Electrostatics

[…] are more likely to be transferred when the plastic rod is rubbed

Electrons (valence electrons - on outer layer)

Identical objects rubber with identical materials […] each other

repel

Electric interactions are [the same as/different from] magnetic interactions

different from

Methods of charging

• friction

• conduction

• induction

• grounding

Friction

two neutral objects acquire opposite charge by rubbing against each other

Conduction

charged object directly transfers some excess charge to another object

• objects touch → charges move

• one object is initially charged

When charges are transferring between objects they [can/cannot] occur in any amount

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

is 4.0 × 10^-19 C a possible electric charge?

no

• divide value by e

• 2.5 charge transferred, not possible

is -0.00072 C a possible electric charge?

yes

• 4.5×10^15 charge transferred, possible

Conductor

• electrons are not bound and can move freely

• can transfer electrons

• excess charge resides on outer surface

Non-Conductors

• electrons bound to protons and cannot move freely

  • can only move around proton

• cannot transfer electrons

• weaker attraction and repulsion than conductors

Induction

• a charged object is brought near a neutral object

• neutral object becomes charges

Grounding

path for excess charge to flow to ground

Water is a […]

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

k =

9×10^9 Nm²/C²

Coulomb’s Law: F_q =

k q₁q₂/r²

Imagine 2 charged balls are hanging from threads. Predict what will happen tot he angle between the strings and the vertical.

Increasing the charge on the balls

increases the angle

Imagine 2 charged balls are hanging from threads. Predict what will happen tot he angle between the strings and the vertical.

Increasing the mass of the balls

decreases the angle

Imagine 2 charged balls are hanging from threads. Predict what will happen tot he angle between the strings and the vertical.

Increasing the string length

decreases the angle

source charge

object that created change in its surrounding space

electric field

change in surrounding space created by source charge; are vectors

test charges

object that interacts with electric field

• assume they’re positive and do not have fields

force and field

field allows force to be exerted as distance

E =

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)

Electric field (E-field) lines

• 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

uniform electric field

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

faraday cage

• 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

Electrostatic equilibrium

• 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

Why are you more likely to get shocked when touching a metal object with your finger as opposed to your entire hand?

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

As you increase the potential on an irregularly shaped conductor, a bluish-purple glow called the cornea forms around a sharp end sooner than a smoother end. Why?

As the sharper ends, more curvature → higher charge density

A person rubs a neutral comb through their hair and the comb becomes negatively charged. Why?

The hair loses electrons to the comb.

Consider four charges, A, B, C, and D, which exist in a region of space. Charge A attracts B, but B repels C. Charge C repels D, and D is positively charged. What is the sign of charge A?

A is negatively charged

Two neutral spheres A and B are near each other. A negatively charged rod is brought near one of the spheres as shown. The far right side of sphere B is

negative

An electroscope is given a positive charge, causing its foil leaves to separate. When an object is brought near the top plate of the electroscope, the foils separate even further. We could conclude

that the object is positively charged.

According to Coulomb’s law, if the electric force between two charges is positive, the force between the charges is [attractive/repulsive]

The force between the charges is repulsive

What methods of charging result in two oppositely charged objects?

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

Identify the type of charging: you touch a charged rod to an electroscope

conduction

• charged object transfers excess charge to neutral object

Identify the type of charging: you connect a charged object to a much large conductor to neutralize the object

grounding

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

Identify the type of charging: you bring a charged object near a neutral soda can. the soda can is attracted to the charged object.

induction