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

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

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

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

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

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

Resistors

Bigger resistors can handle more power w/o overheating.

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

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

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

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

The current just keeps going….

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

Who uses ceramics?Harry Potter!

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

Applications of Superconductivity:1. Medicine

Strong currents easy to maintain strong magnetic fields are used in:

MRI (magnetic resonance imaging)NMR (nuclear magnetic resonance)

2. Transportation: strong currents run forever strong currents produce strong magnetic fields

use repulsion or attraction to levitate train no friction

maglev trains

3. Power Transmission:

no resistance no heat loss more efficient

The Meissner Effect - A superconductor expels a magnetic field.

A magnet is levitating above a superconductor (cooled by liquid nitrogen)