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ElectrostaticsElectrostatics(static electricity)(static electricity)

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Electric Charges:Electric Charges:

Electric charge is a fundamental quantity that Electric charge is a fundamental quantity that is responsible for all electric phenomena.is responsible for all electric phenomena.

Charge is a property of all atomic particles.Charge is a property of all atomic particles.

Charge can be positive, negative, or neutral.Charge can be positive, negative, or neutral.

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Atoms:Atoms:In general, the heavy nuclei of atoms have a In general, the heavy nuclei of atoms have a

positive charge.positive charge.

The small, light, negative electrons reside in The small, light, negative electrons reside in shells or orbits outside the nucleus.shells or orbits outside the nucleus.

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Atoms:Atoms:In general, the heavy nuclei of atoms have a In general, the heavy nuclei of atoms have a

positive charge.positive charge.

Atomic structure

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Electrons and You:Electrons and You:Your body contains an astronomical number of Your body contains an astronomical number of electrons. (Far more than would be needed to electrons. (Far more than would be needed to

electrocute a person.)electrocute a person.)

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However, the negative electrons are balanced However, the negative electrons are balanced by an approximately equal number of positive by an approximately equal number of positive

protons.protons.

This balance gives a net charge of zero This balance gives a net charge of zero (neutral).(neutral).

-

-

---

-+

+ ++

++

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Conservation of Charge:Conservation of Charge:

When one item has “extra” electrons, this When one item has “extra” electrons, this means that something else must have “lost” means that something else must have “lost”

electrons.electrons.

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Just as mass, energy, and momentum are Just as mass, energy, and momentum are conserved, so is electric charge.conserved, so is electric charge.

You can’t create or destroy electric You can’t create or destroy electric charge, it is just transferred or moved.charge, it is just transferred or moved.

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Electrical Forces:Electrical Forces:

Opposites attract.Opposites attract.

This is due to a force arising from the electric This is due to a force arising from the electric charge on particles. charge on particles.

We will learn more about this force later...We will learn more about this force later...

Likes repel.Likes repel.

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Review:Review:

+ +??

RepulsiveRepulsive

+ -??

AttractiveAttractive

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Coulomb’s Law:Coulomb’s Law:

k = 9 x 10 k = 9 x 10 99 Nm Nm22/C/C22

q = one charge q’ = another chargeq = one charge q’ = another charge

d = distance between the chargesd = distance between the charges

This is the law that quantitatively relates the This is the law that quantitatively relates the attraction or repulsion of electric charges.attraction or repulsion of electric charges.

k q q’k q q’FFEE = ------------------- = -------------------

dd22

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Charge Unit:Charge Unit:Since electrons are much too small and Since electrons are much too small and

numerous to be counted individually, they are numerous to be counted individually, they are counted in groups called counted in groups called CoulombsCoulombs..

1 Coulomb, C, = 6.25 x 101 Coulomb, C, = 6.25 x 101818 electrons. electrons.

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Q: Which is stronger, gravitational or electric Q: Which is stronger, gravitational or electric force?force?

A: For atoms and objects on our scale, the A: For atoms and objects on our scale, the electric force is much stronger. On a electric force is much stronger. On a

planetary scale, gravity is much stronger.planetary scale, gravity is much stronger.

??

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Conductors :Conductors :

Conductors:Conductors: Material where electrons are Material where electrons are loosely bound and are able to flow loosely bound and are able to flow

throughout (free electrons). Ex: throughout (free electrons). Ex: metals, metals, impure water, humansimpure water, humans

Don’t try this at home...

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Insulators:Insulators:

Insulators:Insulators: Materials where electrons are Materials where electrons are bound and don’t flow easily.bound and don’t flow easily.

Ex: Ex: glass, rubber, plasticglass, rubber, plastic

Conductors &Insulators

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Semi Conductors:Semi Conductors:

Semiconductors:Semiconductors: Materials in between Materials in between insulator and conductor. Ex: insulator and conductor. Ex: silicon, silicon,

carbon, germaniumcarbon, germaniumUsed in transistors and other electronic Used in transistors and other electronic

components.components.

