ELECTRIC CHARGE

ELECTRIC CHARGE

Just as the mass of the earth exerts an invisible force on you because of your mass.

Any two objects which have an electric charge also exert a force on each other.

The electricity on the comb is not moving so it is called:

STATIC ELECTRICITY

the study of stationary charges is called:ELECTROSTATICS

Which is strongerGravity

Electrical Force

BIG EARTH

little comb

paper

1 mm

If I had a bunch of electrons (1024) in two piles, separated by a millimeter

Their masses would attract each other due to gravity with a force of:

.00000000000000000000000000000000000000006 N

Their Charges would repel each other with an Electrical force of:

230 N

In Mechanics, the primary factor was MASS.

We never really said what it is, just how it behaves.

Gravity

Inertia

Momentum

Energy

With Electrostatics, the primary factor is

CHARGE

We won’t define what it is or why it acts as it does, just how it behaves.

There are ONLY 2 types of charges

Positive (+)

Negative (-)

LIKE CHARGES REPEL

- -

+ +

OPPOSITE CHARGES ATTRACT

- +

All charge in the universe comes from two of the particles that make up an atom

Electrons (-)

Protons(+)-

++

- Neutrons (no charge)

Do the charge particles attract or repel each other in an atom?

-

++

-

Electrons (-)

Protons(+)

BOTH

The neutrons job is to hold the nucleus together

-

++

- Neutrons (no charge)

-

++

-

The electron is the smallest piece of negative charge

The proton is the smallest piece of positive charge

Their charges are EQUAL but OPPOSITE

-

++

-

If an object has the same number of protons as electrons,

It has no NET CHARGE (Electrically Neutral)

The reason that Electrical Forces aren’t very noticeable compared to gravity even though it is MUCH stronger is that:

Objects are electrically neutral for the most part. So the attractive forces pretty much cancel out the repulsive forces

-

++

-

NEUTRAL

-

++

-

++ --

Positive Net Charge (+1)

Negative Net Charge (-1)

- +

Our charge cannot not be split into a smaller piece of charge

Conservation of Charge

Charge is not created or destroyed,but it can move from 1 object to another

Some materials are very good at holding onto their own electrons or stealing them from other objects.

Like: Plastics and Rubber

Others don’t hold on to their electrons well and can’t steal them from other objects.

Like: hair or glass

When two materials are rubbed, usually electrons get stolen by one the of the objects.

+ + +---cloth

glass

+

+-

-

+-

They start out with NO NET CHARGE. Why are they neutral?

After the electrons move. What it the charge on each object?

+ + +---cloth

glass

+

+

+-

-

- Negative

Positive

If a piece of Plastic is rubbed on the same cloth (neutral again)

+ ++-

--cloth

rubber

+

+-

-

+-

If a piece of Plastic is rubbed on the same cloth (neutral again)

+ ++-cloth

rubber

+

+-

-

+-

--

They both end up charged

+ ++-cloth

rubber

+

+-

-

+-

-

-

Negative

Positive

+ + +---

+

+-

-

+-

e-s

CHARGE IS NOT CREATED

+ + +

-

-

-

+

+

--+

-

2e-s

2+

2-

0

It is only transferred between OBJECTS

0

0 + 0 = 0 2+ + 2- = 0

thus CHARGE IS CONSERVED

0 + 0 = 0 (2+) + (2-) = 0

0 = 0

Initial = Final

negatively charged It GAINS Electrons

positively charged

How does an object become

An object DOES NOT gain protons to become positively charged.This would be a NUCLEAR REACTION.

RECAP

It LOSES Electrons

BEN FRANKLIN FOUND THAT

--

+

+

--+

-

+

+

-

+

+

+

-

+

Repel

Repel--

+

+

--+

-

+

+

-

+

--

+

+

--+

-Attract

So he knew that the charges were different

--

+

+

--+

-

+

+

-

+

I dub the charge on the glass rod POSITIVE.

And Rubber rod shall be NEGATIVE

This was arbitrary

+ -

+

The smallest Net charge an object can have is that of

1 electron

1+

1- -

The SI unit of CHARGE is a COULOMB (C)

The smallest possible charge an object can have is

e = 1.60 x 10-19 C

All charge is in multiples of this

e = 1.60 x 10-19 C

How many electrons do I need to have 1 full coulomb of charge?

