The most important part in studying: The most important part in studying:

Participate in class with ClickersParticipate in class with Clickers

Interactive lectures – should Interactive lectures – should keep you alert and thinking.keep you alert and thinking.

““Electricity” – from the Greek word Electricity” – from the Greek word electronelectron ((- - meaning “amber”. The ancients knew that if you rub an meaning “amber”. The ancients knew that if you rub an amber rod with a piece of cloth, it attracts small pieces of amber rod with a piece of cloth, it attracts small pieces of leaves or dust.leaves or dust.

““amber effect”– the object becomes electrically chargedamber effect”– the object becomes electrically charged

All physics to date has led to one primary conclusion: All physics to date has led to one primary conclusion:

• There are four There are four fundamental forces:fundamental forces:

1)1) Gravitational Gravitational 2)2) Electromagnetic Electromagnetic 3)3) Strong nuclear Strong nuclear 4)4) Weak nuclearWeak nuclear

all based on the all based on the Electromagnetic TheoryElectromagnetic Theory

~250 yrs or so since we first learned what electricity is~250 yrs or so since we first learned what electricity is

grand unified theorygrand unified theoryGUTGUT

Electricity & MagnetismElectricity & Magnetism

• static electricity (Electrostatics)static electricity (Electrostatics)– Why do I get a shock when I walk across the Why do I get a shock when I walk across the

rug and touch the door knob?rug and touch the door knob?– Why do socks stick to my pants in the dryer?Why do socks stick to my pants in the dryer?– Why does my hair stick to my comb, and I Why does my hair stick to my comb, and I

hear a crackling sound ?hear a crackling sound ?– Why does a piece of plastic refuse to leave Why does a piece of plastic refuse to leave

my hand when I peel it off a package?my hand when I peel it off a package?– What is lightning?What is lightning?

What is that all about?What is that all about? It’s the CHARGEIt’s the CHARGE

Electric chargeElectric charge is defined by theis defined by the effect (effect (forceforce) it produces. ) it produces.

– positive chargepositive charge– negative chargenegative charge

Benjamin Franklin (1706 (1706 - 1790, American - 1790, American statesman, philosopher statesman, philosopher and scientist)and scientist)

No one has ever seen electric charge; No one has ever seen electric charge; it has no weight, color, smell, flavor, length, or width.it has no weight, color, smell, flavor, length, or width.

Charge is an intrinsic property of matter Charge is an intrinsic property of matter electron has it,electron has it, proton has it,proton has it, neutron doesn’t have it neutron doesn’t have it – and that’s all– and that’s all

Electricity has origin within the atom itself.Electricity has origin within the atom itself.

  

NameName SymbolSymbol ChargeCharge MassMass

ElectronElectron ee - e- e 9.119.11xx1010-31-31 kg kg

ProtonProton pp ee 1.671.67xx1010-27-27 kg kg

NeutronNeutron nn nonenone 1.671.67xx1010-27-27 kg kg

Atom is electrically neutral = has no net charge, Atom is electrically neutral = has no net charge, since it contains equal numbers of protons and electrons.since it contains equal numbers of protons and electrons.

1010-10 -10 mm

1010-15 -15 mm

rratomatom ≈ 100000 x r≈ 100000 x rnucleusnucleus

mmnucleonnucleon ≈ 2000 x m≈ 2000 x melectronelectron

Electric forces• charges exert electric forces on other charges

The repulsive electric force between 2 protons is1,000,000,000,000,000,000,000,000,000,000,000,000times stronger than the attractive gravitational force!

++++ ++

–two positive charges repel each other–two negative charges repel each other–a positive and negative charge attract each other

• charge is measured in charge is measured in Coulombs [C]Coulombs [C]

French physicist French physicist Charles A. de CoulombCharles A. de Coulomb

1736 - 18061736 - 1806

• Every electron has charge -1.6 x 10Every electron has charge -1.6 x 10 -19-19 C, C, and every proton 1.6 x 10and every proton 1.6 x 10-19-19 C C 1C represents the charge of 6.25 billion billion (1C represents the charge of 6.25 billion billion (6.256.25xx10101818) electrons !) electrons ! Yet 1C is the amount of charge passing through a 100-W lightYet 1C is the amount of charge passing through a 100-W light bulb in just over a second. A lot of electrons!bulb in just over a second. A lot of electrons!