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Super Conductors:Super Conductors:Super conductor:Super conductor: a material where electrons a material where electrons

flow without any resistance. Generally, flow without any resistance. Generally, superconductivity only occurs at very low superconductivity only occurs at very low

temperatures. Magnetically levitated superfast temperatures. Magnetically levitated superfast trains are one application of superconductivity.trains are one application of superconductivity.

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Charging by ContactCharging by Contact

Simply rubbing one object against another can Simply rubbing one object against another can transfer electrons. This makes one positive and transfer electrons. This makes one positive and

one negatively charged object.one negatively charged object.

+

+

+

_

_

_++

+__

_

Before: both neutral

++

+__

+

+

+

_

_

_

_

After: Silk positive, glass negative

friction

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Charging by ContactCharging by Contact

Simply rubbing one object against another can Simply rubbing one object against another can transfer electrons. This makes one positive and transfer electrons. This makes one positive and

one negatively charged object.one negatively charged object.

Charging by friction

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Charging by Induction:Charging by Induction:Even without direct contact, you can Even without direct contact, you can

induce the electrons to move due to the induce the electrons to move due to the electric force acting on them.electric force acting on them.

1. Both neutral at

first

2. Negative rod

separates charges.

3. Separate the spheres

4. Remove rod, spheres are now

charged.

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Lightning:Lightning:

When one mass of air moves over another, When one mass of air moves over another, electrons are transferred. This is just like electrons are transferred. This is just like

moving your feet on carpet to build a charge.moving your feet on carpet to build a charge.

A large negative charge on the bottom of a A large negative charge on the bottom of a cloud induces a positive charge on the ground cloud induces a positive charge on the ground

below. ( they are attracted) below. ( they are attracted)

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If the charge imbalance becomes large enough, If the charge imbalance becomes large enough, lightning occurs. This discharges the lightning occurs. This discharges the

accumulated charge.accumulated charge.

-- - - --- - - - -

++

+ + ++

+

+ +

++

Lightning

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Charge Polarization:Charge Polarization:

Just as a charged cloud Just as a charged cloud causes charge to causes charge to

separate in the ground, separate in the ground, this can happen in other this can happen in other

objects.objects.

Notice how the wall is Notice how the wall is still neutral, but the still neutral, but the

charge is separated...charge is separated...Balloon & Wall Another Balloon

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Why Does it Cling?Why Does it Cling?

Although the paper is still neutral, since its Although the paper is still neutral, since its polarized, the (-) is now closer, and the (+) is polarized, the (-) is now closer, and the (+) is farther. This means the attractive force is farther. This means the attractive force is

greater, and the repulsive force is less. ( It sticks!)greater, and the repulsive force is less. ( It sticks!)

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Electric Field:Electric Field:Imagine there is a cluster of – charge as shown. Imagine there is a cluster of – charge as shown.

-

If a small + charge was placed, what force would it feel?

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This “map of force lines” This “map of force lines” shows what a positive shows what a positive test charge will do when exposed to any test charge will do when exposed to any

particular field. When lines are closer together, particular field. When lines are closer together, that means the force is stronger. Here are some that means the force is stronger. Here are some

pictures of various fields:pictures of various fields:

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Electric Field Electric Field Strength/Intensity Strength/Intensity

Electric potential describes how strong the field Electric potential describes how strong the field is per amount of charge.is per amount of charge.

Electric Field Strength = Electrostatic Force Electric Field Strength = Electrostatic Force // chargecharge

E = FE = Fee / q / q

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Question:Question:

Q: Suppose you had a charged rod, and an Q: Suppose you had a charged rod, and an oppositely charged hoop as pictured. oppositely charged hoop as pictured.

+

++

+

+

+

-

- -

-

-

--

-

--

--

Describe the electric field between them. Describe the electric field inside the hoop.

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A: Between, the field points directly from the A: Between, the field points directly from the (+) to the (-). That’s the direction a positive (+) to the (-). That’s the direction a positive

test charge would want to go.test charge would want to go.

However, inside the hoop, there is

surprisingly no field at all!!!

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Shielding:Shielding:

This last question points out an odd, but This last question points out an odd, but important fact: The electric field inside a important fact: The electric field inside a

conducting surface is zero!conducting surface is zero!

