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Electric Charges Forces Fields and Potentials by mikeholy

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• pg 1
```									    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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