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Understanding Electrical Resistance and Resistors

Explore the concept of electrical resistance and how it affects the flow of electric current. Learn about resistors and their role in controlling current and providing a specific amount of resistance. Discover the different factors that affect resistance and the materials that exhibit superconductivity and semiconductivity.

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Understanding Electrical Resistance and Resistors

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  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

  2. 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

  3. 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

  4. 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

  5. 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 =

  6. 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:

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

  8. 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

  9. 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

  10. 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

  11. 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

  12. The current just keeps going….

  13. 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

  14. Who uses ceramics? Harry Potter!

  15. 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

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

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

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

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

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