Electrostatics

Electrons (valence electrons - on outer layer)

repel

different from

• friction

• conduction

• induction

• grounding

two neutral objects acquire opposite charge by rubbing against each other

charged object directly transfers some excess charge to another object

• objects touch → charges move

• one object is initially charged

cannot

• electric charge only comes in multiples of e (charge of an electron). charge is quantized, meaning it only occurs in particular amounts

• e = -1.6 × 10^-19 C

no

• divide value by e

• 2.5 charge transferred, not possible

yes

• 4.5×10^15 charge transferred, possible

• electrons are not bound and can move freely

• can transfer electrons

• excess charge resides on outer surface

• electrons bound to protons and cannot move freely

  • can only move around proton

• cannot transfer electrons

• weaker attraction and repulsion than conductors

• a charged object is brought near a neutral object

• neutral object becomes charges

path for excess charge to flow to ground

conductor

• polar molecule: has positive and negative end

• humidity: tiny water molecules in air → act as conductor → distracting electrons by surrounding charged object

• people are ~60% water, making them conductors

9×10^9 Nm²/C²

k q₁q₂/r²

increases the angle

decreases the angle

decreases the angle

object that created change in its surrounding space

change in surrounding space created by source charge; are vectors

object that interacts with electric field

• assume they’re positive and do not have fields

field allows force to be exerted as distance

F_q /q, q - charge in field

k q₂/r² , q₂ - source charge (only for a point charge; one source charge and one test charge)

• represents electric field

• number of lines is proportional to strength of object’s charge

• can never cross or touch other lines

• when there is a positively charged object and a negatively charged object, lines start on positively charged object

• positive charge: arrows point away from charge

• negative charge: arrows point towards charge

occurs when E-field vectors and E-field lines in a given area are parallel to each other

• external electric field causes electric charges in cage (hollow metal box) to be distributed

  • electrons move from one part of box to another

  • part where electrons left is now positive

• distribution of charges cancel the field’s effect in the cage’s interior

  • inside, fields cancel out

• state reached by charged conductors

• when excess charge is placed on a conductor, charges spread out until they reach the lowest energy state possible

• there is no net movement of charge

For objects in electrostatic equilibrium…

1. excess charge migrates to and remains on the outer surface of conductor

2. there is no electric field inside the conductor (neutral)

3. the direction of the electric field is perpendicular to the surface of conductor

4. there is a greater surface charge density at location of higher curvature

the finger is smaller and more curved, therefore there is a greater surface charge density

As the sharper ends, more curvature → higher charge density

The hair loses electrons to the comb.

A is negatively charged

negative

that the object is positively charged.

The force between the charges is repulsive

Friction and induction

• friction: 2 neutral objects acquire opposite charges via rubbing (one gains electrons, other loses electrons)

• induction: a charged object is brought near neutral objects, resulting in the separation of charges in the neutral objects

conduction

• charged object transfers excess charge to neutral object

grounding

• connecting charged object to larger conductor, which can add or accept excess electrons

induction