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An overview of electrostatics, including coulomb's law, charged particles, and the concept of electric fields. Learn about the forces between charges, the role of charge conservation, and the differences between conductors and insulators.
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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
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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-^
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.
rd
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…
(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
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…
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.
-^ 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,
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: