# 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.

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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. קובץ זה נועד אך ורק לשימושם האישי של מורי הפיזיקה ולהוראה בכיתותיהם. אין לעשות שימוש כלשהו 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! 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. Force of B on A = kq A q B r 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 B C Answer is A. Look at forces from each charge and add them up. - -- - - - A Color-code 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.

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. Pom-pom demo ------ - Belt moves free electrons up to metal ball… Vandegraff --------- -------------------- ----------------------- -------------------------- ----------------------- -----------------

Demo of pie plates stacked on Vandegraff. Turn on VG – put lots of extra electrons on it. What will happen to the plates? Answer: c. The topmost will fly off, then the one underneath, then the next one underneath. 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. Charges and Fields PhET demo. - - - -- -- -- --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Cross-section view Electrons pile up on outer surface Force of charges on VG on top plate = k q TopPlate q VandegraphBall r 2

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.

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