Attractive force between protons and electrons cause them to Attractive force between protons and electrons cause them to form atoms. form atoms. Electrical force is behind all of how atoms ond…chemistry…Electrical force is behind all of how atoms ond…chemistry…

The smallest amount of the The smallest amount of the free positive free positive chargecharge is the charge on the proton. is the charge on the proton.

The smallest amount of the The smallest amount of the free negative free negative chargecharge is is thethe charge on the electron. charge on the electron.

ChargeCharge of the single proton is of the single proton is qqprotonproton = e = e . .

Charge of the single electron is Charge of the single electron is qqelectronelectron = - e = - e   

•Charge is quantized:Charge is quantized: cannot divide up charge into cannot divide up charge into smaller units than that of electron (or proton) i.e. all objects smaller units than that of electron (or proton) i.e. all objects have a charge that is a whole-number multiple of charge of have a charge that is a whole-number multiple of charge of the smallest amount (a single e). the smallest amount (a single e).

•The net charge is the algebraic sum of the The net charge is the algebraic sum of the individual charges (+ 5 - 3 = 2).individual charges (+ 5 - 3 = 2).

quarks have 1/3, but quarks have 1/3, but they come in tripletsthey come in triplets

let e = 1.6 x 10let e = 1.6 x 10-19-19 C C

Objects can be charged – there can be net Objects can be charged – there can be net charge on an object. How?charge on an object. How?

Everyday objects - electronically neutral – Everyday objects - electronically neutral – balance of charge – no net charge.balance of charge – no net charge.

The only type of charge that can move around is the negative The only type of charge that can move around is the negative charge, or electrons. The positive charge stays in the nuclei. So, charge, or electrons. The positive charge stays in the nuclei. So, we can put a NET CHARGE on different objects in two wayswe can put a NET CHARGE on different objects in two ways

Add electrons and make Add electrons and make the object the object negatively negatively charged.charged.

Remove electrons and make Remove electrons and make the object the object positively positively charged.charged.

Some materials have atoms that have outer electrons (farthest Some materials have atoms that have outer electrons (farthest from nucleus) loosely bound. They can be attracted and can from nucleus) loosely bound. They can be attracted and can actually move into an outer orbit of actually move into an outer orbit of anotheranother type of atom. The type of atom. The atom that has lost an electron has a net chargeatom that has lost an electron has a net charge ++ee ( (positive positive ionion).). An atom that gains an extra electron has a net charge ofAn atom that gains an extra electron has a net charge of – – ee ( (negative ionnegative ion).).

This type of charge transfer often occurs when two different This type of charge transfer often occurs when two different materials (different types of atoms) come into contact.materials (different types of atoms) come into contact.

• Which object gains the electrons depends on theirWhich object gains the electrons depends on their electron affinity:electron affinity:

During that process, the net charge produced is zero. During that process, the net charge produced is zero. The charges are separated, but the sum is zero. The charges are separated, but the sum is zero. The amount of charge in the universe remains constant The amount of charge in the universe remains constant (we think!!) It is(we think!!) It is CONSERVEDCONSERVED!!

electrons can be transferred from one object to anotherelectrons can be transferred from one object to another

Another Law of Conservation: Another Law of Conservation:

Charge is always conserved:Charge is always conserved: charge cannot be charge cannot be created or destroyed, but can be transferred from created or destroyed, but can be transferred from one object to another.one object to another.

Conclusion:Conclusion:

When objects are charged by rubbing, they don’t stay charged for When objects are charged by rubbing, they don’t stay charged for ever. They eventually return to neutral state – very often the ever. They eventually return to neutral state – very often the charge will “leak off” onto water (polar) molecules in the air. charge will “leak off” onto water (polar) molecules in the air. Sometimes they will be neutralized by charged ions in the air Sometimes they will be neutralized by charged ions in the air (formed, for example, by collisions with charged particles known (formed, for example, by collisions with charged particles known as cosmic rays).as cosmic rays).