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Anywhere inside a conducting surface, the forces Anywhere inside a conducting surface, the forces on you cancel out, giving no electric field. on you cancel out, giving no electric field.

Even though you may be closer to one end ( less distance), on the other

end, there is more charge pulling on you. These effects cancel out

and give 0 field.

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Lightning Strike?Lightning Strike?Q: If you are inside your car when it is struck Q: If you are inside your car when it is struck

by lightning, you will survive. Why?by lightning, you will survive. Why?

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A: A: The electrons repel themselves to the The electrons repel themselves to the extreme outside of the car. extreme outside of the car.

Two electrons wouldn’t want to be near each Two electrons wouldn’t want to be near each other on the inside by you. other on the inside by you.

The electricity flows around the outside of the The electricity flows around the outside of the car, not through you.car, not through you.

This would happen even if there were no tires This would happen even if there were no tires at all on the car!at all on the car!

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Work and Work and the Electric the Electric

FieldField

Work is done by the electric field in moving the Work is done by the electric field in moving the positive charge. If the charge were moved positive charge. If the charge were moved

counter to the field, work would be required.counter to the field, work would be required.

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Field between plates is uniformField between plates is uniform

  

- (same strength all over)- (same strength all over)

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Electric Potential Electric Potential

How much work is done, depends on the How much work is done, depends on the amount of charge. amount of charge.

Electric potential describes how much work is Electric potential describes how much work is done, per amount of charge.done, per amount of charge.

Electric potential = potential energy Electric potential = potential energy // charge charge

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Electric Potential Unit:Electric Potential Unit:A Joule per Coulomb is defined as a Volt. A Joule per Coulomb is defined as a Volt.

1 V = 1 J / 1C 1 V = 1 J / 1C

A volt is the basic unit of electric potential.A volt is the basic unit of electric potential.

Named after Alessandro Volta, he invented the Named after Alessandro Volta, he invented the electric battery.electric battery.

DieHard

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Mechanical and Electrical Mechanical and Electrical EnergyEnergy

Gravitational potential energy is derived from Gravitational potential energy is derived from the earth’s gravitational field. Electrical the earth’s gravitational field. Electrical

potential energy, is derived from an electric potential energy, is derived from an electric field.field.

Despite the name similarity, electric potential is Despite the name similarity, electric potential is NOT the exact same as potential energy. NOT the exact same as potential energy.

Electric potential describes how much work Electric potential describes how much work could be done per amount of charge.could be done per amount of charge.

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Mechanical / Electrical EnergyMechanical / Electrical Energy

Two rocks are at the same height, the larger one Two rocks are at the same height, the larger one has more PE. Two charges have the same has more PE. Two charges have the same

electric potential, the larger charge has more electric potential, the larger charge has more PE.PE.

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V=V=W(W(JoulesJoules))  

q(q(CoulombsCoulombs))

WW = = workwork done against field done against field oror energy acquiredenergy acquired working with field (Joules or eV)working with field (Joules or eV)

  qq - amount of charge moving through field - amount of charge moving through field (Coulombs)(Coulombs)

VV - Potential Difference (volts) - Potential Difference (volts)

Ex) It takes 6 Joules of work to move 2 Coulombs Ex) It takes 6 Joules of work to move 2 Coulombs of charge between 2 points in an electric field. of charge between 2 points in an electric field. What is the potential energy difference (voltage) What is the potential energy difference (voltage) between these 2 points?between these 2 points?

  

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Ex) It takes Ex) It takes 6 Joules of work6 Joules of work to move to move 2 coulombs2 coulombs of of chargecharge between 2 points in an electric field. between 2 points in an electric field. What is the What is the potential energy differencepotential energy difference (voltage) (voltage) between these 2 points?between these 2 points?

  

V=V=W(W(JoulesJoules))  

q(q(CoulombsCoulombs))

V =6 Joules / 2 coulombsV =6 Joules / 2 coulombs

  

  

V = 3 Volts or J/CV = 3 Volts or J/C

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Both groups of charges on the side are at the Both groups of charges on the side are at the same electric potential (voltage). However, it same electric potential (voltage). However, it

would take much more work to move the lower would take much more work to move the lower one closer since it has a larger charge...one closer since it has a larger charge...