Chapter 5 Conductance and Resistance

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Chapter 5Conductance and Resistance
Readpp 68 - 78
WRITE and ANSWER Review Main Ideas1-26
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Chapter 5Conductance and Resistance
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Chapter 5Conductance and Resistance
Conductors a material through which electrons
flow easily voltage easily removes valance
electrons
Good conductors
Gases
Metals
GoldSilverCopperAluminum
NeonArgonMercury VaporSodium Vapor
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Electron Flow - from negative to positive
Atom looses an electron from its outer shell and
attracts another from another atom
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Electrical Impulse -
Individual electrons travel less then 1 inch
per second
After voltage is applieda free flow of
electrons occurs close to the speed of light
Speed of light - 186,000 miles per second 7
times around the equator in one second
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Resistance an opposing force between one thing
and another
Basic Unit of resistance OHM ?
The symbol for resistance is R
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Resistance an opposing force between one thing
and another
Resistance in metal conductors is dependent on
four things
1. The kind of metal
2. Its temperature
3. Its length
4. Its cross-sectional area
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Resistance an opposing force between one thing
and another
Properties of different metals
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Resistance an opposing force between one thing
and another

• Its temperature resistance increases when
  temperature increases
• a decrease in temperature decreases
  resistance.

If temperature is reduced to Absolute Zero
(-273.16o C or 459.69o F) the resistance of a
conductor will be Zero This is called a
SUPERCONDUCTOR
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Resistance an opposing force between one thing
and another
Its LENGTH if wires of two different lengths
have the same cross sectional area the longer
wire will have a higher resistance If two wires
are the same length, the wire with the larger
cross sectional area will have less resistance
the larger wire has more free electrons
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Resistance an opposing force between one thing
and another
Insulators a material that does not easily
conduct electricityThese have valance electrons
that are tightly bound the nuclei of their
atomTo make them conduct it takes extremely
high amounts of voltage to produce significant
amounts of free electrons
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Resistance an opposing force between one thing
and another
Insulators a material that does not easily
conduct electricityCommon Insulators
dielectrics glass paper porcelain wood mica
rubber plastics
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Resistance an opposing force between one thing
and another
Resistors a device with a know value of
resistance. Fixed Resistor single value
remains constant under normal conditions Carbon-
composition Wire wound Film resistors
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Resistance an opposing force between one thing
and another
Variable Resistor provide an
adjustable amount of resistance in the
circuit A.K.A. potentiometers
Schematic Symbol
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Resistance between B and A increases. Resistance
between B and C decreases
Resistance between B and A decreases. Resistance
between B and C increases
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Resistance an opposing force between one thing
and another
Variable Resistor have an adjustable amount of
resistance in the circuit rheostat
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Rheostat used to control current in a lamp circuit
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Resistance an opposing force between one thing
and another
Ratings Tolerance greater or less
than its rated value e.g. or 5 , 10
, 20
Power rating- how much heat a
resistor can handle before burning in half
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Resistance an opposing force between one thing
and another
Resistor color code 1st Stripe first number
of the resistance value
2nd Stripe second number of the resistance
value
3rd Stripe number of zeros
3rd Stripe if it is gold the number given
by the first two stripes is divided by ten
3rd Stripe if it is silver the number given
by the first two stripes is divided by 100
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Resistance an opposing force between one thing
and another
Resistor defects To much current passing
through Carbon resistors will often burn
completely apart Scorched cracked or
buldged out Wire wound resistors will burn
apart at one point
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VOM - Digital
Red Lead
Black Lead
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Resistance Formulas
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Series
Rt R1 R2 R3
Resistor total Resistor 1 Resistor 2
Resistor 3
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Parallel
If all the resistors are equal in value
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Parallel
If resistances are not all of equal values
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Parallel
If there are two unequal resistor values
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Measuring Resistance
Solder-less Circuit boards
DMM
6 - 220 O Carbon Resistors
6 - various Carbon Resistors
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The Solder-less Circuit Board
Not connected
All connected
Not connected