Given enough time, the particles in the air will remove the excess Given enough time, the particles in the air will remove the excess charge from the object leaving it neutrally charged. This explains charge from the object leaving it neutrally charged. This explains why on dry days we tend to have more trouble with static why on dry days we tend to have more trouble with static electricity build-up than on humid (moist) days. On moist days electricity build-up than on humid (moist) days. On moist days there are more water molecules in the air to steal charge more there are more water molecules in the air to steal charge more rapidly. On dry days there are fewer particles in the air to steal rapidly. On dry days there are fewer particles in the air to steal charges so we accumulate charge until we touch something and charges so we accumulate charge until we touch something and get discharged (shocked).get discharged (shocked).

Any material that allow charges to move about more or less freely Any material that allow charges to move about more or less freely is called is called conductorconductor. So, if you transfer some electrons to the . So, if you transfer some electrons to the

metal rod, that excess of charge will distribute itself all around rod. metal rod, that excess of charge will distribute itself all around rod. Tap water, human body and Tap water, human body and metalsmetals are generally good are generally good

conductors.conductors.

That’s all very nice, but why is that so?That’s all very nice, but why is that so?

Electrical conductors, insulators, Electrical conductors, insulators, semiconductors and superconductorssemiconductors and superconductors

- distinction - distinction basedbased on their ability to conduct electric charge. on their ability to conduct electric charge.

What makes conductors conduct?• Atoms have equal numbers of positive and negative charges,

so that a chunk of stuff usually has no net charge the plusses and minuses cancel each other.

• However, in metal atoms the valence electrons – the electrons in the outermost orbits - are loosely bound, so when you put a bunch of metal atoms together (to form a metal) an amazing thing happens valence electrons from each atom get confused and forget which atom they belong to.

• They now belong to the metal as the whole. As the result, positive ions which are tightly bound and can only oscillate around their equilibrium positions, form a positive background. All the homeless electrons - “Free electrons”

wander around freely keeping ions from falling apart – metallic bond!!

Electrons in insulators are tightly bound to atomic nuclei and so cannot be easily made to drift from one atom to the next. Only if a very strong electric field is applied, the breakthrough (molecules become ionized resulting in a flow of freed electrons) could result in destruction of the material.

The markings caused by electrical breakdown in this material – look

similar to the lightening bolts produced when air undergoes

electrical breakdown.

Materials like amber, pure water, plastic, glass, rubber, wood… are called insulators. They do not let electricity flow through them. Electrons are tightly bound to nuclei, so it is hard to make them flow. Hence, poor conductors of current and of heat.

Semiconductors

• Materials that can be made to behave sometimes as insulators, sometimes as conductors.

Eg. Silicon, germanium. In pure crystalline form, are insulators. But if replace even one atom in 10 million with an impurity atom (ie a different type of atom that has a different # of electrons in their outer shell), it becomes an excellent conductor.

• Transistors: thin layers of semiconducting materials joined together.

Used to control flow of currents, detect and amplify radio signals, act as digital switches…An integrated circuit contains many transistors.

The movement of electrons in semiconductors is impossible to describe without the aid of quantum mechanics. As the conductivity of semiconductors can be adjusted by adding certain types of atomic impurities in varying concentrations, you can control how much resistance the product will have. ADVANTAGE – A HUGE ONE

Superconductors

• Have zero resistance, infinite conductivity• Not common! Have to cool to very very low temperatures.

• Current passes without losing energy, no heat loss.

• Discovered in 1911 in metals near absolute zero (recall this is 0oK, -273oC)

• Discovered in 1987 in non-metallic compound (ceramic) at “high” temperature around 100 K, (-173oC)

• Under intense research! Many useful applications eg transmission of power without loss, magnetically-levitated trains…

Conductors and Insulators – and how to charge them

Most things are in between perfect conductor/ insulator

Electrons are free to move in a conductor

Electrons stay with their atom in an insulator

REMEMBER:

ELECRTOSTATIC CHARGINGELECRTOSTATIC CHARGING 1.1. Charging by Friction:Charging by Friction:

The transfer of charge is due the rubbing - friction The transfer of charge is due the rubbing - friction between two previously neutral materials. between two previously neutral materials.

When you move your comb through your hair, the friction When you move your comb through your hair, the friction between the comb and hair can pull some of the electrons between the comb and hair can pull some of the electrons out of your hair and onto the comb. As a result your comb out of your hair and onto the comb. As a result your comb ends up with a net negative charge and attracts your hair ends up with a net negative charge and attracts your hair which is now positive.which is now positive.

