Transcript Electricity: It`s All Around Us

Electricity: It’s All Around Us
Courtesy Dr. Joseph Alward, University of the Pacific Physics Dept.
Electricity
• In innumerable gadgets & high technology
•It holds atoms together
•It’s in lightning & other sparks
Electricity Comes From Charged
Particles in Atoms
Nucleus contains positive protons
Negative electrons buzz
around nucleus
Nucleus also contains
neutral neutrons
Net charge of atoms is
zero - neutral
Courtesy Science Museum, London
Courtesy www.energy.ca.gov/education/story/ story-html/chapter07.html
Electricity Basics
Like charges repel
Unlike charges attract
Courtesy psych.hanover.edu/Krantz/
neural/charge2anim.html
Attraction and Repulsion
Evidence for Two Kinds of Charge
Acrylic rod charged by rubbing with wool attracts pith
ball
After touching, rod repels ball
Rubber rod rubbed with silk attracts same ball
Conservation of Charge
In atoms positive and negative charges balance
Removing electron produces positive ion
Ions are charged atoms
Imbalance can also come from adding an electron to
make negative ion
Electrons are neither created nor destroyed, merely
transferred
No one has ever witnessed electric charges, + or -,
destroyed or created
Positive or Negative?
You rub cloth and add electrons. Are you positive or
negative?
Negative
- - - - - - - - -
You rub something else and lose electrons. Are you
positive or negative?
Positive + + + + + + +
Coulomb’s Law
F = k q1 q 2 /d2
Like gravity law) F = Gm1m2/d2
q1 is amount of charge of one particle
q2 is amount of charge of other particle
d is distance between particles
Unit of charge is the coulomb, abbreviated C
k = 9 000 000 000 N-m2 /C2 = 9 x 109 N-m2 /C2
___________
Repelling Students
Two students one meter apart each carry a charge of
one Coulomb. What is the force between them?
F = k q1 q 2 /d2 = 9 x 109 N-m2 /C2 x 1C2 /1 m2 =
= 9 x 109 Newtons
What would happen to our school if this were actually the
case?
How would they accelerate?
a = F/m = 9 x 109 N/(6 x101 kg) = 1.5 x 108 m/s2
v = at = 1.5 x 108 m/s2 (1 sec) = 1.5 x 108 m/s
Speed of light = 3 x 108 m/s
Conductors and Insulators
Conductors are materials in which electrons are free
to move around, especially metals.
•Good conductors of
electricity
In insulators such as rubber, paper, glass and styrofoam electrons are tightly bound to atoms.
• Poor conductors of electricity
Electrons Are Free To Move in
Metals
Semiconductors
Good insulators when pure
Become much better conductors when tiny amounts of
impurities are added
Ex. Germanium and silicon, used in transistors and
microchips
Superconductors
Metals that become infinitely conducting at low
temperatures
Electric current can flow forever without energy input
Methods of Charging
•Friction – rubbing transfers charge
•Contact – touching leads to charge transfer
•Induction – bringing charged object near causes
redistribution of charge
•Grounding – charges repel or attract to or from
an effectively infinite reservoir of charge such as
the ground
Charging by Friction
Charging by Contact
Some electrons transfer from rod to ball
Charging by Induction(conductors)
Courtesy of the Physics Classroom
Charging by Induction(insulators)
Such separation of charge is called charge
polarization
Polarizing Atoms
Explain This
Explain This
A charged comb attracts little bits of paper
Courtesy Dr. Joseph Alward, University of the Pacific Physics Dept.
Explain This
Applications of Electrostatic Charge
Negatively charged paint adheres
to positively charged metal
Application
Fine mist of negatively charged gold
particles adhere to positively charged
protein on fingerprint.
(From Eugene Hecht's Physics, 2nd
Edition Brooks/Cole Publishing)
Electrostatic Air Cleaner
Xerox machine
You find out how it
works
Charge Coupled Device (CCD)
A semiconductor device used to record light and
make images.
Capacitor
A device used to store charge
Basically consists of two metal plates oppositely
charged, and not touching
+
-
In between there may
be an insulating
material called a
dielectric to boost the
amount of charge it
can store
A Capacitor Stores Electric Energy
A battery produces electric energy bit by bit
A capacitor is NOT a type of battery
A battery can be used to charge a capacitor
The energy stored is the work done to charge it
Capacitor Design
One style is like a rolled up sandwich of foil and
wax paper
Capacitance Units
Defined by Q = CV
Symbol C
Unit: coulombs per volt = farad
1 pf = 1 picofarad = 10-12 farad
1nf = 1 nanofarad = 10-9 f
1mf = 1 microfarad = 10-6 f
C is Constant for a Given Capacitor
Does not depend on Q or V
Proportional to area
Inversely proportional to distance between plates
C = e0 A/d
If dielectric like oil or paper between plates use e = Ke0;
K is called dielectric constant
e is called permittivity. e0 is permittivity of free space
Capacitors
Photos courtesy Illinois Capacitor, Inc
Charging and Discharging
Capacitors can charge slowly and discharge
quickly, as in an electronic flash
This is why your digital camera flash needs time
in between shots
Applications
In automotive ignitions
In strobe lights
In electronic flash
In power supplies
In nearly all electronics
Capacitor Safety
A high voltage capacitor can kill
Do not open a computer monitor or old style TV
set
Electronic technicians know to discharge large
capacitors with a screwdriver or other insulated
tool
How Big Are Capacitors?
Previously capacitors were usually no bigger
than millifarads. A one Farad capacitor would be
as big as a refrigerator.
But today’s supercapacitors can pack several
Farads into a few cubic centimeters.
The biggest can be thousands of Farads
Supercapacitors* use double layer
electrolyte technology, usually with activated carbon
The carbon has huge surface area
*Also called ultracapacitors
Supercapacitor Advantages
Charge much more quickly than batteries
Deliver energy much more quickly than batteries
Store much more energy than regular capacitors
Supercapacitor applications
Power hybrid electric motor vehicles for short
bursts
Backup power for computers
Operate emergency doors and slides in
commercial aircraft
Lower energy density but greater power density
than batteries
More info