TR3.5

Physics 250 Day 1

Dylan Wallace 2021-09-27 Mon 12:00

1 Experiments

Basically, we just rubbed a bunch of things on each other and checked the resulting charge with an electrometer.

1.1 Interesting results

  • Combs are great for static electricity
  • Rubbing some objects on others caused similar charges, while other object caused different charges
  • These notes are in hindsight so I legit don't remember too much

2 Explanation

  • Opposite charges attract; similar charges repel
  • When charged object is brought close to a conductor, electrons in the conductor will flow and polarize the conductor
  • When charged object is brought close to an insulator, atoms inside the insulator will be polarized. With small objects, this can make the whole object be basically polarized.
  • When a charged object makes contact with a conductor, the electrons will be shared between objects.

3 Homework

3.1 Lecture Notes

Might not be complete.

3.1.1 Electrostatics Basics

  • There are Insulators and Conductors
    • Insulators: Don't share electrons
    • Conductors: Share electrons
    • Learn why this is in solid state physics
  • List of charges when rubbed
    • Plastics usually become negative
    • Fur, elastics usually become positive
  • Electrons can be shared between materials
  • Electrons can move somewhat freely (depending on the material) within an object
    • Especially when close to another charged object!
  • Even in materials where electrons can't move freely (e.g. paper, other insulators), polarization can cause a "chain reaction" and "polarize" the object as a whole

3.1.2 Quantification

  • Coulomb's Law
    • Given two point charges, Q1 and Q2, and a distance r
    • \(F = k \frac{q_1 q_2}{r^2}\)
      • \(k\) is \(8.99\times 10^{9}Nm^{2}C^{-2}\)
      • \(r\) is in meters
      • \(q_1\), \(q_2\) in Coulombs (\(C\))
      • if \(F > 0\), then force is repulsion
      • if \(F < 0\), then force is attraction

    • Sample Problem: Find distance (\(r\)) given \(q_1\), \(q_2\), and \(F\) \[

      \begin{aligned} q_1 &= 50uC &= 50\times 10^{-6}C \\ q_2 &= 1uC &= 1\times 10^{-6}C \\ F_{12} &= 2N \\ k &= 8.99\times 10^{9}Nm^{2}C^{-2} \\ F &= k \frac{q_1q_2}{r^2} \\ r^2 &= k \frac{q_1q_2}{F} &= 8.99\times 10^{9}Nm^{2}C^{-2} \cdot 50\times 10^{-12}C^{2} \div 2N \\ &= 224.75 \times 10^{-3}m \\ r &= \sqrt{224.75 \times 10^{-3}}m \\ &= 474\times 10^{-3}m \end{aligned}

      \]

    • In more complicated setups, certain things such as acceleration won't be constant because it is determinant on force, which is determined by distance from other charges.
      • This complicates things so don't expect it to be simple.

3.1.3 Vector Fields

  • Fields of vectors
    • Vector magnitude is in \(NC^{-1}\) (Newtons per Coulomb)
    • Behave in interesting ways i guess i dunno
    • Calculate using a hypothetical proton