If an object has 2 billion If an object has 2 billion protons and 2 billion protons and 2 billion

electrons what is its charge?electrons what is its charge?

A.A. Positive!Positive!

B.B. Negative!Negative!

C.C. Its not charged!Its not charged!

D.D. Huh?Huh?4]

The top secret, terribly The top secret, terribly mysterious, and totally mysterious, and totally

easy to understand world of easy to understand world of the Greenies.the Greenies.

a.k.a Electricity

The Truth

• I was approached by the Greenies several years ago.

• I’m waiting for the world to be ready for the truth.• What is a Greenie:

– Strange, previously unknown species.– Unimaginably small.– Live in another dimension that slightly overlaps ours.– Invisible, often green, like to party.– All my Greenie knowledge was gained through

interviews with a Greenie named Dave.

Greenie society

• Greenies like to party.• Female Greenies Pick out choice

locations and throw parties– These usually don’t move (so you can

order pizza for the after-party).– Female Greenies blast Greenie music to

“advertise” the party.

Greenie society

• Male Greenies are nomadic party animals. Spend their entire lives looking for girls and their parties.– They are always moving around looking for

girls and avoiding other guys (too many guys ruins the party dynamic).

– When guy greenies hear music they go nuts. They NEED TO PARTY! The more music, the greater the need to party!

– The more guys around the bigger the party has to be to keep them happy.

Electric Force and ChargesProtons• Positive electric charges• Repel positives, but attract negatives

Electrons• Negative electric charges• Repel negatives, but attract

positives

Neutrons• Neutral electric charge

When you brush your hair and scrape electrons from your hair, the charge of your hair is

A. positive.

B. negative.

C. Both A and B.

D. Neither A nor B.

What “They” think

• The world is made up of very tiny objects that have “charge”.

• “They” have no idea what “charge” is! They let some kite-flying nut decide that there are both positive and negative versions of this imaginary “charge”!

• Opposite charges attract• Like charges repel• No one knows why! (except us)

WRONG! It’s really Greenie guys

(negative) and Greenie girls (positiv

e)!

++

+ --

-

--

---

-

Bringing Charges Together

++

+ ++

+

Attract

RepelRepel

Conductors and Insulators• Conductor: Materials in which one or more of the

electrons in the outer shell of its atoms are not anchored to the nuclei of particular atoms but are free to wander in the material– Example: Metals such as copper and aluminum

• Insulators: Materials in which electrons are tightly bound and belong to particular atoms and are not free to wander about among other atoms in the material, making them flow– Example: Rubber, glass

Conductors and Insulators• Semiconductors: A material that can be made to

behave sometimes as an insulator and sometimes as a conductor.– Fall in the middle range of electrical resistivity between

insulators and conductors.– They are insulators when they are in their pure state.– They are conductors when they have impurities.

• Semiconductors conduct when light shines on it.– If a charged selenium plate is exposed to a pattern of

light, the charge will leak away only from the areas exposed to light.

When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper

wire, electrons

A. must be supplied by you, just as water must be supplied for a water pipe.

B. are already in the wire.

C. may fall out, which is why wires are insulated.

D. None of the above.

Superconductors• Superconductors: Materials acquire zero

resistance (infinite conductivity) to the flow of charge.– Once electric current is established in a

superconductor, the electrons flow indefinitely.– With no electrical resistance, current passes

through a superconductor without losing energy.

– No heat loss occurs when charges flow.

Triboelectricity• Charging by friction and contact.

Example:

Stroking cats fur, combing your hair, rubbing your shoes on a carpet

• Electrons transfer from one material to another by simply touching. For example, – when a negatively charged rod is placed in

contact with a neutral object, some electrons will move to the neutral object.

+ -

+-

+

+-

-

-+

+

+

-

-

+

+

+-

-

-

Charging Up Rods: inducing a charge

Gla

ss

Acr

ylic

wool

“Official Story”- It’s all about affinity for electrons and the triboelectric series

wool

Charging• Charging by induction

– If you bring a charged object near a conducting surface, electrons are made to move in the surface material, even without physical contact.

– Example: The negative Example: The negative charge at the bottom of charge at the bottom of the cloud induces a the cloud induces a positive charge on the positive charge on the buildings below.buildings below.

ChargingInduction: Consider two insulated metal spheres A and B.a. They touch each other, so in effect they form a single

uncharged conductor. b. When a negatively charged rod is brought near A, electrons in

the metal, being free to move, are repelled as far as possible until their mutual repulsion is big enough to balance the influence of the rod. The charge is redistributed.

c. If A and B are separated while the rod is still present, each will be equal and oppositely charged.

Charge Polarization• One side of the atom or molecule is induced into becoming

more negative (or positive) than the opposite side. The atom or molecule is said to be electrically polarized.

