Physics 102 lab notes

Electrostatics

Bring:
- Safety sign in, e-scope
Syllabus:
- Go through bold text points.
  - Contact: hours = guaranteed to find me, email = any time
  - Grading: check on OAKS. (85 gw, 15 q, 100 reports)
  - Attendance: all absences are equal, one drop, one makeup
  - Rules: work together on all parts!
  - Tutoring: CSL, free tutors, paid.
- Safety rules / sign in.
  - If you are unwell and need attention, let me know and I will call for help.
  - I will give instructions and cautions for safe equipment use. Follow them.
  - If you break something glass, let me know, and I will clean it up.
  - Wear closed toe shoes.
  - Practice common sense with regards to other lab attire, and when moving about the room.
  - Food and drink (lidded cups / bottles) are allowed until the day I have to clean up after someone.
Coulomb's law
- Electrical force between point charges
- |F_e| = k_e |q₁ q₂| / r²
- diagram explaining meanings (2 charges)
- same sign charges → repulsion; opposite → attraction
- k_e = 9.0 × 10⁹ N m²/C²
- unit of electric charge = C (fundamental unit)
- charge of one electron = e = -1.6 × 10^-19 C (minus is important)
Example problem
- Two pieces of tape, carrying the same electrical charge, are hung, connected at the top
- diagram
- Goal: estimate total charge on tapes from angle, length, mass
- [diagram of actual, and approximate (all charge and mass concentrated at midpoint)]
- Forces, Newton's 2nd, solution
- Spend time reviewing vectors
Demo - difficulty
- Demonstrate what expected interactions look like (crete U, L - show interactions with styrofoam)
- Electroscope, electrophorous
- Charge electroscope (+) using electrophorous until needle is deflected
- Bring L tape towards electroscope
- Far away → repulsion
- nearby → attraction
- Moral: if you have to bring tapes closer than a couple inches apart to see an interaction, chances are you will only see attraction, regardless of actual charge.
Notes
- U = -, L = +

Electrostatics answers

Follow instructions in the lab manual. Record your results in the space below. Hand in this worksheet at the end of lab.

1- Your hand and a U tape

The tape should either attract (A), repel (R), or not react (N) with your hand. In the top left empty cell of the table (hand / U tape), record the type of interaction.

Does the interaction appear to depend in any way on which side of the tape you approach?

object	U tape	L tape
hand
charged U tape
charged L tape
neutral metal
neutral paper
charged plastic (-)
charged glass (+)

2-5 (Two U tapes, Unlike Charges, Other Objects, What kind of charge?)

Fill in the rest of the table.

Do the interactions you see seem consistent with the statement "Like charges repel; unlike charges attract"? Explain, giving examples from what you observed.

Was there anything you observed to be attracted to both tapes? How can you explain something being attracted to both U and L tapes, if U and L tapes have opposite charge?

What type of charge (+ or -) corresponds to each type of tape (U and L)?

6 - How many excess electrons are on your negative tape?

Record the following measurements

tape length (l)	angle between tapes (θ)	tape mass

What is the gravitational force on the tape?

What is the distance between the midpoints of the tapes (r)?

What is the electrical force on the tapes (F_e)?

What is the charge on one of the tapes (Q)?

How many excess electrons are on one tape?

How many molecules are on the surface of the tape?

What fraction of molecules have become charged (if each has either 0 or 1 excess electrons)?

7-Electric field near a tape

What is the charge density on tape (σ)?

What is the electric field near the tape (E)?