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Resistance R - ______________________________________________________________________________________________________________________________________________
2. R is a __________________ . It has _________________ .
units of R: ___________________
It is a ______________________ unit.
1. Resistance occurs as a result of ________________
colliding with ___________________ and with the
__________________________ , resulting in ____________ .
This converts __________________energy to ___________.
the opposition that a device or conductor offers to the flow of electric current.More resistance less current, and vice versa.
friction
heat
electrons
scalar
ohms, derived
other electronsmaterial of the conductor
no direction
3. Any factor that makes it more _________________for
_______________ to move will through a material will
__________________________________ of the material:
difficult
electrons
increase the resistance
electrical
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length L:
more R
cross-sectional area A:
less R
R
L
R
A
A. __________________
B. __________________
AA
For _____________, there are four factors that affecthow much resistance it has:
metals
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temperature:
more R
R
T
C. __________________
Higher T atoms of the metal _________________
________________________ for e-'s
to move through the metal
more ____________________
vibrate fastermore difficult
resistance
D. ______________________ : Different metals have different numbers of ____________________ .
______ electrons ______ current _______ resistance
The material
free electrons
R
# of free electrons
more
more
less
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These 4 factors are summed up in:
(rho) is called the _________________ of a material. depends on the ___________________ of a metal
and is different for different _____________ .
units of :
_________________
_________________
L/AR =
resistivity
temperature
metals
ohm·meter, ·m
(derived)
Lowest = _______________Highest = _______________Metals that have more free_____________ will have a_________ and _________ R.
silvernichrome
electrons
lower lower
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Ex. Calculate the resistance of 100 meters of copper wire that has a cross-sectional area of 3.44 x 10-6 m2.
L A
R =
= (1.72 x 10-8 ·m) (100. m)
(3.44 x 10-6 m2)
= 0.500
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A _________________is a device that is designed to
have a definite amount of _________________.
Resistors are used to
1. control _____________ flow; and
2. provide a _____________________
of a certain amount.
Symbols:
1. resistor:
2. variable resistor:
resistor
resistance
current
potential difference
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Resistors
Bigger resistors can handle more power w/o overheating.
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Variable resistors:
Justturn the knob!(It's that easy.)
As you turn the knob, this "arm"
swivels around and connects more
and more wire into the circuit.More wire more R
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Semiconductors (like ___________ and ______________ )
have ____________ resistance at higher temperatures.
Here’s why:
silicon germaniumless
___________ silicon (Si) is
an _______________________ .
It _____________ its outer e-’s
with 4 other silicon atoms in
a ___________________ bond,
so that its own electrons
_______________________
electricity.
shares
Pure
insulator
covalent
cannot conduct
Two materials that do not follow these rules for metals
are _____________________ and ______________________ .semiconductors superconductors
= a ________ of shared e-s
= Si atom
bond
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Phosphorus P and arsenic Ashave __________ outer e- than Si.
Boron B and gallium Ga have __________ outer e- than Si.
If you add _________________ of P, As, B or Ga to pure Si, it creates extra charge carriers. This is called _____________ . Higher temps “free up” more of these extra charges and allows for more __________ and so less _____ . And because of the extra charge carriers, semiconductors have _________________ resistancesthat can be ______________ . They are now used in making almost all _______________________________ .
1 more
1 less
tiny amounts
doping
outer
e-’s3 4 5
B C N
Al Si P
Ga Ge As
current
Raverage
controlledtiny electrical devices
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Superconductors:
The resistance R of superconductors is _________
as long as the material is _____________________________.
Because they have no _____ , electrons can travel through
them __________ , and so they can carry ________ currents
for _________________ without producing large amounts
of ___________ . This is useful in the ___________________
___________ and _________________________________________
Originally (around 1911), only certain ____________
were found to be superconducting. But they had to be
cooled to near ___________________ using liquid helium
(boiling point about _______ ) for this to happen.
This is very expensive.
R
freely large
long times
heatof power creating strong magnets (medical use).
transmission
0below a "critical" temperature
metals
absolute zero
4 K
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The current just keeps going….
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Materialmetal=mceramic=
c
critical temp.
(K)
absolute zero 0
Zinc m 0.88
Aluminum m 1.19
Tin m 3.72
Mercury m 4.15
liquid nitrogen
YBa2Cu3O7 c 90
TlBaCaCuO c 125
room temp. 293
In _______, a new type of superconductor was discovered whose makeup is similar to ________________ . These become superconductors at higher temperatures. This makes them much more ____________________.
ceramics1986
much ___________to use liquid N
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Who uses ceramics?Harry Potter!
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Materialmetal=mceramic=
c
critical temp.
(K)
absolute zero 0
Zinc m 0.88
Aluminum m 1.19
Tin m 3.72
Mercury m 4.15
liquid nitrogen
YBa2Cu3O7 c 90
TlBaCaCuO c 125
room temp. 293
In _______, a new type of superconductor was discovered whose makeup is similar to ________________ . These become superconductors at higher temperatures. This makes them much more ____________________.
ceramics
affordable
1986
much ___________to use liquid N
cheaper77
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Applications of Superconductivity:1. Medicine
Strong currents easy to maintain strong magnetic fields are used in:
MRI (magnetic resonance imaging)NMR (nuclear magnetic resonance)
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2. Transportation: strong currents run forever strong currents produce strong magnetic fields
use repulsion or attraction to levitate train no friction
maglev trains
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3. Power Transmission:
no resistance no heat loss more efficient
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The Meissner Effect - A superconductor expels a magnetic field.
A magnet is levitating above a superconductor (cooled by liquid nitrogen)