Demo Equipment

Force review

Details
  • For any force, there is an experiencing object, and a causing object.
  • Pull → force on experiencing object is towards causing object.
  • Push → force on experiencing object is away from causing object.
  • Drag → force resists motion of experiencing object relative to causing object.
  • Tension, normal, friction require contact between causing and experiencing objects.
  • Gravity is caused by the Earth, does not require contact.
  • *** This applies to forces so far, and electric force. We will eventually learn about magnetic force, which is a non-contact force that is neither push, pull, nor drag.
Examples
  • Forces on me (standing on floor): Normal from floor, gravity from Earth. Directions: Floor is below me, and normal is a push (away from floor) → Normal is up. Earth is below me and gravity is a pull (towards Earth) → gravity is down.
  • Forces on me (leaning on wall): Same + normal from wall, static friction. Direction: Wall is to the right of me, and normal is a push (away from the wall) → normal from wall is left. Friction is a drag, and normal from wall would cause me to move left → friction is to the right.

Demo: electric force

  • styrofoam, confetti
  • Bring styrofoam close to (but not touching) confetti - what happens?
  • Rub styrofoam on hair, bring close to confetti - what happens?
  • Rub styrofoam with hand / let sit for a few minutes, bring close to confetti - what happens?
Repeat as necessary
Neutral styrofoam
  • When I first bring the styrofoam close to the confetti, what forces act on the confetti? Are any of the forces caused by the rod?
  • Gravity from Earth (down), normal from table (up), nothing from styrofoam
  • styrofoam has no effect on paper
Charged styrofoam
  • Styrofoam is able to lift confetti (without touching it)
  • Diagram of styrofoam above confetti (on table)
  • Forces: gravity and normal...
  • If there are no other forces on the confetti, can the confetti rise? (no)
  • What direction must the 3rd force be in? (to rise, up)
  • Have students make a list of forces from 101. (gravity, normal, tension, friction)
  • Which forces above did not require the object causing the force to be in contact with the object experiencing it? (gravity)
  • Q: Could the force lifting the confetti be gravity? Why?
  • A: The force on the confetti is upwards. Since gravity is a pull, the causing object would have to be above the confetti (the styrofoam). But, the mass of the styrofoam does not change, but its ability to lift the confetti does.
Styrofoam + hand
  • Styrofoam is no longer able to lift confetti
  • Rubbing styrofoam with hair alters it, allowing it to exert a mysterious new (electrical) force
  • ↑ charging
  • Rubbing styrofoam with hand / letting it sit restores it to its original condition
  • ↑ discharging

Electric force

  • Causing and experiencing objects are electric charges.
  • A charge can be + or -.
  • +/+, -/- = push
  • +/- = pull
  • Leave space for charge conservation
  • -- The net charge (positive charge minus negative charge) of a region doesn't change, except by charge entering or leaving that region
  • Leave space for charge quantization
  • -- The charge of anything can only be n * e, where n is an integer (..., -1, 0, 1, ...), and e is the fundamental charge (e = +1.6×10-19 C)

Charge diagram

  • Only depict accumulation of net charge (+ or -)
  • A single + can not disappear (nor a single -)
  • If a single + and a single - come together, they will both disappear from the diagram. They will not represent charge accumulations anymore.
  • A single + and a single - can come from the same originally neutral point.
  • + and - charge can travel along a conductor (metal), but remains stuck in place on an insulator (plastic, paper, glass, rubber, ...)
Why
  • All matter is made up of subatomic particles (protons, neutrons, electrons, ALL 3). Even neutral matter.
  • The total number of protons, electrons, and neutrons in the entire univers remains the same*** (not true in nuclear processes)
  • Atomic structure: Protons and neutrons are strongly bound in a very small nucleus, and electrons are loosely bound (or unbound) far away from the nucleus.
  • Protons will not leave a nucleus. Electrons can move around within a metal. Or, + or - ions (atoms / molecules with electron - proton imbalance) can travel through liquid / gas / vacuum.
Implication
  • Add charge conservation to electric force
  • Add charge quantization to electric force

Example charge diagram

  • Charging styrofoam
  • Charged styrofoam and confetti
Charging diagram
  • Neutral hair, neutral styrofoam
  • Hair and styrofoam are both insulators
  • Styrofoam acquires - charge. Does the hair acquire charge? (Yes, +)
  • Where must the charge be on the styrofoam? On the hair? (They must both be at the point of contact)
Styrofoam and confetti
  • Does the confetti acquire net charge? (no, paper and styrofoam are insulators)
  • What happens? (+ accumulation at top, - at bottom)
  • How is there attraction? (+ is closer to styrofoam than -, experiences greater force)

Notes: Braun electroscope

  • Top + post + needle are all conductors, in contact.
  • Post + needle will have equal charges
  • If post and needle are charged, needle is deflected (more charge → more deflection)
Details
  • Needle rotates almost freely (little force is required)
  • Needle is slightly off balance, so it wants to be vertical
  • Any charge on needle and post → they repel each other

Demo: electroscope

  • 1 - neutral electroscope + charged styrofoam
  • 2 - neutral conductor + charged styrofoam + hand
  • 3 - charged conductor + electroscope
1
  • a - styrofoam is charged, electroscope is neutral
  • b - styrofoam is placed on electroscope, needle deflects
  • c - styrofoam is removed, needle no longer deflects
  • b - why? Because 6 - charges at the top are pushed onto the needle / post, leaving 6 +s at top.
  • c - why? Because 6 - charges remain on the styrofoam (insulator means charge is stuck), leaving the electroscope neutral everywhere.
2
  • a - styrofoam is charged, conductor is neutral but has opposite charges accumulated
  • b - minus charge moves to hand, leaving only + on conductor
  • c - conductor is left with + charge
3
  • b - needle deflects → some charge (+) moves from conductor to electroscope
  • c - needle remains deflected → some charge (+) remains on electroscope
  • d - needle deflects less → + charge on electroscope is attracted to the top, near the styrofoam