F =

The force exerted between two charged objects is

k Q1 Q2

r2

Q2Q1

r

Charge on object #1 (C)

Charge on object #2 (C)

distance between (m)

F =k Q1 Q2

r2

+2+1

r F12 = F21 (N)

Which charge feels the greater force?

SAME!!!

F =k Q1 Q2

r2

Fudge Factor = 8.988 109 N m2

C2

This equation is known as COULOMB’S LAW

What force is felt between, two objects each with 1 Coulomb of NET Charge separated by 1 m?

F =k Q1 Q2

r2

k = 8.988 109 N m2

C2

F =k Q1 Q2

r2F =

G m1 m2

r2

Force of Gravity Coulombic Force

Proportional to mass Proportional to charge

8.988 109(BIG)6.667 10-11 (little)

BOTH FOLLOW INVERSE SQUARE LAW

F =k Q1 Q2

r2F =

G m1 m2

r2

Force of Gravity Coulombic Force

Always attractive Can be attractive or repulsive

Mass is always positive Charge can be positive or negative

What force is felt by the electron in a hydrogen atom from its nucleus. The radius of a hydrogen atom is 5.3 x 10-11 m

How would the force change if the radius was doubled?

How would the force change if the nucleus had two protons?

What if the charge on both were doubled?

Electrons can travel easily through some materials

but not other materials

It is easier for electrons to travel hundreds of miles through metal wire than it is a few centimeters of rubber

Conductor- electrons are free to move around.

Insulator- electrons are held in place strongly

If a charge is placed on a conductor, the like charges repel each other and migrate to the surface to get as far as possible from other like charges.

-

-- -

---

Negatively charged metal sphere

+

++

+

++

Positively charged metal sphere

- - - - - - - - -- - - - - - - - - - --

- ----- --

----

-

like your black slate desk top

If a charge is placed on a INSULATOR, electrons are stuck where they land. So charged patches can be seen.

-- ----- -- ----- -- ----

What would have happened to cause a positive patch your desk?

Friction (rubbing) is not the only an object can be charged

-

-- -

--

Note: the extra electron on this sphere repel each other and spread out

Negative Metal Sphere Neutral Metal Sphere

Starting with a negatively charged sphere and a neutral oneseparated by air

-

-- -

--

If they are brought into contactThey electrons (repelling) are able to spread out further,charging the other sphere!

The second sphere was charged by CONDUCTION.CONDUCTION- Charging by contact.

-

-- -

--

COPPER

-- -

-

-COPPER

-The charge was CONDUCTED, through the copper

-

-- -

--

Glass

Glass

-

-- -

--

NO CONDUCTION THROUGH AN INSULATOR

If a negative object is brought near a neutral one

-

--

--

-+-

-- -

--

+ ++

++

NeutralNegative

The neutral object will end up with a positive side and a negative side.

This movement of charge (without contact) is called induction

If a negative object is brought near a neutral one

-

--

--

-+-

-- -

--

+ ++

++

NeutralNegative

The neutral object will end up with a positive side and a negative sideIf the sphere moves back, the sphere is still neutral.

The process of induction will cause a charged object to attract a neutral one.

-

-- -

--

+-

-- -

--

+ ++

++

--

- -

-+-

-- -

--

++

+

++

Attract

Why are the attractive forces stronger than the repulsive ones?

Distance

                                                              Charged comb attracts neutralbits of paper.

Charged comb attractsneutral water molecules.

Neutral objects are attracted to charged objects

If the spheres touch what will happen

-

-- -

-- -

-- -

-+

++

+

++

Attract

--

- -

-+

++

+

++-

-- -

--

Both negatively chargedREPEL

-

--

--+

+ ++

++ -

INDUCTION can be used to create a “permanent” charge on an object

2 neutral metal spheres in contact, and a charged rod is brought close.

-

--

--+

+ ++

++

-

--

-

-- -

If the spheres are separated before the negative rod is removed, both spheres are charged

++ +

+

++

-

--

--+

+ ++

++ ---

- --

-

If they right sphere has a charge of -.56 C, what is the charge on the left sphere?

-

--

-

--

What would happen if the negative rod was removed before the spheres were separated?

