Sticky Tape Lab - PowerPoint Presentation

Slide1

Sticky Tape LabSlide2

The Set-upSlide3

We Left These HangingSlide4

Possible Results

Top

Bottom

Foil

Paper

Top

REPEL

ATTRACT

ATTRACT

ATTRACT

Bottom

ATTRACT

REPEL

ATTRACT

ATTRACT

Foil

ATTRACT

ATTRACT

NEUTRAL

NEUTRAL

Paper

ATTRACT

ATTRACT

NEUTRAL

NEUTRAL

Plastic

(Other)Slide5

Possible Results 2Slide6

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. Slide7

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” Slide8

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.Slide9

Let’s Apply the

CCCD Model

to our Sticky Tape Lab

CCCD? Chocolate Chip Cookie DoughSlide10

Tape: Before

adhesion

Neutral atoms (+ = -)

dot = negative electronSlide11

Tape:

In contact

negative can move (positive can’t)

top(+)

bottom (-) Slide12

Tape:

Separated

top(+)

bottom (-)

Why is the top now positive and bottom piece now negative? Think about the comparison of positive and new amount of negative charge.Slide13

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”. Slide14

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. Slide15

Neutral

Foil

(how do + and – compare?)Slide16

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 objectsSlide17

How would B-tape approaching foil look?

See the Balloon and Static Electricity simulation on the class site:

http://sciencewithhoffman.weebly.com/unit-6-particles-w-internal-structure.html

for another look at the polarization of materials causing attraction/repulsionSlide18

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. Slide19

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).