Lesson #15Lesson #15Topic: ElectrostaticsTopic: Electrostatics

Objectives: Objectives: (After this class I will be (After this class I will be able to)able to)

1.1. Observe an example of electrostaticsObserve an example of electrostatics2.2. Define and state the units of chargeDefine and state the units of charge3.3. Describe electric forceDescribe electric force

10/19/06

Warm Up: What makes the atoms of the desk stay together? Why doesn’t gravity make all of the atoms fall to the earth?

Assignment: Hewitt Ch32 RQ # 2,3,4,6,9,13,17,21,24; T&E # 1;

Exploring Exploring 1.1. Why does the balloon stick to the wall? Why does the balloon stick to the wall?

Form a hypothesis and be able to Form a hypothesis and be able to explain your reasoning.explain your reasoning.

2.2. Create a demonstration that shows that Create a demonstration that shows that like charges repel one another.like charges repel one another.

3.3. The mass of a proton = 1.673x10The mass of a proton = 1.673x10-27-27kgkgThe mass of a neutron= 1.675x10The mass of a neutron= 1.675x10-27-27kgkgThe mass of an electron = 9.11x10The mass of an electron = 9.11x10-31-31kgkgThere are approximately equal numbers There are approximately equal numbers of each particle in your body. Find the of each particle in your body. Find the number of protons in your body.number of protons in your body.

ElectrostaticsElectrostatics Electrostatics means “electricity at rest”Electrostatics means “electricity at rest”

In chemistry we learned that all atoms are made In chemistry we learned that all atoms are made of protons, neutrons, and electrons.of protons, neutrons, and electrons.

Protons have a positive charge, electrons have a Protons have a positive charge, electrons have a negative charge, and neutrons have no charge.negative charge, and neutrons have no charge.

So an atom with an equal number of protons and So an atom with an equal number of protons and electrons has no charge. electrons has no charge.

Atoms have charge when they have a deficiency Atoms have charge when they have a deficiency or surplus of electrons.or surplus of electrons.

ElectrostaticsElectrostatics Example: Cl Example: Cl -1-1 = a chlorine atom with an extra = a chlorine atom with an extra

electron. electron. Example: NaExample: Na+1+1 = a sodium atom missing an = a sodium atom missing an

electron.electron. These “ions” attract one another and create These “ions” attract one another and create

molecules.molecules. Sounds like a bunch of icky chemistry to me…Sounds like a bunch of icky chemistry to me… In physics we deal not with charged atoms, In physics we deal not with charged atoms,

but the overall charge of a regular size object.but the overall charge of a regular size object. That means millions and millions of charged That means millions and millions of charged

atoms. atoms.

ElectrostaticsElectrostatics So a ClSo a Cl-- atom will attract and bond with a Na atom will attract and bond with a Na++

atom… What makes them move together?atom… What makes them move together? Newton’s laws hold true not only for galaxies, Newton’s laws hold true not only for galaxies,

planets, and people, but for atoms as well.planets, and people, but for atoms as well. For an atom at rest to move to another atom, For an atom at rest to move to another atom,

some force must act on it.some force must act on it. The force that makes these two objects bond The force that makes these two objects bond

together is called the together is called the electromagnetic forceelectromagnetic force. . This is our second fundamental force of nature.This is our second fundamental force of nature. We will later learn why it is called We will later learn why it is called

electroelectromagneticmagnetic, but for now we will treat it as , but for now we will treat it as purely electric.purely electric.

Electric ForceElectric Force The electric force is what bonds and holds all The electric force is what bonds and holds all

molecules together. molecules together. This force is tremendously stronger than the This force is tremendously stronger than the

force of gravity, but has a very similar force of gravity, but has a very similar relationship.relationship.

This relationship was found in the late 18This relationship was found in the late 18thth century by French physicist century by French physicist Charles CoulombCharles Coulomb..

The force of attraction (or repulsion) between The force of attraction (or repulsion) between any two charged objects is directly any two charged objects is directly proportional to the product of the charges proportional to the product of the charges and inversely proportional to the square of and inversely proportional to the square of the distance between them.the distance between them.

Electric ForceElectric Force q = charge of an objectq = charge of an object r = distance between chargesr = distance between charges k = Boltzman constantk = Boltzman constant The constant k was found with a very similar The constant k was found with a very similar

apparatus as the one used to determine the apparatus as the one used to determine the gravitational constant G. It just used charged gravitational constant G. It just used charged metal balls instead of heavy ones.metal balls instead of heavy ones.

k = 8.99 x 10k = 8.99 x 1099 Nm Nm22/C/C22

Looks a lot like the law of Universal Looks a lot like the law of Universal Gravitation, but what the heck is q? Gravitation, but what the heck is q?

221

rqkqFe

ChargeCharge How do we know how much charge an object How do we know how much charge an object

holds?holds? By determining how much excess or lack of By determining how much excess or lack of

electrons it has.electrons it has. 1 Coulomb of charge is determined to be 1 Coulomb of charge is determined to be

6.2415x106.2415x101818 excess electrons or lack of excess electrons or lack of electrons (protons).electrons (protons).