Rubbing: rubber rod with fur or cloth, glass rod with silk, hair Rubbing: rubber rod with fur or cloth, glass rod with silk, hair

with balloon, shuffling across a carpeted floor.with balloon, shuffling across a carpeted floor.

Insulated Insulated standstand

2. Charging by Conduction (Contact):2. Charging by Conduction (Contact):

2.1 Conductors2.1 Conductors::When a charge is placed on a conductor, the When a charge is placed on a conductor, the mutual repulsion of the individual charges mutual repulsion of the individual charges causes them to move as far away from each causes them to move as far away from each other as possible. Thus, a charge deposited on a other as possible. Thus, a charge deposited on a conductor quickly spreads out over its surface.conductor quickly spreads out over its surface.

2. 2 Insulators:2. 2 Insulators:

When a charge is placed on an insulator, it remains When a charge is placed on an insulator, it remains where it is deposited and surrounding molecules where it is deposited and surrounding molecules become polarized. become polarized. An external (negative) charge distorts the shape of An external (negative) charge distorts the shape of an atom by forcing its negatively-charged electron an atom by forcing its negatively-charged electron clouds to shift away from the charge and the clouds to shift away from the charge and the positively charged nuclei to shift toward the charge. positively charged nuclei to shift toward the charge. Such a distorted atom is said to be polarized.Such a distorted atom is said to be polarized.

Glass Glass spheresphere

Insulated Insulated standstand

Metal Metal spheresphere

Consider a negatively charged rod touching a conductor Consider a negatively charged rod touching a conductor versus touching an insulator.versus touching an insulator.

What is the difference between how the electrons are What is the difference between how the electrons are arranged on the conductor and insulator?arranged on the conductor and insulator?

Would spread out evenly on a good conductor, Would spread out evenly on a good conductor, because the transferred e’s repel each other. because the transferred e’s repel each other.

But on insulator, or poor conductor, would be But on insulator, or poor conductor, would be more localized at where the rod touched.more localized at where the rod touched.

Question:Question:

• charges can be transferred from/to conductors or non-charges can be transferred from/to conductors or non-conductors but they can only move through conductors.conductors but they can only move through conductors.

3. Charging by Induction3. Charging by Induction

a. Neutral conductor with free electronsa. Neutral conductor with free electrons

b. b. freefree electrons in the metal are electrons in the metal are repelled as far as possible from the repelled as far as possible from the charged object.charged object.

d. Object is left positively chargedd. Object is left positively charged

Charge has been Charge has been separated, but metal separated, but metal sphere is still neutral.sphere is still neutral.

Or you can touch Or you can touch it with finger, it with finger, electrons flow electrons flow from your finger, from your finger, through you, to through you, to the groundthe ground. .

3.1 Conductors3.1 Conductors::

c. The Earth is reservoir of any charge. It c. The Earth is reservoir of any charge. It can easily accept or give up electrons. can easily accept or give up electrons. Connect conductor with a conducting wire to Connect conductor with a conducting wire to the ground - many of free electrons in the ground - many of free electrons in metal are able to move even further from metal are able to move even further from charged object down the wire into the Earth. charged object down the wire into the Earth.

e. cut the wire, remove the rod and the e. cut the wire, remove the rod and the metal sphere has evenly distributed metal sphere has evenly distributed positive charge.positive charge.

Insulators:Insulators:When insulator is charged When insulator is charged by inductionby induction, there , there will be no change of charge on that object. will be no change of charge on that object. Instead of that charge is moved within the Instead of that charge is moved within the molecule/atom (the net charge is kept zero) molecule/atom (the net charge is kept zero)

Therefore we call it rather:Therefore we call it rather:

Positive surface Positive surface chargecharge

Even though sphere is neutral Even though sphere is neutral there is attraction force acting between the rod and sphere.there is attraction force acting between the rod and sphere.

Charging by PolarizationCharging by Polarization

A charge placed near an insulator polarizes its atoms. A charge placed near an insulator polarizes its atoms. While the insulator’s interior remains electrically neutral, a net charge While the insulator’s interior remains electrically neutral, a net charge appears on the surface, and can produce force on other charges near appears on the surface, and can produce force on other charges near the insulator. the insulator.

