Resistance R - _____________________________________ ____________________________________________________ _____________________________________________________ the opposition that a device or conductor offers to the flow of electric current. More resistance  less current, and vice versa. 1. Resistance occurs as a result of ________________ colliding with ___________________ and with the __________________________ , resulting in ____________ . This converts __________________energy to ___________. electrons other electrons material of the conductor friction electrical heat 2. R is a __________________ . It has _________________ . units of R: ___________________ It is a ______________________ unit. scalar no direction ohms, W derived 3. Any factor that makes it more _________________for _______________ to move will through a material will __________________________________ of the material: difficult electrons increase the resistance

A metals For _____________, there are four factors that affect how much resistance it has: metals length L: A. __________________ R more R L cross-sectional area A: B. __________________ R A A less R A

Higher T  atoms of the metal _________________ temperature: C. __________________ R T more R 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 more R more less # of free electrons

These 4 factors are summed up in: rL/A (rho) is called the _________________ of a material. depends on the ___________________ of a metal and is different for different _____________ . resistivity temperature metals units of r: _________________ ohm·meter, W·m (derived) Lowest r = _______________ Highest r = _______________ Metals that have more free _____________ will have a _________ r and _________ R. silver nichrome 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. R = rL A (1.72 x 10-8 W·m) (100. m) = (3.44 x 10-6 m2) = 0.500 W

A _________________is a device that is designed to have a definite amount of _________________. resistor resistance Resistors are used to 1. control _____________ flow; and 2. provide a _____________________ of a certain amount. current potential difference Symbols: 1. resistor: 2. variable resistor:

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

Two materials that do not follow these rules for metals are _____________________ and ______________________ . semiconductors superconductors 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. Pure = Si atom insulator shares covalent cannot conduct bond = a ________ of shared e-s

3 4 5 B C N Al Si P Ga Ge As Phosphorus P and arsenic As outer e-’s 3 4 5 B C N Al Si P Ga Ge As Phosphorus P and arsenic As have __________ outer e- than Si. Boron B and gallium Ga 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 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. below a "critical" temperature R freely large long times transmission heat creating strong magnets (medical use). of power metals absolute zero 4 K

The current just keeps going….

In _______, a new type of superconductor was discovered whose makeup is similar to ________________ . These become superconductors at higher temperatures. This makes them much more ____________________. 1986 ceramics Material metal=m ceramic=c critical temp. (K) absolute zero Zinc m 0.88 Aluminum 1.19 Tin 3.72 Mercury 4.15 liquid nitrogen YBa2Cu3O7 c 90 TlBaCaCuO 125 room temp. 293 much ___________ to use liquid N

Who uses ceramics? Harry Potter!

In _______, a new type of superconductor was discovered whose makeup is similar to ________________ . These become superconductors at higher temperatures. This makes them much more ____________________. 1986 ceramics affordable Material metal=m ceramic=c critical temp. (K) absolute zero Zinc m 0.88 Aluminum 1.19 Tin 3.72 Mercury 4.15 liquid nitrogen YBa2Cu3O7 c 90 TlBaCaCuO 125 room temp. 293 cheaper much ___________ to use liquid N 77

Applications of Superconductivity: Medicine Strong currents easy to maintain  strong magnetic fields are used in: MRI (magnetic resonance imaging) NMR (nuclear magnetic resonance)

Transportation:  strong currents run forever  strong currents produce strong magnetic fields  use repulsion or attraction to levitate train  no friction maglev trains

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)