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Electric Charges Forces Fields and Potentials

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					    Electric Circuits


  Day #3


RESISTANCE

           Lyzinski Physics
Current flows freely through _____________ but will not flow
                               Conductors
through _____________. Some materials allow current to flow
            Insulators
easier than others. A conductor’s “resistance” is defined as the
                Voltage
ratio of its ____________ to its ________________. In other
                                   Current Flow
words,
                           V
          Resistance = R =          OR   V = IR
                            I

 The units of Resistance are _______, which comprise a new
                                V /A
 SI unit known as the OHM, which is represented by
     W
 _______, the Greek letter Omega, or “W”. A material is
                      V / I
 called “Ohmic” if _________is constant. This means that no
 matter what the potential difference (or Voltage) across a
 resistor is, the resistance remains constant. Some materials
 are non-ohmic (ie. the resistance varies as Voltage is
 changed).
 Factors Affecting the Resistance
             of a Wire
•Length of the wire
                                       MORE
                      (longer length, ________ resistance)

•Cross-Sectional Area
                                      MORE
                        (less area, _________ resistance)

•Material:
                    LESS                        MORE
             Gold ( _____ ), Copper, Aluminum ( _____ )

•Temperature:
                                 UP
                Resistance goes ______ with temperature.
American Gladiator Analogy
Certain materials have ZERO resistance below a certain
temperature. This temperature is called the
_______________________. Materials that exhibit this
    Critical Temperature
behavior are called ____________________. So what?
                       Super Conductors
What’s so special about a material with NO resistance.
Well…..if a material has no resistance, then current continues
forever (almost) even if the potential difference (the pressure
driving the current) is removed. This has great potential for
technological progress. But….most materials that exhibit
superconductivity only do so at VERY low temperatures.
Higher temperature (like room temperature) superconductivity
is being investigated and could …. dare I say it ….. change the
world!!!
 Current level
                             Probable effect on human body
(in milliamperes)


                    Perception level. Slight tingling sensation. Still
    1 mA
                       dangerous under certain conditions.
                    Slight shock felt; not painful but disturbing.
                        Average individual can let go. However, strong
    5 mA
                        involuntary reactions to shocks in this range
                        may lead to injuries.
                    Painful shock, muscular control is lost. This is called
   6-30 mA
                       the freezing current or "let-go" range.
                    Extreme pain, respiratory arrest, severe muscular
  50-150 mA            contractions. Individual cannot let go. Death is
                       possible.

                    Ventricular fibrillation (the rhythmic pumping action
1000-4300 mA           of the heart ceases.) Muscular contraction and
                       nerve damage occur. Death is most likely.


  10,000 mA         Cardiac arrest, severe burns and probable death.
Wet conditions are common during low-voltage
electrocutions. Under dry conditions, human
skin is very resistant. Wet skin dramatically
drops the body's resistance.
   Dry Conditions: Current = Volts/Ohms =
                          ________
         120/100,000 = .0012A = 1.2 mA

   Wet conditions: Current = Volts/Ohms =
          120/1,000 = .________
                         12A = 120 mA

				
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