So the charge of an electron = -1.60218x10So the charge of an electron = -1.60218x10--

1919CC And the charge of a proton = +1.60218x10And the charge of a proton = +1.60218x10-19-19CC Ok… so how does one go about getting Ok… so how does one go about getting

“charged”?“charged”?

Lesson #16Lesson #16Topic: Net Charge and Electric ForceTopic: Net Charge and Electric Force

Objectives: Objectives: (After this class I will be (After this class I will be able to)able to)

1.1. Explain net charge and the Explain net charge and the conservation of chargeconservation of charge

2.2. Describe polarization and charging by Describe polarization and charging by contact and inductioncontact and induction

3.3. Solve electric force problemsSolve electric force problems

10/22/06

Warm Up: What causes certain objects to become charged?Assignment: “Electric Force”

Net ChargeNet Charge All objects have a tremendous amount of All objects have a tremendous amount of

charge within them, however most of the charge within them, however most of the positive charge is canceled by the presence of positive charge is canceled by the presence of an equal amount of negative charge.an equal amount of negative charge.

A A net chargenet charge occurs when there is an occurs when there is an imbalance of positive and negative charge imbalance of positive and negative charge within an object.within an object.

What would be your net charge if 10% of all the What would be your net charge if 10% of all the electrons in your body suddenly disappeared? electrons in your body suddenly disappeared?

A net charge can be formed by A net charge can be formed by contactcontact, , frictionfriction, , or or inductioninduction. (Not by disappearing electrons). (Not by disappearing electrons)

Charging by FrictionCharging by Friction An insulator is made of material that An insulator is made of material that

has bound electrons that cannot move. has bound electrons that cannot move. Examples: Rubber, plastic, glass.Examples: Rubber, plastic, glass. Any objects made of this material can Any objects made of this material can

be rubbed against something to build be rubbed against something to build up charge.up charge.

Electrons cannot easily move Electrons cannot easily move throughout the material and electrons throughout the material and electrons pile up on top of one another.pile up on top of one another.

Demo: Rub a plastic rod with fur and Demo: Rub a plastic rod with fur and notice the effects.notice the effects.

Charging by ContactCharging by Contact Conductors and semi-conductors are objects Conductors and semi-conductors are objects

made of a material that allows electrons to made of a material that allows electrons to flow freely throughout the material.flow freely throughout the material.

Examples: Metal, carbon, some forms of rock.Examples: Metal, carbon, some forms of rock. Any objects made of this material can be Any objects made of this material can be

touched to another object of this material, touched to another object of this material, and electrons will flow from one object to and electrons will flow from one object to another.another.

This will distribute the charge evenly This will distribute the charge evenly throughout both objects.throughout both objects.

Demo: Van De Graph generatorDemo: Van De Graph generator

Charging by InductionCharging by Induction Conductors can charge other Conductors can charge other

conductors with an opposite charge conductors with an opposite charge through through polarizationpolarization and and groundinggrounding. .

Polarization is when electrons flee to Polarization is when electrons flee to one side of an object or the other side one side of an object or the other side of an object depending on where they of an object depending on where they are being forced by another charged are being forced by another charged object.object.

Grounding is when the conductor is Grounding is when the conductor is attached to the ground such that the attached to the ground such that the electrons can escape into the earth. electrons can escape into the earth.

The earth’s charge is always neutral.The earth’s charge is always neutral.

Contact vs. Induction when Contact vs. Induction when charging a conductor.charging a conductor.

- - - - - -

+ + + +- - - -

- - -- + - + --+ - + - -

Contact

- - - - - -

+ + + +- - - -

- - - - - -

+ + + + - -

--

+ + + +

Induction

Lesson #17Lesson #17

Topic: Lab: Charging an ElectroscopeTopic: Lab: Charging an Electroscope Objectives: Objectives: (After this class I will be (After this class I will be

able to)able to)1.1. Experiment with an electroscopeExperiment with an electroscope2.2. Write a report on the different ways to Write a report on the different ways to

charge an object.charge an object.3.3. Describe how the electric force is Describe how the electric force is

both attractive and repulsive both attractive and repulsive

10/24/06

Task: Charge an electroscope by contact and then by induction. Question: How can you tell when the electroscope is charged? Assignment: Lab report due tomorrow

Electroscope labElectroscope lab On top of the usual lab report, be sure to include On top of the usual lab report, be sure to include

the following…the following… How do you know when the electroscope is How do you know when the electroscope is

charged?charged? How did you charge it by contact? By induction?How did you charge it by contact? By induction? Explain what is happening on an atomic level.Explain what is happening on an atomic level. Explain why the electroscope reacts as it does. Explain why the electroscope reacts as it does. When does it have a positive charge? When does it have a positive charge? When does it have a negative charge?When does it have a negative charge? How did you discharge it?How did you discharge it? What did you learn from these experiments?What did you learn from these experiments?