Electrostatics: Understanding Charges, Coulomb's Law, and Electric Fields, Slides of Physics

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Finish Ch. 20 on Sound

Start Ch. 22 on Electrostatics

Chapter 22:

Electrostatics

Charge

•^

Fundamental quantity in all electrical phenomena: positive andnegative particles carry “charge”

Recall, protons

electrons

-^ Attractive force btn protons and electrons cause them to form atoms, aswe saw in Ch.11.• Electrical force is behind all of how atoms bond i.e. behind chemistry…• Every electron has charge -1.6 x 10 -

C, and every proton 1.6 x 10

C

i.e. -1 C represents the charge of 6.25 billion billion electrons !Yet 1C is the amount of charge passing through a 100-W light bulbin just over a second. A lot of electrons!

-^ Charge is always conserved:

charge cannot be created or destroyed, but

can be transferred from one object to another.Eg. Rubbing a rod with fur – electrons transfer from fur to rod, leaving rodnegatively charged, and fur with exactly same magnitude of positive charge.

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More on charge

-^

Note that in everyday charging processes (like rubbing objects), it is the electrons

that transfer (not the protons). A negatively charged object has

an excess of e’s, whereas positively charged one has deficiency (bysame amount)

-^

Which object gains the electrons depends on their

electron affinity:

Eg. Rod has greater affinity than fur, so rod becomes –, fur +Eg. Silk has greater affinity than rod

when rubbed together, rod

becomes +, silk - Eg. Combing hair

Comb becomes –, hair + (e’s go from hair to comb)

•^

Charge is

quantized:

cannot divide up charge into smaller units than

that of electron (or proton) i.e. all objects have a charge that is a whole-number multiple of charge of a single e.

Clicker Question

So the electrical attraction is by far dominant in providing

the centripetal force that keeps the electron in orbitaround the proton. How about the force the electronexerts on the proton?

A)

it’s larger than the force on the electron

B)

It’s the same

C)

It’s smaller

D)

It depends

Answer: B, Newton’s 3

rd

law

The electrical force is an interaction (as any force is) andeach pulls on the other equally.

Conductors and Insulators

•^

How easy is it to get an electric current to flow across a material?Property called

electrical conductivity

•^

Depends on how strongly the electrons are anchored to the nuclei: Good

conductor

: e.g. metal. Electrons freely wander in the material, they

are “loose”. Good conductors of electrical current are also good heatconductors. Good

insulator

: e.g. rubber, glass, wood. Electrons tightly bound to nuclei,

so hard to make them flow. Hence, poor conductors of current and ofheat.

-^

Electrical resistivity –

quantifies how much a material resists current

flow. Insulator has very high resistance (or resistivity), conductor very low. There

is a range, depending on the material. (More on this in Ch 23)

Superconductors

•^

Have

zero resistance, infinite conductivity

below a critical

temperature

-^

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 0

o K,

o C)

•^

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

o C)

•^

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

Charging

(1) Charging by friction and contact Already discussed a lot (rubbing materials together, see earlier slide on

charge). Often can see or hear the sparks when the charges move.eg. Walk across a rug – feel tingle when touch door knob: electrons

transferred from rug to your feet, then to the door knob.

charging byfriction

charging bycontact – simplytouch

(2) Charging by induction Bring a charged object

near

a conducting surface, electrons will move

in conductor even though

no physical contact

: Due to attraction or

repulsion of electrons in conductor to the charged object – since free tomove, they will!Charge redistribution until forces between all charges balance to 0.Then if you separate parts of conductor – they will be charged.Eg. Here, in (b), e’s in A-Brepelled away from rod, soget excess on B, leaving Apositively charged:

Note, the charged rod never touched them, and retains its original charge.Question: Must the resulting charges on spheres A and B be equal andopposite?

Yes, because each + charge on A is from an electron

leaving it and moving to B. Charge is conserved – no charge is added fromrod as no contact.

Charging by induction continued… • Charge induction by

grounding

: Here, can induce charge on a

single

neutral sphere hanging from a non-conducting string:

When touch with finger, electrons flow fromyour finger, throughyou, to the ground

.

The earth is a hugereservoir of charge... (More in Ch 23)

…so thathere, sphereis left +.Remove rod: Steps a-dyield a+chargedsphere.

Here, chargeredistributes,but netcharge onsphere still 0

If touch rod tosphere, getcharging bycontact – electronsflow onto sphere.Remove rod: Steps a-f yield a-charged sphere.

Charge polarization

Instead, if bring a charged object near an insulator, electrons are not free tomigrate throughout material. Instead, they redistribute

within

the

atoms/molecules themselves: their “centers of charge” move Here, usual atom,with center ofelectron cloud atpositive nucleus

When a –ve charge isbrought near the right,electron cloud shifts tothe left. Centers of +and – charges no longercoincide.

Atom is

electrically polarized

Surfaces ofmaterial look likethis. A – chargeinduced on left,and + on the right.(Zero net chargeon whole object)

Charge polarization continued

•^

DEMO:

Rub balloon on your hair – it will

then stick to the wall! Why?Balloon becomes charged (by friction) when rub

on hair, picking up electrons. It then inducesopposite charge on the wall’s surface closestto it (+ve), and the same charge as itself (-)on side of wall furthest away. So balloon is attracted to + charges and repelled

by – charges in wall – but the – charges arefurther away so repulsive force is weakerand attraction wins. (Argument applies generally – key thing is

-^ Charge polarization is why difference in distance btn + and -)

a charged object can attract a

neutral

one

-^ Eg. Charge a comb by rubbing it throughyour hair, and then see it attracts bits ofpaper and fluff…

1. opposite direction. 2. same direction. 3. … but it won’t bend at all. A thin stream of water bends toward a negativelycharged rod. When a positively charged rod isplaced near the stream, it will bend in the

Like compasses that align along a magnetic field, H

Os align 2

Answer: 2 If you answered 1, you likely thought thebending was due topositively charged water. But the waternormally has no appreciable net charge.The interaction between the charged rodand the water stream is mainly due to thedipole nature of water molecules. Halong the electric field of the nearby rod—whether the rod ispositive or negative. For both magnets and charges,the closest aligned pole or charge is always opposite in sign.Opposites attract, so net attraction is the result.

O 2

molecules are electric dipoles, positive onthe hydrogen side and negative on theoxygen side.

Electric Field

-^ Just like we defined grav field, we’ll define electric field: both forcesact on objects they are not in contact with

.

The orbiting bodiesinteract with the forcefields (grav for planet,electric for proton). i.e. think of the force as interaction between one body and field set up by theother. Electric field,

E=

F q

And field lines have arrowindicating direction a

positive test

charge

would be pushed.

So always point away from+charges, towards – charges…

Eg. For a– charge:and for a(larger)+ charge: