacitors
3. Current Resistivity and
Resistance
4. DIRECT CURRENT
CIRCUITS
5. MAGNETIC FIELD AND
MAGNETIC FORCES