++ +

+

++

-

--

--+

+ ++

++ ---

- --

--

--

-

--

Demo soda can- Charging by induction

The earth is neutral (and really big). So it acts like a charge reservoir. If a negatively charged object touches a conductor which is “grounded”….

---

neutral

Electrons will flow to the earth. The earth is so big it is still essentially neutral & so is the object

neutral-- -

e-’s

If a positively charged object touches a conductor which is “grounded”….

neutral

+++

Electrons from the earth flow to the positive object. Again both objects are now neutral

neutral

+++- --

e-’s

+ +--+

+

+

-

-

-+

+

+

-

-

-

-

----

--

+ +--+ +--

+

+

-

-

-+

+

+

-

-

-

What would the charge be on the can if a positively charged rod was used instead?

+++++++

+

+

-

-

-+

+

+

-

-

-

What would the charge be on the can if a positively charged rod was used instead?

+++++++

+ +--+ +--

The hand acts as a GROUND in this experiment

Glass Flask

Rubber Stopper

INSULATORS

Metal ball & Rod

Thin Metal Foil Strips

an ELECTROSCOPE is a device used to detect charge

It can start out neutral

- -

--

-

++

+

++

If a negatively charged object is brought near

- --

--

++

+

+ +

- -- - -- - - - - --

Electrons are repelled down to the foil strips

The negatively charged foil strips repel each other(charged by induction)

If the rod is taken away, electrons redistribute themselves again

- -

--

-

++

+

++

If a negatively charged object is brought near again. And then touches

- --

--

++

+

+ +

- -- - -- - - - - --

Electrons will move from the rod to the electroscope

- --

--

++

+

+ +

-

--

-

--

- - -- --Now the electroscope is negatively charged by:

contactor

conduction

Even if the rod is removed, the negative charge remains and the leaves STILL repel each other

- --

--

++

+

+ +

-

--

-

-

If a positively charged rod is brought near

- --

--

++

+

+ +

-

--

-

-+ + + + + + +

If a positively charged rod is brought near

-

--

-

-++

+

++

-

--- -+ + + + + + +

Electrons migrate from the leaves toward the positive rod reducing their repulsion

Powder Painting

Copy Machine (animation)

PHOTOCOPIERS LASERPRINTERS

Electrostatic Precipitators

Problems involving static chargeElectronics

lightningwinter and door knobs

Lightning Damage

Formation of a lightning strike(animation)

INDUCED

Static Electricity Lab

Triboelectric series Human Hands (if very dry) Leather Rabbit Fur Glass Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel (neutral) Wood Amber Hard Rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (scotch tape) Polypropylene Vinyl (PVC) Silicon

Teflon

ELECTRON GIVERS (Positive)

ELECTRON STEALERS (Negative)

WHAT 3 types of things can exert a force without physical contact with the other object

Electric Charge MagnetsGravity(mass)

The space around a black hole is not a good place to be.It has a strong gravitational field.

A piece of metal near a magnet is attracted due to a magnetic field.

If you walk near a Van de Graaff generator, you can feel theelectric field.

Electric Charge MagnetsGravity(mass)

That really bugs me.

Exert a force without contact through a vacuum

A charge creates a FIELD, and the field exerts a force on objects in it

- +

Electric Field

Electric, Magnetic, and Gravitational Fields are FORCE FIELDS

electrons around a nucleus applet

- +

An ELECTRIC FIELD cannot be seen directly. But it can be felt.

An electric field is mapped out by placing a positive “test particle” in it and feeling the force on the test particle.

+

+

1

2

3

Compare the magnitude and direction of the force felt by the particles

applet

To show each Force vector at every possible location would be too messy.

Electric Field Vector Map (applet)

applet

FIELD LINES are used to simplify the picture

Arrows point in the direction of force of a positive test charge

Lines are CLOSE where Field is STRONG

and FURTHER where field is weaker

Why do field line arrow point TOWARDS a negative CHARGE

Compare the two sets of Field Lines

++

Field around 2 positive charges

Field around 2 opposite charges

Electric Field applet

Field Lines are like a map. How does is a topography map show elevation changes

They indicate the magnitude and direction of the Field (vector).

The direction FIELD LINE is also the direction of the FORCE ON A POSITIVE TEST PARTICLE

Field Lines around charged parallel plates

- -

-

-

-

-

-

--

-

- -

--

-

-

What would field lines look like outside of a charged metal ring?

