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 electrons material 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. __________________ A A For _____________, there are four factors that affect how 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 faster more difficult resistance D. ______________________ : Different metals have different numbers of ____________________.  ______ electrons  ______ current  _______ resistance The material free electrons R # of free electrons 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/A R = resistivity temperature metals ohm·meter,  ·m (derived) Lowest  = _______________ Highest  = _______________ Metals that have more free _____________ will have a _________  and _________ R. silver nichrome electrons lower

Ex. Calculate the resistance of 100 meters of copper wire that has a cross-sectional area of 3.44 x m 2.  L A R = = (1.72 x  ·m) (100. m) (3.44 x m 2 ) = 

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: Just turn 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 germanium less ___________ 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 ______________________. semiconductorssuperconductors = a ________ of shared e - s = Si atom bond

Phosphorus P and arsenic As have __________ 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 _________________ resistances that can be ______________. They are now used in making almost all _______________________________. 1 more 1 less tiny amounts doping outer e - ’s 345 BCN AlSiP GaGeAs current R average controlled tiny 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 heat of power creating strong magnets (medical use). transmission 0 below a "critical" temperature metals absolute zero 4 K

Material metal=m ceramic=c critical temp. (K) absolute zero0 Zincm0.88 Aluminumm1.19 Tinm3.72 Mercurym4.15 liquid nitrogen YBa 2 Cu 3 O 7 c90 TlBaCaCuOc125 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 1986 much ___________ to use liquid N

Material metal=m ceramic=c critical temp. (K) absolute zero0 Zincm0.88 Aluminumm1.19 Tinm3.72 Mercurym4.15 liquid nitrogen YBa 2 Cu 3 O 7 c90 TlBaCaCuOc125 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 cheaper 77

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)