Understanding Electrical Resistance and Resistors

1. the opposition that a device or conductor offers to the flow of electric current. More resistance  less current, and vice versa. ResistanceR - _____________________________________ ____________________________________________________ _____________________________________________________ 1. Resistance occurs as a result of ________________ colliding with ___________________ and with the __________________________ , resulting in ____________ . This converts __________________energy to ___________. electrons other electrons friction material of the conductor heat electrical no direction scalar 2. R is a __________________ . It has _________________ . units of R: ___________________ It is a ______________________ unit. 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

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

6. temperature: C. __________________ R T more R vibrate faster Higher T atoms of the metal _________________  ________________________ for e-'s to move through the metal  more ____________________ more difficult resistance The material D. ______________________ : Different metals have different numbers of ____________________ .  ______ electrons  ______ current  _______ resistance free electrons more R more less # of free electrons

8. These 4 factors are summed up in: rL/A R = • (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

11. Ex. Calculate the resistance of 100 meters of copper wire that has a cross-sectional area of 3.44 x 10-6 m2. rL A R = (1.72 x 10-8W·m) (100. m) = (3.44 x 10-6 m2) 0.500 W =

12. 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:

13. Resistors Bigger resistors can handle more power w/o overheating.

15. 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

16. 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: germanium silicon 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

17. 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 current R average controlled tiny electrical devices

19. 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. 0 below a "critical" temperature R large freely long times transmission heat creating strong magnets (medical use). of power metals absolute zero 4 K

20. The current just keeps going….

23. 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 much ___________ to use liquid N

24. Who uses ceramics? Harry Potter!

25. 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 cheaper much ___________ to use liquid N 77

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

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

28. Power Transmission: no resistance  no heat loss  more efficient

29. The Meissner Effect - A superconductor expels a magnetic field. A magnet is levitating above a superconductor (cooled by liquid nitrogen)