PHYS 102 Lecture 2: Electrostatics

Posting my lecture notes for reference (hence why an English title)

Lecture 2: Electrostatics

Charge:

-          Fundamental Quality of matter (as is mass)

-          Comes in positive (e.g. proton, 1.60 x 10^-19 C) or negative (e.g. electron, -1.60 x 10^-19 C)

-          Unit of charge: Coulomb

-          Charge is quantized (specific “packages” allowed)

-          Charge is conserved e.g. balancing oxidation / reduction reactions

-          Charge can be transferred

-          Charge cannot be converted into anything else

-          Symbol for charge: q or Q

Definitions:

Conductor – a material in which charges are mobile, e.g. metal, blood (sea water)

-          Will conduct electricity

Insulator – a material in which charges are immobile, e.g. rubber, glass, paper, distilled water (all ions removed)

-          Also: Non-conductor or dielectric

Neutral object – contains equal number of positive and negative charges

Charged object – has an excess of positive or negative charge

*Demo: Rubber balloon

1.      - Rub fur on a balloon, makes it negatively charged
- Electrons come from fur
- Bring a rubber rod rubbed with fur to the balloon; they repel

2.      - Rub glass on plastic, makes rod positive
- Electrons transfer from glass to plastic
- Bring the rod to the balloon; they attract

Opposites attract, likes repel

3.      - Neutral insulator and negative balloon experience electric attraction
- Neutral objects attract charged objects due to polarize-ability of neutral objects

*Demo: Metal can

-          Metal can is a neutral conductor

-          Negative rubber rod put next to can

o   Neutral conductor experiences an attraction to the rod

-          Hold positively charged glass rod next to can

o   Also an attraction

F_e: Varies with distance

Force is stronger when charged objects are closer

F_e  α  1/r^2

F_e α q₁q₂ ; q₁ = charge of object 1 ; q₂ = charge of object 2

Average distance between negative rod of positive charges in the aluminum can is less than the average distance between negative rod of negative charges in the aluminum can

∴ Attractive electric forces > repulsive electric forces

F_e (electric force) between point (very small size) charges

Vector quantity: |F_e| = magnitude, length of vector

Also need to specify direction

Coulomb’s Law:

|F_e |=|(q_1 q_2)/r^2 |

k = Coulomb force constant

~  1/(4πε_0 )

or 

k = 9.0 x 〖10〗^9   (N ∙m^2)/c^2

Practically: First determine whether F_e is attractive (< 0) or repulsive (> 0)

Direction:

(+) -> <- (-) arrows lie along the line joining 2 point charges

<- (+) (-) ->