3.2 Insulators3.2 Insulators::

Charge polarizationCharge polarization

When bring a charged object near an insulator, electrons are not free to When bring a charged object near an insulator, electrons are not free to migrate throughout material. Instead, they redistribute migrate throughout material. Instead, they redistribute within within the the

atoms/molecules themselves: their “centers of charge” moveatoms/molecules themselves: their “centers of charge” move

Here, usual atom, Here, usual atom, with center of with center of electron cloud at electron cloud at positive nucleuspositive nucleus

When a When a – – charge is charge is brought near the right, brought near the right, electron cloud shifts to electron cloud shifts to the left. Centers of the left. Centers of ++ and and –– charges no longer charges no longer coincide.coincide.

Atom is Atom is electrically polarizedelectrically polarized

Surfaces of material look like this. A – charge Surfaces of material look like this. A – charge induced on right, and + on the right.induced on right, and + on the right.

(Zero net charge on whole object)(Zero net charge on whole object)

EXAMPLE - QUESTION Charging by inductionEXAMPLE - QUESTION Charging by induction

Bring a charged object Bring a charged object near near a conducting surface, electrons will a conducting surface, electrons will move in conductor even though no physical contact: Due to move in conductor even though no physical contact: Due to

attraction or repulsion of electrons in conductor to the charged attraction or repulsion of electrons in conductor to the charged object – since free to move, they will!object – since free to move, they will!

Note, the charged rod never touched them, and retains its original charge.Note, the charged rod never touched them, and retains its original charge.

Once separated from each other with rod still close they’ll Once separated from each other with rod still close they’ll remain charged. Charge is conserved, so charges on spheres remain charged. Charge is conserved, so charges on spheres

A and B are equal and opposite.A and B are equal and opposite.

QUESTION:

A metal ring receives a positive charge by contact. What happens to the mass of the ring?Does it increase, stay the same, or decrease?

Will the object have deficiency or excess of electrons?When the positively charged ball touches the ring, electrons inside it are attracted to the ball. Some will leave the ring trying to neutralize the ball. Only a tiny fraction leaves the ring. The mass of the electrons is so small compared to the atoms, so although the mass of the ring decreases, measuring it would not be possible. (By the way, both will be positively charged, but the ball will be less then before)

EXAMPLES AND CONSEQUECIES:

Van de Graaff

The sphere gives the girl a large negative charge. Each strand of

hair is trying to:1) Get away from the charged sphere.2) Get away from the ground.3) Get near the ceiling.4) Get away from the other strands of hair.5) Get near the wall outlet.

Like charges attached to the hair strands repel, causing them to get away from each other.

Example:

What is his secret?

Seeing the effects of charge: the electroscope

• the electroscope is a simple device for observing the presence of electric charge

• it consists of a small piece of metal foil (gold if possible) suspended from a rod with a metal ball at its top

• If a negatively charged rod is placed near the ball, the electrons move away because of the repulsion.

The two sides of the metal foil then separate.

++ ++

Charging by Induction

• Bring a charged rod in near

Positive charged rod

results in positive leaves.

Attracting uncharged objects

unchargedmetal sphere

+

+

+

+ • A negatively chargedrod will push the electrons to the far side leaving the nearside positive.

• The force is attractivebecause the positivecharges are closer tothe rod than the negativecharges

Why? Balloon becomes charged by friction when rub on

hair, picking up electrons. It then polarize molecules on the surface, induces + charge layer on the wall’s surface closest to it , and next negative furthest away.

So balloon is attracted to + charges and repelled by – charges in wall, but the – charges are further away so repulsive force is weaker and attraction wins.

Charge polarization is why a charged object can attract a neutral one :

• Charge a comb by rubbing it through your hair, and then see it attracts bits of paper and fluff…

•DEMO: Rub balloon on your hair – it will then stick to the wall !

You can bend water with charge!

charged rodThe water moleculehas a positive end and

a negative end.

When a negative rod isbrought near the streamof water, all the positiveends of the water mole-cules turn to the right

and are attracted to thenegative rod.

stream of waterWhat happens if the rod is

charged positively?

As we said Like charges repel, and opposite charges attract.

This is the fundamental cause of almost ALL electromagnetic behavior.

But how much?

How Strong is the Electric Force between two charges?

