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Phys 100, How Things Work Lecture 14, Electric Charge
Electric Charges
Triboelectricity and Static Electricity
Electric charges
Entertaining things to do with
orphans
Why do batteries and electric
air cleaners work?
How do Lightning Rods work?
Sherlock Holmes: A shocking habit …
Phys 100, How Things Work Lecture 14, Electric Charge
Triboelectric Effect
Aka static electricity
On contact between any
two substances, the one
above becomes
positively charged (loses
some electrons), the one
below it becomes
negatively charged
(gains those electrons)
Dry human skin
Leather
Rabbit pelt
Glass, mica
Human pelt
Nylon
Wool
Lead
Cat pelt
Silk
Aluminum
Paper
Cotton
Steel, wood
Lucite, Acrylic
Rubber (balloon)
Nickel, copper, …
Acetate, rayon
Styrofoam (polystyrene)
Saran wrap
Scotch tape (polyethylene)
Silicon
Teflon
Silicone rubber
Phys 100, How Things Work Lecture 14, Electric Charge
Charges
Charge is measured in Coulombs (and a bunch of other units,
but no one in hisher right mind uses anything but Coulombs)
Negative charge means too many electrons on (part of) the
material
Positive charge means too few electrons
Triboelectric hierarchy is about who gives up electrons to whom
(but -- like friction -- it’s details are not understood)
Phys 100, How Things Work Lecture 14, Electric Charge
Touching
Charge transfers on contact by triboelectric
effect
Electrometer
metal
Charging
Glass
Rubber
Phys 100, How Things Work Lecture 14, Electric Charge
No Longer Touching
Charge will remain until contact with metal
discharges the material
Electrometer
Rubber
Glass
metal
Charged
Phys 100, How Things Work Lecture 14, Electric Charge
No Contact Required
Neutral means an equal, uniform mix of + and -
If a charge gets near it then like charges flee
and unlike charges accrue
Material is polarized
Even a neutral object can attract a charge object!
Phys 100, How Things Work Lecture 14, Electric Charge
Fraternity Initiation ca 1750
Charity boys as
entertainment for the
upper classes
Phys 100, How Things Work Lecture 14, Electric Charge
Coulomb’s Law
Charge can be positive or negative
Else it is just like gravity
Like charges repel
Unlike charges attract
€
F = k
q
1
q
2
r
2
Phys 100, How Things Work Lecture 14, Electric Charge
Who Sez?!?
Coulomb worked on this
230 years ago using a
spring balance
Phys 100, How Things Work Lecture 14, Electric Charge
Lightning Rod
Sharp point “concentrates” charge and electric field
Like houses, trees, etc
Lightning rod concentrates
charge dump from clouds
Ben Franklin, left this as his
“gift to mankind”
Phys 100, How Things Work Lecture 14, Electric Charge
What’s Coulomb’s Law Good For?
All electronics use Coulomb’s Law but
Here we will talk about batteries and air cleaners
If the dust is around something that has charges it will stick
just like socks in an electric dryer --> air cleaners
Some materials “want” to give up electrons to other materials
--> batteries
Phys 100, How Things Work Lecture 14, Electric Charge
Electrostatic Potential Energy & Voltage
Just like F = mg
Electric force is
F = qE
Work = F × Δx
= qE × Δx
= q × ΔV
P.E. = q × ΔV
Phys 100, How Things Work Lecture 14, Electric Charge
Conductors
Lots of empty seats ⇐⇒ theater is always about half full
Patrons prefer to sit near the stage and electrons fill seats up
to the lowest row that includes enough seats for all the
electrons, but there are plenty of
seats just one row back.
So electrons are mobile ⇒ hop to
nearby empty seat and move
easily towards the exit
⇒ conductor
Phys 100, How Things Work Lecture 14, Electric Charge
Insulators
Orchestra level is completely full so no one can move unless
they climb all the way to the balcony level
Electrons completely fill states up to a point where there are
no more states available.
Electrons cannot move without a BIG boost in energy to
allow them to hop to an empty seat at much higher energy.
So: charge does not
flow
⇒ insulator
(Orchestra)
Phys 100, How Things Work Lecture 14, Electric Charge
Semiconductor
If energy difference is not too large then light or heat can give
a boost into the balcony level where electron moves freely
This makes the material a sensor for light or temperature
because the “insulator” converts into a “conductor”
Solar cells and, motion sensors, thermostats, etc can work this
way.
(Orchestra)
Phys 100, How Things Work Lecture 14, Electric Charge
Light Can Turn Insulators into Conductors
Light can excite electrons into balcony
states
Mobile electrons can move when pushed by
other charges (or a battery)
In the dark ⇒ insulates -- no charge flows
In the light ⇒ conducts -- charge flows
Phys 100, How Things Work Lecture 14, Electric Charge
Photoconductor to Photocopier
Where light hits, static charge flows away leaving charged
pattern and patterned ink sticks ⇒ photocopies
Phys 100, How Things Work Lecture 14, Electric Charge
Take home messages
Charges repel or attract for like or opposite signs according to
Coulomb’s Law
Voltage is potential energy per charge
Charges rearrange in Polarization
Triboelectric effect creates “static electricity”
Light can excite charge to make it mobile
€
F = k
q
1
q
2
r
2
