Intro to Electrostatics

Electrostatics Intro

By Nicole Woltschlaeger

First let's see...

​...what you remember from chemistry.

The next series of slides are just to see what you remember from last year (or two years ago).

Chemistry

Make sure you take notes on this...

Multiple Choice

The smallest unit of matter that has the properties of the element.

• atom

• ion

• molecule

• formula unit

Multiple Select

Atoms are made up of subatomic particles called (choose all that apply)

• protons

• neutrons

• orbitals

• electrons

Multiple Choice

neutrons have a charge of

• +1

• 0

• -1

Multiple Choice

protons have a charge of

• +1

• 0

• -1

Multiple Choice

electrons have a charge of

• +1

• 0

• -1

Multiple Select

Which subatomic particle(s) are in the nucleus of the atom?

• proton

• neutron

• electron

Multiple Select

Which subatomic particle(s) are outside of the nucleus?

• proton

• neutron

• electron

Protons and electrons

• ​Protons and electrons are the fundamental particle that carry a charge.

• ​The charge of the proton is exactly the opposite the charge of the electron.

• ​The SI Unit of the electric charge is the Coulomb, C.

• ​The proton has a charge of +1.602 x 10^-19 C

• ​The electron has a charge of -1.602 x 10^-19 C

• The variable we assign to charge is the letter q. ​

Charges of protons and electrons.

Conservation of Charge

• Just like mass and energy, charge cannot be created or destroyed.

• If you select your system boundaries so that no charges cross into or from your system, then

  • q_initial = q_final

  • where q_initial is the net charge before the interaction

  • q_final is the net charge after the interaction

Charge is always conserved!

What do I mean by interaction?

I am going to release a simulation in Google Classroom called John Travoltage. Go there next

​

BUT DON'T CLOSE THIS TAB!​

Subject | Subject

John Travoltage

Open Ended

Charge cannot be created or destroyed. After exploring John Travoltage, what did you discover about charge?

Movement of charge

• We have learned through a lot of experimentation that it is the negatively charge particles, the electrons, that move.

• When an object is rubbed, electrons can either be added or removed from that object​.

• In uncharged objects, the ​amount of positive charge is equal to the amount of negative charge.

• For simplicity, we say the charge of an electron is -1, and the charge of a proton is +1.​

Here is a representation of charges using​ simple blocks.

A negative charge is represented by a white box;

a positive charge is represented by a black box.​

​Here we have five positive charges (5 black boxes), and two negative charges (2 white boxes).

The net charge is +3.​

Multiple Choice

A neutral object gains two electrons. Its new charge is

• +2

• 0

• -2

Multiple Choice

A neutral object loses 5 electrons. Its new charge is

• +5

• 0

• -5

Multiple Choice

An object that initially has a charge of +4 gains four electrons. Its new charge is

• +8

• 0

• -4

Multiple Choice

An object that initially has a charge of +3 gains four electrons. Its new charge is

• +1

• -4

• -1

Multiple Choice

An object that has an excess charge of -3 is neutralized. Its new charge is

• +3

• 0

• -3

Multiple Choice

An object that has a charge of -5 loses four electrons. Its new charge is

• -9

• -1

• +1

​Electrons can be transferred in several ways

• Bringing two different materials close together, especially by rubbing, transfers electrons from one material to the other.

• You can also transfer electrons by conduction. When Jon Travoltage touches the doorknob, the electrons "flow" off his body because like-charges repel. They want to get as far from each other as possible.

• We can also charge by induction​

Some text here about the topic of discussion

• Also called dielectrics.

• ​Charged particles cannot move freely, but can shift slightly.

• This shift of charge can result in ​a dipole.

Insulators

• Charge can easily move through the material.​

• In metals, these particles are electrons.​

Conductors

Conductors vs. Insulators

dipole - an excess of negative charge on one end; an excess of positive charge on the other end

​Source: Openstax High School Physics

(a) represents a conducting sphere with excess negative charge. The charge spreads out on the surface of the sphere since the electrons repel each other.

​

(b) represents an insulating sphere with excess negative charge. ​The electrons cannot move, so they are stuck in their original positions.

​

Source: Openstax High School Physics

(a) Two neutral conducting spheres are touching.

(b) a positively charged rod is brought close to the spheres. The electrons are attracted to the + charge on the rod, resulting in a net - charge on the left sphere and net + charge on the right sphere.

(c) The spheres are separated.

(d) They retain their non-neutral charges.​

​

Photo: Openstax High School Physics

Charging by induction

Multiple Choice

In a physics lab, you charge up three metal spheres, two with +3 nC and one with -5nC. (nC is nanocoulomb). When you bring all three spheres together so that they all touch, what is the total charge on the three spheres?

• +6 nC

• +5 nC

• +1 nC

• -1 nC

Multiple Choice

How does a conductor differ from an insulator?

• Electric charges move easily in an insulator but not in a conducting material.

• Electric charges move easily in a conductor but not in an insulator.

• A conductor has a large number of electrons.

• More charges are in an insulator than in a conductor.

Coulomb's Law

Describes the electrostatic force between two charged objects.

k = 9 ×10⁹ N⋅m²/C²​

Like charges; opposite charges

• Like charges repel each other.

• ​Opposite charges attract.