• An electron buzzing around the atomic nucleus An electron buzzing around the atomic nucleus produces an electron cloud.produces an electron cloud.

a. The center of the negative a. The center of the negative cloud normally coincides with cloud normally coincides with the center of the positive the center of the positive nucleus in an atom. nucleus in an atom.

b. When an external negative b. When an external negative charge is brought nearby to the charge is brought nearby to the right, the electron cloud is right, the electron cloud is distorted so that the centers of distorted so that the centers of negative and positive charge negative and positive charge no longer coincide. The atom is no longer coincide. The atom is now electrically polarizednow electrically polarized

Charge Polarization• If the charged rod is negative, If the charged rod is negative,

then the positive part of the then the positive part of the atom or molecule is tugged in a atom or molecule is tugged in a direction toward the rod, and direction toward the rod, and the negative side of the atom or the negative side of the atom or molecule is pushed in a molecule is pushed in a direction away from the rod. direction away from the rod.

• The positive and negative parts The positive and negative parts of the atoms and molecules of the atoms and molecules become aligned. They are become aligned. They are electrically polarized.electrically polarized.

Charge Polarization• When a charged comb is When a charged comb is

brought nearby, molecules brought nearby, molecules in the paper are polarized. in the paper are polarized.

• The sign of charge closest The sign of charge closest to the comb is opposite to to the comb is opposite to the comb’s charge. the comb’s charge.

• Charges of the same sign Charges of the same sign are slightly more distant. are slightly more distant. Closeness wins, and the Closeness wins, and the bits of paper experience a bits of paper experience a net attraction.net attraction.

Charge Polarization• Rub an inflated balloon on your hair, Rub an inflated balloon on your hair,

and it becomes charged. and it becomes charged.

• Place the balloon against the wall, Place the balloon against the wall, and it sticks. and it sticks.

• This is because the charge on the This is because the charge on the balloon induces an opposite surface balloon induces an opposite surface charge on the wall. charge on the wall.

• Again, closeness wins, for the Again, closeness wins, for the charge on the balloon is slightly charge on the balloon is slightly closer to the opposite induced closer to the opposite induced charge than to the charge of same charge than to the charge of same signsign

Charge Polarization• Many molecules—HMany molecules—H22O, for O, for

example—are electrically example—are electrically polarized in their normal polarized in their normal states. states.

• The distribution of electric The distribution of electric charge is not perfectly even. charge is not perfectly even.

• There is a little more There is a little more negative charge on one side negative charge on one side of the molecule than the of the molecule than the other.other.

• Such molecules are said to Such molecules are said to be be electric dipoleselectric dipoles..

Example: Greeniepalooza Example: Greeniepalooza (aka lightning)(aka lightning)

Imagine you are a greenie guy…..• Your floating along on a water molecule in a

cloud using you “greenie sense” to find a party.

• You notice that more and more Greenie Guys are hanging out on your part of the cloud. So many that you become VERY uncomfortable.

• Then you hear it…. An entire planet made of Greenie Girls jammin some tunes below you. The more Guys that crowd on your cloud the louder the music gets! You can’t take it!!!

• What do you DO!!!!!!!!• Zap!!!

Uses of Static ElectricitySpraying a Car

Positive Car

Negative Spray gun

The paint spreads out as each negative drop repelsNo paint is wasted as the positive car attracts the negative paint

Removing Smoke from Power Station Chimneys

Positive Plate

Negative Plate

Charge up the Smoke

No smoke leaves the chimney

Earthing Fuel Tankers

Fuel rubbing against the pipe can build up a static charge which could cause an explosionThe tanker is joined to the ground with a wire to stop a charge building up

An atom has 10 protons and 12 electrons. What is the net charge on the atom?

A. 10

B. -12

C. 2

D. -2

Charges are placed on two identical metal spheres. The charge on one sphere is -6 and the charge on the other sphere is 0. If

the two spheres are touched together, what will be their resulting charge?

A. Both will have -6 charge

B. Both will have -3 charge

C. One will have -6 and the other 0

D. One will have -3 and the other O

Now the spheres have a charge of +2 and -4. What will be the resulting charge after touching?

A. -1

B. -2

C. -6

D. +6

Charges are placed on two identical metal spheres. The charge on one sphere is -6 and the

charge on the other sphere is 4. If the two spheres are touched together, what will be their resulting

charge on each sphere?

A. -2

B. 2

C. 1

D. -1

In a good conductor, electrons are usually __________.

A. tightly bound in place.

B. not moving at all.

C. free to move around after an impurity has been added.

D. free to move around.

In a good semiconductors, electrons are usually

__________.

A. tightly bound in place.

B. not moving at all.

C. free to move around after an impurity has been added.

D. free to move around.

Coulomb’s Law

Coulomb’s law• Relationship among electrical force, charge, and

distance discovered by Charles Coulomb in the 18th century

• States that for a pair of charged objects that are much smaller than the distance between them, the force between them varies directly, as the product of their charges, and inversely, as the square of the separation distance

Coulomb’s Law

Coulomb’s law (continued)• If the charges are alike in sign, the force is repelling;

if the charges are not alike, the force is attractive.• In equation form:

k = 9,000,000,000 Nm2/C2

• Unit of charge is coulomb, C• Similar to Newton’s law of gravitation for masses• Underlies the bonding forces between molecules

F = kq

1q

2d2

According to Coulomb’s law, a pair of particles that are placed twice as far apart will experience forces

that are

A. half as strong.

B. one-quarter as strong.

C. twice as strong.

D. 4 times as strong.

A +1μC charge is placed 0.1 m from a -1 μC charge.How much electric force is between the charges?