- -

-

-

-

-

-

--

-

- -

--

-

-

What about on the inside?

+

Lets put a test charge in and see which way it is pulled.

Inside of a conductor, there is no NET FORCE and NO FIELD(no matter the shape of the object)

No Electric Field on the inside

- -

-

-

-

-

-

--

-

- -

--

-

-

+

What if the electron is moved closer to one side?

It is closer to the electrons on the right sideBUT has more electrons pushing from the left side

Notice that field lines are not seen in the middle of this conductor

No field = No force on an electron put in there.

An electric field does not affect the inside of a conductor. It is SHIELDED

You are surrounded by a conductor and so the electric field inside is ZERO.

The rubber tires DO NOT PROTECT YOU in a car. The lightning just jumped over 1,000 ft through air (a few inches of rubber are no problem).

When determining the field around a charge, why should we use a small test charge?

- +

Object creating field to be measured

small test charge

BECAUSE the test charge creates its own field.

The field is created by Charge #1, but in order to measure it at some location we have to use a “test charge” and measure the force on it.

+

1 E =FQ

Force felt on test charge (N)

Charge on test charge (C)

Electric Field (N/C)

A 2 C charge in a particular spot experiences 12 N of force on it, what is the strength of the electric field?

+

1 E =FQ

How much force would be on a 4 C test charge at the same spot?

All living marine organisms generate an electric field around their body and some animals possess a sensory system (the Ampullae of Lorenzini) which enables them to detect weak electric fields and use them to orient to cryptic prey.

a hammerhead shark biting at an electrode

Why does a rock lifted up on earth gain POTENTIAL ENERGY

You sure do have potential,

son!!

Gravitational energy can be converted into Kinetic Energy

Weeeee

The PE is also the amount of Work done to lift the object against the force of gravity.

h

PE = mgh = W = Fd

Gravitational Potential Energy

Electric Potential Energy

-

+

Both can be converted to KE

-

+

Work must be done to move against a force to put them back in the same spot

-+

- -

- -

1

2

Does the Electric Potential Energy increase or decrease?

-

-

1

2

Does the Potential Energy increase or decrease?

+

+

2-

+ +

1-

Compare the PE of the two negatively charged objects.

More work would have to be done to separate the “2-” because there is a greater force on it

Electric Potential =

The Electric Potential Energy of a charged object depends on its charge and location

Electric Potential EnergyCharge

1 volt = 1 Joule

1 Coulomb

Some Voltages ( Electric Potentials)

Lightning 100,000,000 VoltsOur VDG

120 VoltsResidential Electric

50,000 Volts

D battery 1.5 Volts

Electric Potential = Electric Potential Energy

Charge

How much potential energy does 2 C of electrons have in house hold voltage of 120 V?

When shuffling along a carpeted floor in the winter you may reach 15,000 volts!!

Why are you not killed when you touch a door knob?

15,000 Joules of EnergyCoulomb of Charge

you probably only build up a millionth of a coulomb of charge

Capacitors store energy in an electric fieldThey come in all shapes and sizes

Capacitors store the energy for a camera flash

the energy is stored slowly and released QUICKLY

the capacitor from a camera being discharged by “shorting the circuit”

Capacitors are simply to thin pieces of metal, separated by a non conducting material like air or plastic.

Initially the metal pieces are neutral but when connected to a battery (or other voltage source)

+- +

-+- +

-

1.5 V

1.5 V

The electrons will stay put when the battery is removed.But now there is an electric field. And the electric have ENERGY.

+-

+-

+- +

-

1.5 V

When connected again the electrons flow (& do work)

+-

+-

+- +

-

1.5 V

BIG capacitors store energy to be used when you Air conditioner starts up.

They are used in most electronic device like TV’s.They retain their energy even when the power is off.So be careful when digging around in one.

More Charge & Energy be stored in a capacitor if:plates have a large areaplates are close togetherbattery is high voltage

To increase the area of the plates without making the capacitor huge,the “plates” are sandwiched between a dielectric material and rolled up.

Energy Can be Stored in an electric Field

If the spheres touch what will happen

-

-- -

-- -

-- -

-+

++

+

++

Attract

--

- -

-+

++

+

++-

-- -

--