2. QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Have you ever walked across the carpet and
gotten “shocked” when you touched the
doorknob?
3. What about static
cling? Have you
ever gotten to
school only to be QuickTime™ and a
embarrassed when
TIFF (Uncompressed) decompressor
are needed to see this picture.
someone points out
the sock sticking to
your back?
6. Charge
Protons & Electrons have a property
called electric charge
– Protons: positive electric charge (+)
– Electrons: negative electric charge (-)
– The strength of the positive charge on a
proton is the same as the strength of the
negative charge on the electron
7. Charge cont.
Neutrons do not have charge
– Neutrons: neutral
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QuickTime™ and a
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Neutrons must be
from Switzerland
8. Can you tell the difference?
Can you tell the difference between an
electron in an oxygen atom and an
electron in a sodium atom?
Theelectrons of ALL atoms are identical.
Each has the same quantity of charge and
the same mass.
9. Protons & Neutrons
Protons and neutrons compose the nucleus.
Protons are about 1800 times more massive
than electrons but carry an amount of charge
equal to the negative charge of electrons.
Neutrons have slightly more mass than the
protons and have no charge.
10. Particle Charge Summary:
Particles
may be charged (positive or
negative) or neutral (not charged)
Particle Charge
Protons positive (+)
Neutrons neutral
Electrons negative (-)
12. Basically…
Oppositely charged particles attract each
other
– Ex: Protons (+) and Electrons (-) attract
Particles with the same charge repel each
other
– Ex: 2 Electrons (-) would repel each other
– Ex: 2 Protons (+) would repel each other
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13. Continued…..
Particles with neutral charge do not
interact
– Neutrons do not attract or repel each other
– Neutrons do not attract or repel electrons
or protons
14. In the real world, we rarely deal
with individual charged particles
Everyday objects are made of lots of
atoms
Most atoms have an equal number of
protons and electrons
Therefore, most atoms are neutral
– Remember, even though most atoms may
be neutral, they are still made of charges
15. What does it mean to say that an
object is neutral?
A neutral object has no net (overall)
charge
A neutral object has equal amounts of
positive and negative charge
16. What does it mean to say that an
object is charged?
A charged object has a net charge
A positively charged object has a
greater quantity of positive charge than
negative charge
A negatively charged object has a
greater quantity of negative charge than
positive charge
17. What does it mean for an object
to be polarized?
When an object is polarized, its charges
have shifted so that one side of the
object has a net negative charge and
the other side of the object has a net
positive charge.
A polarized object might be neutral (no
net charge) or have a net charge.
18. Electrons move, Protons don’t!
Protons don’t move!
– Protons are very massive. They have too
much inertia.
– They are in the center of the atom.
Electrons are outside the nucleus.
– It is easier to move particles on the
perimeter.
19. What do you have to do to make
an object positively charged?
You need to take
electrons away
from the object
20. What do you have to do to make
an object negatively charged?
You need to transfer electrons to
the object.
21. The “normal” configuration of
the atom.
Most atoms are neutral.
Thus most atoms have an equal
number of protons and electrons.
Remember that the number of protons
determines the element.
22. What is an ion?
When an atom gains an extra electron,
– it is negatively charged. It is then a
negative ion.
When an atom loses an electron,
– it is positively charged. It is then a
positive ion.
23. So, a charged sock can stick to
my shirt… Does that mean that
my shirt is charged?
Not necessarily
Remember that a neutral object is
made up of innumerable positively and
negatively charged particles.
A charged object (positive or negative)
will be attracted to a neutral object.
– We’ll discuss exactly why later…
24. Conductors vs. Insulators
Conductors: Insulators:
– Loosely bound – Tightly bound
electrons electrons
– Allow the flow of – Slow the flow of
electrons electric charge
– Examples: – Examples:
metals rubber, plastic,
and styrofoam
25. So what is happening when you
rub a balloon on your head and it
becomes charged? Is friction
creating charge?
No! Charge cannot be
created or
destroyed. QuickTime™ and a
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– Conservation of Charge
Charges are being
exchanged…
26. There are 3 Charging Methods
Objects can be charged by
– Conduction
– Induction
– Friction (triboelectricity)
27. Conduction
Requires the objects to be in contact
(or close enough for a spark)
Electrons are exchanged
Works best from conductor to conductor
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28. Charging by Conduction
Suppose you have two identical metal spheres, imaginatively
named A and B. Each sphere is on an insulating stand.
Originally, A has 10 units of charge, and B is neutral. If the two
spheres are briefly touched together, then separated, what
is the charge on each?
