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Resistor
Potential Drop
 Within a material there is a
+
-
voltage drop associated with a
field.
 The material can be a weak
conductor.
• Charges may flow
• Field and equipotentials exist
 The material will have a
current I and voltage drop V.
+
-
V  Ed
Ohm’s Law
 Many materials have a linear relationship between
current and voltage.
 This relationship is called Ohm’s law.
Resistance
 The ratio of the change in
DP
v
d
voltage to the current is the
resistance.
• Like resistance to water flow
by stones
A
 Resistance is measured in
ohms (W).
• 1 W = 1 V/A
R V / I
V  IR
I V / R
Non-Ohmic
 Some materials have non-linear relations between
current and voltage.
• Non-ohmic materials
• Fluorescent and incandescent lamps
• Diodes and transistors
Resistor
 Material can be fashioned to
have a specific resistance.
 The symbol for a resistor is a
zigzag line.
• This is a resistor
 Resistors can be made from
very long conductors.
• Wound wire in a coil
• Very precise
 Carbon composites make
good resistors.
 Like other electrical symbols
the orientation doesn’t matter.
Resistivity
 The resistance of a material
depends on the shape.
• Length
• Cross-sectional area
 The resistivity (r) is the
material-dependent part of the
resistance.
 Resistivity also depends on
temperature.
L
Rr
A
r  r0 (1 +  0 DT )
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