What Do All These Pictures Have In

Common?

Electrostatics

?Did You Figure It Out?

Electrostatics

Electrostatics involves electrical charges at rest and the forces that exist between them.

In order to understand these forces we first need to discuss the basic parts of an atom.

The Atom

The atom consists of a nucleus at the center made up of protons and neutrons. The nucleus is orbited by electrons.

Protons are positively charged particles located in nucleus of the atom.

Neutrons are neutral particles (particles that have no charge) are located in the nucleus of an atom. The neutron has a mass equal to that of a proton.

Electrons are negatively charged particles located in orbit around the nucleus of the atom. The mass of an electron is approximately 1/1837 that proton.

The electrical charge on protons and electrons are equal in magnitude but opposite in charge.

Rules for working with charges

Like charges repel

Different charges attract

Working with neutral chargesA neutral charge (no charge), is different than both negative and positive charges.

Therefor a neutral charge will actually be attracted to both positive charges and negative charges.

Total charge of a bodyBecause an electron and proton have equal and opposite charges the net charge of atom is zero.

(2 negative charges) (2 positive charges) (2 neutral charges)

= 0

+ +

However if and Atom somehow loses an electron, the net charge of the atom is no longer balanced, thus resulting in a net positive charge.

This positively charged atom is known as a positive ion.

Because electrons are located very far away from the nucleus of an atom it is reasonably easy to remove them from the atom.

The same cannot be said for proton. It is very difficult to remove a proton from an atom.

A negatively charged atom is known as a negative ion.

Therefore if a net negative charge is desired one must add electrons to an atom, so that there is an abundance of electrons.

Negative Ion

Conservation of Charge

We now know that it is much easier to move an electron than a proton.

Therefore electrostatics is the result of the movement of

electrons not protons.

The conservation of charge states that charge cannot be created or destroyed simply transferred from one location to another.

Electrons can move more easily through some substances than others.

Substances where electrons can move very easily, are called conductors.

Substances where the electrons cannot move easily are called insulators.

Most metals are very good conductors because they have many loosely attached electrons.

Some common insulators are glass, rubber, and silicon.

The question now is how do we transfer electrons to or from a body

There are three ways:

1) Charging by friction

2) Charging by conduction

3) Charging by Induction

Charging by friction When two objects are made of different materials,

their atoms will hold onto their electrons with different strengths.As they pass over each other the atoms with weaker bonded electrons are “ripped” off of that material and collect on the other material.

This is known as charging by friction.

There is a net transfer of electrons from one material to the other!

Example:Rub a glass rod with a piece of silk.

In this case the silk holds onto the electrons more strongly than the glass.

Electrons are ripped off of the glass and go on to the silk. The glass is now positive and the silk is negative.

-

- --

++

++

Sweater and Balloon

Charging by Conduction

Once something is charged either positively or negatively, it tends to lose its excess charge to another object if they come in contact

Charging by conduction involves the contact of a charged object to a neutral object.

Conduction just means that the two objects will come into actual physical contact with each other (this is why it is sometimes called “charging by contact”).

If the two objects are brought close enough that an arc of electricity jumps between them, it counts as conduction also.

Let's assume we have a negatively charged metal object and an uncharged metal sphere. The uncharged sphere is on an insulating stand so that it will not interact with anything else.

We bring the two objects close together. We will see a separation of charge happen in the neutral object as negative electrons are repelled to the right hand side.

At this time, they are not touching and no charges have been transferred.

We now allow the two objects to touch.

Some of the negative charge will transfer over to the uncharged metal object.

This happens since the negative charges on the first object are repelling each other... By moving onto the second object they will be able to spread away from each other.

When the negative object is removed, it will not be as negative as it was.

Both of the objects have some of the negative charge… how much depends on the size of the objects and the materials they are made of.

If they are the same size, made of the same materials, then the charge will be the same on both.

Charging by Conduction Video

Charging by InductionWhen a charged object is brought close to an object without physical contact, the charged object will induce a movement of electrons in the uncharged object.

In this case the electrons in the neutral object will will be repelled from the electrons in the charged object.

If the ground wire is then attached to the neutral object electrons will flow to or from ground in order to stabilize the electrical forces surrounding the neutral object.

In this case the electrons will leave to an object and go to ground.

Once the electrons have been removed from the neutral object, remove the ground wire and then the negatively charged object.

Thus leaving the originally neutral object with fewer electrons than protons, and resulting in a net positive charge.

It is crucial that the ground wire is removed first, otherwise the electrons will simply return to the metal sphere.

Charging by Induction Video

Concept Development (32-2)

What’s the Charge Activity

Charging an Electroscope Computer Simulation

Do

Charles Augustin de Coulomb • Born in 1736, died in

1806• Created a device that

helped him develop his theories on charges and electric force and field.

• Discovered Coulomb’s Law

What is Coulomb’s Law• The magnitudes of the electrostatic force between two

point electric charges are directly proportional to the product of the magnitudes of each charge and inversely proportional to the square of the distance between the

charges.

Fkq q

r 1 2

2

Where “k” is Coulomb’s constant and is equal to 9.0x10 9 Nm2/C2

When working with Coulomb’s law a negative solution means that there is an attractive force.

A positive solution means that there is a repulsive force. Such as two like charges.

This would be the case for finding the force between two opposite charges such as a positive and negative charge.

+ & - = -

+ & + = + - & - = +

Each charged object exerts an equal but opposite force on the other charged object.

q1 q2

F21

F12

For a two point source there is actually two forces at play.

For example, you have the force that the first charge exerts on the second (F12)

and the force that the second charge exerts on the first (F21)

The Coulomb is a SI unit of charge.

The Coulomb

One Coulomb is the charge on 6.25 x 1018 electrons.

The magnitude of charge on an electron is called the elementary charge and is equal to 1.6 x 10-19C.

ExampleSuppose that two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. Determine the magnitude of the electrical force of repulsion between them.

F = 9.0 x 109 N

ExampleTwo balloons with charges of +3.37 µC and -8.21 µC attract each other with a force of 0.0626 N. Determine the separation distance between the two balloons.

d = 1.99 m

Example

a) Positive

b) 0.85 m

A small sphere, carrying a charge of –8.0 C, exerts an attractive force of 0.50 N on another sphere carrying a charge with a magnitude of 5.0 C.

a) What is the sign of the second charge?

b) What is the distance of separation of the center of the spheres?

Page 638 #’s 1-5 (pdf 81)

Concept Development (32-1)

Do

ExampleThree charges A(+5.0 C), B(-2.0 C), and C(+3.0 C), are arranged at the corners of a right triangle as shown. What is the net force on charge C?

150 N @ 22° clockwise from the horizontal

Page 640 #’s 6-10 (pdf 81)

Do

THE END