A. A has 10 units, B has zero.
B. B has 10 units, A has zero.
C. A has 10 units, and B also has 10 units.
D. A and B each have 5 units.
E. A and B each have zero charge.
29. Induction
A charged object charges a neutral conductor
without contact
The conductor does make contact with a
neutral object (often the ground)
The conductor ends up with a charge opposite
that of the charged object brought near
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30. Charging by Induction
Notice that we are
charging by
induction,
grounding the
side of the
sphere next to
the positive
object.
What will the final
charge on the A. Positive
sphere be? B. Negative
C. Neutral
D. It depends
31. Triboelectric (friction)
Two dissimilar
materials are
brought into contact
and charge is QuickTime™ and a
exchanged TIFF (Uncompressed) decompressor
are needed to see this picture.
The contact often
involves friction
Works best with
insulators
32. More on Triboelectricity
Charge separation occurs when two
insulators are rubbed together
One of the insulators is more likely to
grab electrons and the other insulator is
more likely to donate electrons
CHARGE IS NOT CREATED! Electrons
are simply being transferred.
33. The Triboelectric Series
Electron Donors
(objects that give electrons
become positive)
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(objects that take electrons
become negative)
Electron Grabbers
34. Triboelectric Charging
You rub a balloon against your hair, and the hair
becomes positively charged. This means that
A. Electrons moved from the balloon to your
hair.
B. Protons moved from the balloon to your hair.
C. Protons moved from your hair to the balloon.
D. Electrons moved from your hair to the
balloon.
E. The rubbing destroyed electrons in your hair,
leaving it positive.
35. Triboelectric Series
A Triboelectric Sequence
ELECTRON GRABBERS ELECTRON DONORS
Rubber Amber Cotton Silk Cat fur Wool Glass Rabbit fur
If you rub cotton with amber, which becomes positive?
A. Amber B. Neither C. Cotton
Which of the following can make glass negative?
A. Amber B. Cat fur C. Rabbit fur
36. Neutralizing/Grounding Objects
When a charged object comes in contact
with a very large, neutral conductor, the
object becomes neutralized.
Earth itself is a large, neutral
conductor, so it neutralizes charged
objects quite well.
37. Coulomb’s Law
Two charged objects exert a force on
each other.
The magnitude of the force is directly
proportional to the product of the
quantities of charge.
The magnitude of the force is inversely
proportional to the distance between the
objects squared.
38. More Coulomb’s Law (We’ll
come back to this later…)
kq1q2
Fe = 2
R
k = constant
q = quantity of charge
R = distance between the centers
of the objects.
39. Consider two identical charged particles
near each other as shown.
The force exerted by q1
on q2 points
A. Left
B. Right
C. Up
D. Down
E. Nowhere; there is
no force.
40. Two air pucks each carry a
charged sphere.
The sphere on the right
carries three times
as much charge as
the sphere on the
left.
Which force diagram
correctly shows the
direction and
magnitude of the
electrostatic forces?
41. What if one object is neutral?
Note that a neutral object and a charged
object will attract each other.
A charged object will polarize a
neutral (or weakly charged)
object.
Animation of Polarization
Illustrations of Polarization
42. Three pith balls hang from
threads…
A pith ball is a small,
light sphere coated
Wait a minute! with conductive
material.
What’s a The coating allows it to
be easily charged or
“pith ball”? grounded, and its
small mass allows it
to respond easily to
small forces.
May we go on?
43. Three pith balls each hang from
an insulating thread.
1 and 3 have charges of
PB 1 and PB 2 repel A.
opposite sign.
each other. B. 1 and 3 have charges of
the same sign, 2 has
PB 2 and PB 3 repel
opposite charge.
each other. C. All three have charges of
the same sign.
Which of these D. One of the objects carries
statements is true no charge.
E. We need more data to
about the charges determine the signs of the
on the PBs? charges.
44. And now for a similar,
yet different, problem…
45. Three pith balls each hang from
an insulating thread.
1 and 3 have charges of
PB 1 and PB 2 A.
opposite sign.
attract each other. B. 1 and 3 have charges of
the same sign.
PB 2 and PB 3 repel
C. All three have charges of
each other. the same sign.
D. One of the objects carries
Which of these no charge.
statements is true E. We need more data to
determine the signs of the
about the charges charges.
on the PBs?
46. Review
Charged objects exert forces:
Like repels like
Opposites attract
Charged objects and neutral objects attract
How objects get charged or neutralized:
Conduction
Induction
Triboelectric
Grounding