A. .9 N attracting

B. .9 N repelling

C. .09 N attracting

D. .09 N repelling

A +5 μC charge is placed 23 centimeters from a -2 μC charge. Find the force between the charges.

A. 189 N

B. 1.70 N

C. .000170

D. .391 N

A pencil is given a charge of +8 μC and is placed 50 cm from a cup that has an unknown charge. The magnitude of the attractive electrical force

between the cup and pencil is 3.2 Newtons. What is the charge on the cup?

A. 1.1 C

B. -1.1 C

C. 11 μC

D. -11 μC

Electric Field

Electric Field

Electric field direction• Same direction as the force on a positive charge• Opposite direction to the force on an electron

A charge of +6.5 μC is placed in an electric field that has a magnitude of 2.3x105 N/C. What is the magnitude of the force exerted on this charge?

A. 15 N

B. 1.5 N

C. 1500000

D. .00015

A -9μC charge is placed in an electric field of 1.5x105 N/C. How much force is felt by the

charge?

A. .135 N

B. 1.35 N

C. 7.8 N

D. 78 N

According to Coulomb’s law, a pair of particles that are placed half as far apart will experience forces

that are

A. half as strong.

B. one-quarter as strong.

C. twice as strong.

D. 4 times as strong.

The Superposition Principle

• When a number of separate charges act on the charge of interest, each exerts an electric force. The electric forces can be calculated separately and added as vectors.

The Superposition Principle

• Each charge exerts a force on the other.• The net force on a charge is the sum of all the forces acting on it by other charges.

• Fnet = F12 – F23

Remember: if vectors are

pointed in the same

directio

n, you add them. B

ut

if they are pointed in

opposite dire

ctions, you

subtract!

Practice

1. Three charges are arranged in a line as shown.

What is the net force on q2?

Electric Potential

Electric potential energy• Energy possessed by a charged particle due to

its location in an electric field. Work is required to push a charged particle against the electric field of a charged body.

Electric Potential

Electric potential energy• Energy possessed by a charged particle due to

its location in an electric field. Work is required to push a charged particle against the electric field of a charged body.

Electric Potential

(a) The spring has (a) The spring has more elastic PE when more elastic PE when compressed. (b) The compressed. (b) The

small charge small charge similarly has more similarly has more

PE when pushed PE when pushed closer to the charged closer to the charged

sphere. In both sphere. In both cases, the increased cases, the increased

PE is the result of PE is the result of work input.work input.

Electric potential (voltage)• Energy per charge possessed by a charged

particle due to its location• May be called voltage—potential energy per

charge• In equation form:

Electric Potential

electric potential electric potential energyenergyamount of chargeamount of charge

Electric Electric potential potential

Electric PotentialElectric potential (voltage) (continued)

• Unit of measurement: volt,

Example:• Twice the charge in same location has twice the

electric potential energy but the same electric potential.

• 3 times the charge in same location has 3 times the electric potential energy but the same electric potential (2 E/2 q = 3 E/3 q = V)

1 1 voltvolt 1 1 coulombcoulomb

1 joule1 joule

Electric potential energy is measured in joules. Electric potential, on the other hand (electric potential energy per

charge), is measured

A. in volts.

B. in watts.

C. in amperes.

D. also in joules.

Electric Potential

Electric potential (voltage) (continued)• High voltage can occur at low electric potential

energy for a small amount of charge.• High voltage at high electric potential energy

occurs for lots of charge.

What “They” think

• “charged” objects send out “electric lines of force” or “electric fields”

• If another charged object is in an area with an electric field it will experience a force. This force is described by Coulomb’s law.

• The greater the charge, or smaller the distance from the charge the greater the Electric Potential or “voltage”

WRONG! Still

Greenies.

•The positive electric

field is really the

music the girls play.

•The negative electric field is the guys

“looking” for parties.

•Electric Force is how hard the Greenies

are trying to move (away or to

ward) each

other.

• Voltage is really just a measure of the

greenies “need to party”.

The Electric FieldThis is a region where a charge experiences a

force

Positive Plate

Negative Plate

Neutral Point

The field is from + to -

Electric Potential (Voltage)

Girl Girl greeniegreenies (+)s (+) Guy Guy

greeniegreenies (-)s (-)

Strong Strong attractive attractive force, High force, High voltagevoltage

medium medium strength strength attractive attractive force, force, medium medium voltagevoltage

Weak Weak attractive attractive force, low force, low voltagevoltage

Story So far…..• Greenies like to party.

– Too many guys in one place, not good for the guys.– Too many girls in one place, not good for the girls.– Scientists call these “+ and – charges”

• Each greenie sends out a “greenie field”– Girls send out music– Guys look and listen– Scientists call these “electric fields”

• Greater the attraction the greater the “need to party”– Scientists call this “voltage” or “Electric Potential”