Charges Activity w/ PhET SIMs
This is an activity pulled from a lecture covering electric
charges from PHYS1010 at CU Boulder exploring the
nature of charges and forces. The applets used as part
of the activity are the Electric Field Hockey, Charges and
Fields, Balloons and Static Electricity, and John
Travoltage. Five concept questions are integrated into
the activity to check student understanding.
‫ אין לעשות שימוש כלשהו‬.‫קובץ זה נועד אך ורק לשימושם האישי של מורי הפיזיקה ולהוראה בכיתותיהם‬
‫בקובץ זה לכל מטרה אחרת ובכלל זה שימוש מסחרי; פרסום באתר אחר (למעט אתר בית הספר בו‬
.‫מלמד המורה); העמדה לרשות הציבור או הפצה בדרך אחרת כלשהי של קובץ זה או כל חלק ממנו‬
Electric Hockey Simulation!
Forceof B on A = kqAqB
r2
Place charge (B), see charged “puck” (A) fly away when 2 cm away.
Now place charged “puck” (A) 1 cm away from placed charge (B).
Compared to previous situation, force on puck (A) will be:
a. half as large. b. the same. c. twice as large. d. four times larger
e. something else.
Demo and discuss
Answer: d. four times larger since force depends on
1/(distance between charges)2 …
distance smaller, force larger
Add another charge to B, right on top of the first. Force on
“puck” (A) will be:
a. /2. b. the same. c. * 2. d. * 4. e. something else.
demo and discuss
Answer: c. * 2 because force on A follows (charge of A x
charge of B); we changed the charge of B from 1 to 2.
Blue charges are “nailed down” negative charges.
The green charge is the puck (free moving)
For which of these choices is puck most likely not to move (e.g. net
force on puck is zero)?
A
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B
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C
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-
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Answer is A. Look at forces from each charge and add them up.
Color-code
A
- force from each
charge.
Explains WHY charges pile up on points
Charges and Fields applet on PhET
If we put bunch of electrons in a box. They will
a. clump together.
b. spread out uniformly across box.
c. make a layer on walls.
d. do something else.
Experiment with the applet and discuss.
Pom-pom demo
Demo of pie plates stacked on Vandegraff.
Turn on VG – put lots of extra electrons on it.
What will happen to the pie plates?
a. Nothing. The plates will stay there.
b. They will fly off at the same time and
stick together.
c. The topmost will fly off, then the
one underneath, then the next one
underneath.
d. They will all fly off at the same time and
separate.
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Vandegraff
Belt moves free electrons
-up to metal ball…
-
Demo of pie plates stacked on Vandegraff. Turn on VG –
put lots of extra electrons on it. What will happen to
the plates?
- - - -- -- -- --- - -
- -- --
-
-
Charges and Fields PhET demo.
Electrons pile
up on outer surface
Forceof charges on VG on top plate = k qTopPlate qVandegraphBall
r2
-
Like charges repel!
Just like pom-poms, the topmost
plate flies off first because charges
pileup on its edges. The negative
charges on the topmost plate repel
negative charges on Vandegraph and
it flies off.
Cross-section view
- --
-
-
Answer: c. The topmost will fly off,
then the one underneath, then the next
one underneath.
- --
Balloon demo – rub a balloon stick it to a wall.
What attracts the balloon to the wall?
Sweater and Balloon applet
Predict what charges are on the balloon and the sweater.
a. Both have extra plus charges.
b. Both have extra minus charges.
c. Balloon has extra plus or minus charges, sweater is neutral.
d. Sweater has extra plus or minus charges, balloon is neutral.
e. Either sweater has extra minuses and balloon extra pluses
or balloon has extra minuses and sweater extra pluses.
Experiment with applet and find out.
Rub a second balloon on the sweater.
The two balloons will
a. attract. b. repel. c. Do nothing; there is no force between them.
"Lightning" on a smaller scale
(John Travoltage applet)
Suppose we put a charge on Mr. Travolta by rubbing his
foot on the carpet more times. Will his finger be closer or
further away from the doorknob when the sparks fly?
a) Closer.
b) Further.
c) Same distance.
Moral: Don't get back in your car while filling the
tank, especially on a dry winter day.