Sticky Tape Lab
The Set-up
We Left These Hanging
Possible Results
Top
Bottom
Foil
Paper
Top
REPEL
Bottom
ATTRACT REPEL
Foil
ATTRACT ATTRACT NEUTRAL NEUTRAL
Paper
ATTRACT ATTRACT NEUTRAL NEUTRAL
Plastic
(Other)
ATTRACT ATTRACT ATTRACT
ATTRACT ATTRACT
Possible Results 2
What’s Going On?
• To explain the source of these charges, we
need to expand our model of the atom to
have some internal structure. We will assume
that each atom contains both positive and
negative charges that normally cancel each
other.
What’s Going On?
• J. J. Thomson proposed that in
solids, only the negative charges
are free to move, and that these
charges are much smaller than
an atom and carry only a
negligible fraction of its mass.
• We will use the Thomson model
of the atom – a massive positive
core associated with a small
number of mobile, negatively
charged particles we call
“electrons”
What’s Going On?
• A visual representation of this model is the
“plum pudding” model. I’ll reference this as
chocolate chip cookie dough.
• The positive cores are represented by bowls
of “dough”, which attract the negative
electrons represented by chocolate chips.
• The attraction of the chips to some bowls is
stronger than to others, and they can move
from one bowl to another because of such
differences in attraction.
• However, since “chips” also repel one another,
you cannot cram too many “chips” into the
same bowl of dough.
Let’s Apply the CCCD Model to our Sticky
Tape Lab
CCCD? Chocolate Chip Cookie Dough
Tape: Before adhesion
Neutral atoms (+ = -)
dot = negative electron
Tape: In contact
negative can move (positive can’t)
• top(+)
• bottom (-)
Tape: Separated
• top(+)
Why is the top now positive and bottom piece now negative? Think about the
comparison of positive and new amount of negative charge.
• bottom (-)
•
When two objects of different substances come into
contact, some electrons move from one substance to
the other (some chips creep from one set of bowls
into the other).
• If the objects are then quickly separated, an excess of
electrons remains in one object (creating overall
negative charge), counterbalanced by a deficiency of
electrons in the other (overall positive)
– I.e.: one set of bowls is now “chip rich”, while the other is
“chip deficient”.
• This microscopic imbalance of charges
translates to an overall macroscopic charge on
the object.
• The T tape becomes positively charged
because electrons are transferred to the B
tape. The overall number of electrons does
not change, just their distribution on the
tapes.
Neutral Foil
(how do + and – compare?)
T-tape approaches foil
Electrons become polarized and move toward the Top Tape (or some
other positive object). The macroscopic effect is an attraction of objects
How would B-tape approaching foil look?
• See the Balloon and Static Electricity simulation
on the class site:
for another look at the polarization of materials
causing attraction/repulsion
http://sciencewithhoffman.weebly.com/unit-6-particles-w-internal-structure.html
• The attraction between both T and B tapes and the
aluminum foil is greater than that observed with the
paper.
• In metals, electrons can be compelled to move from
one core to another by the application of an external
electric field. The same electric field does not result
in movement of electrons in non-metals. We
conclude that the attraction of the cores to the
electrons is weaker in metals.
T-tape approaches Paper
The effect of polarization in an
insulator is less pronounced
because the electrons are not so
free to move about (each chip
stays in its own bowl).