Electric Fields “forces at a distance”

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1 Electric Fields Electric Fields “forces at a distance” “forces at a distance”

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Electric Fields “forces at a distance”. Electric Charges:. Electric charge is a fundamental quantity that is responsible for all electric phenomena. Charge is a property of all atomic particles. Charge can be positive, negative, or neutral. Electrical Forces:. Likes repel. - PowerPoint PPT Presentation

Transcript of Electric Fields “forces at a distance”

Page 1: Electric Fields “forces at a distance”

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Electric FieldsElectric Fields“forces at a distance”“forces at a distance”

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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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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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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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Electric Field: Electric Field: The region around a charged particle through The region around a charged particle through which a force is exerted on another charged particle.which a force is exerted on another charged particle.

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?

E Fields!

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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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Here are some pictures of various fields:Here are some pictures of various fields:

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Electric Field Strength/Intensity Electric Field 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

Gravity Field Strength = Gravity Force / massGravity Field Strength = Gravity Force / mass

g = Fg = Fgg / m / m

Field Strength

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ExampleExample

• What is the magnitude of the electric field What is the magnitude of the electric field strength at a point in a field where an strength at a point in a field where an electron experiences a 1.0-newton force?electron experiences a 1.0-newton force?

Use E = Fe/qFe=1.0N q =1.60 x 10-19 C

= 6.3 x 1018 N/C

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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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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.

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 conducting important fact: The electric field inside a conducting surface is zero!surface is zero!

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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 the Work and the Electric FieldElectric Field

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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•If If dd between two is relatively small compared to area, between two is relatively small compared to area, electric field is electric field is uniformuniform!.!.•Parallel to each other. Parallel to each other.

•Electric field strength is same at every point Electric field strength is same at every point between plates.between plates.

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•Magnitude of electric force on an electron or a proton Magnitude of electric force on an electron or a proton located at any point between two given oppositely located at any point between two given oppositely charged parallel plates is the same.charged parallel plates is the same.

•Particle speed increases as it approaches plates of Particle speed increases as it approaches plates of opposite sign.opposite sign.

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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 Electric potential = potential energy / chargeenergy / 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 EnergyMechanical and Electrical Energy

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=WV=W((JoulesJoules)) /  / qq((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...

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• If an elementary charge is moved against If an elementary charge is moved against an electric field through a potential an electric field through a potential difference of one volt, the work done on the difference of one volt, the work done on the charge is:charge is:

• W=Vq =(1.00V)(1.60 x 10 W=Vq =(1.00V)(1.60 x 10 -19 -19 J)J)

»=1.60 x 10 =1.60 x 10 -19 -19 JJ

• (this gain in potential energy is called the (this gain in potential energy is called the electronvolt, eV!)electronvolt, eV!)

ExampleExample

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ExampleExample• Moving a point charge of 3.2 x 10Moving a point charge of 3.2 x 10-19-19 coulomb coulomb

between points A and B in an electric field between points A and B in an electric field requires 4.8 x 10 requires 4.8 x 10 -18 -18 joules of energy. What is joules of energy. What is the potential difference between these points?the potential difference between these points?

• V=W/qV=W/q4.8x10 4.8x10 -18 -18 J / 3.2x10 J / 3.2x10 -19 -19 C =C =

15V15V