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1
Basics Concepts and Laws
Unit 1.1:
Voltage and Power
Voltage
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Voltage
Energy is expended to move the charges in
an electric circuit
The total energy per unit of charge
associated with the motion of charge
between two points
is called
voltage
.
v =
Δ
w/
Δ
q
where
v
= the voltage in Volts
w
= the energy in Joules
q
= the charge in Coulombs
1 Volt = 1 Joule/Coulomb
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Allesandro Volta
The Volt is named after
Allessandro Volta
(17451827) who
developed the electric
battery (voltaic pile) in
1799.
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Voltage Continued
Voltage is also known as potential
difference
The voltage between two points in a
circuit indicates the energy required
to move charge from one point to the
other
An electric field is what moves the
charge
Does not necessarily mean that the
charges are moving between these two
points
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Analogy for Voltage
In a gravitational field, we speak of
“up” which implies a reference and
direction
Voltage and gravity share the same
mathematical properties
You can think of voltage using a
gravity analogy
F
g
=
k
g
m
1
m
2
r
2
F
e
=
k
e
q
1
q
2
r
2
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Reference Direction for Voltage
When a positive charge moves from point
1 to point 2, it sees a REDUCTION in
potential of 5 V
When a positive charge moves from point
2 to point 1 it sees an INCREASE in
potential of 5 V.
+
5 V
1
2

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Voltage References
Voltage requires:
Definition of two points in space
Reference polarity.
Otherwise it is undefined and
meaningless!
Circuit
Element
v
x
= 3V
Circuit
Element
+
v
x
= 3V

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Voltage References
Voltage is a signed quantity.
Reverse the polarity
Reverse the sign.
The reference polarity is arbitrary.
Negative voltages are OK.
Circuit
Element
+
v
x
= 3V

Circuit
Element

v’
x
= 3V
+
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Voltage References
Voltage polarity may also be indicated using
subscript notation
The first subscript is the positive terminal
The pair of subscripts define the two points
in space
a
b
Circuit
Element
+
v
ab
= 3V

Circuit
Element

v
ba
= 3V
+
a
b
v
ab
is the voltage at point a with respect to point b.
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 Fall '06
 MCCANN
 Energy, Power, Volt, Watt, Joule

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