ELECTROSTATIC – ELECTRIC - COULOMB FORCE

The force between two point charges is proportional to the product of the amount of the charge on each one,

and inversely proportional to the square of the distance between them.

221

r

qqkF

229 /1099.8 CmNk

Force is a vector, therefore it must always have a direction.

SHE accumulates a charge q1 of 2.0 x 10-5 C

(sliding out of the seat of a car). HE has accumulated a charge q2 of -8.0 x 10-5 C

while waiting in the wind.

a) They exert equal forces on each other only in opposite directionNr

qqkF 40.0

221

b) r’ = 0.5 r

Nr

qqkF 40.0

221 (“-“ = attractive force)

FNr

qqkF 46.1

''

221 At very small separation - spark

How many electrons is 2.0 x 10-5 C ?

C5100.2

electrons 10C106.1C100.2 14

19

5

What is the force between them a) when she opens the door 6.0 m from him and b) when their separation is reduced by a factor of 0.5?

When you comb your hair with a plastic comb, some electrons from your hair can jump onto it

making it negatively charged.

Suppose that you could borrow all the electrons from a friend’s body and put them into your pocket. The mass of electrons would be about 10 grams (a small sweet). With no electrons your friend

would have a huge positive charge. You, on the other hand, would have a huge negative charge in your pocket.

If you stood 10 m from your friend the attractive force would be equal to the force that 1023 tons would exert sitting on your

shoulders – more 100,000 times greater than the gravitational force between the earth and the Sun. Luckily only smaller charge imbalances occur, so huge electrical forces like the one described

simply do not occur.

Your body contains more than 1028 electrons.

Three point charges : q1= +8.00 mC; q2= -5.00 mC and q3= +5.00 mC.

(a) Determine the net force (magnitude and direction) exerted on q1 by

the other two charges. (b) If q1 had a mass of 1.50 g and it were free to move, what would be

its acceleration?

1.30 m

1.30 m

230

230 q1

q2

q3

Force diagram

F2 F3 q1

N213.0r

qqkF

221

2 N213.0

r

qqkF

231

3

Force Force diagramdiagram FF22 FF33

qq11 x-components will cancel, x-components will cancel, because of the symmetrybecause of the symmetry

0 02 323 23sin sinyF F F F

0 00 213 23 0 213 23. sin . sinF F = 0.166 NF = 0.166 N

F

23

016615 10

. /.

a m sma = Fma = F a = 111 m/sa = 111 m/s22

in y - directionin y - direction

electric force is very-very strong force, and resulting electric force is very-very strong force, and resulting acceleration can be hugeacceleration can be huge

++ ----

Positive charge is attracted (force to left)Positive charge is attracted (force to left)

Negative charge is repelled (force to right)Negative charge is repelled (force to right)Positive charge is closer so force to left is larger.Positive charge is closer so force to left is larger.

A positive and negative charge with equal magnitude are A positive and negative charge with equal magnitude are connected by a connected by a rigid rodrigid rod, and placed near a large negative , and placed near a large negative charge. In which direction is the net force on the two charge. In which direction is the net force on the two connected charges?connected charges?

1) Left1) Left 2) Zero2) Zero 3) Right3) Right

Three Charges

Q=-3.5 CQ=+7.0C

Q=+2.0C

6 m

4 m

• Calculate force on +2C charge due to other two charges– Calculate force from

+7C charge– Calculate force from

–3.5C charge– Add (VECTORS!)

F7

5 m

1 22

kq qF

r

N25

)107)(102)(109( 669

7

F

N105 37

F

F3

N25

)105.3)(102)(109( 669

3

F

N105.2 33

F

Q=-3.5 CQ=+7.0C

Q=+2.0C

6 m

4 m

F7

5 m

F3

F

Adding Vectors F7+F3

•Decompose into x and y components.

F7x = F7 cos = F7(3/5) = 3x10-3 N

F7y = F7 sin = F7(4/5) = 4x10-3 N

F3x = F3 cos = F3(3/5) = 1.5x10-3 N

F3y = F3 sin = F3(4/5) = -2x10-3 N

Fx = 3 10-3 N + 1.5 10-3 N

Fy = 4 10-3 N – 2.0 10-3 N

Fx = 4.510-3 N Fy = 2.010-3 N

22yx FFF N 109.4 3