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1
Passive Sign Convention
1
2
Conventions
Sometimes there is more than one
valid
way to express a physical
concept.
It is handy if everyone does it the same
way to avoid confusion.
In these situations, it is common to
define a standardized convention
.
The definition of current is an example.
We will now introduce another:
The Passive Sign Convention
3
The Issue
We know that:
The battery supplies
power
⇒
Power is
negative
by definition
The lamp dissipates
power
⇒
Power is
positive
by definition
The current through the battery is the same as the current
through the lamp.
The voltage across the battery is the same as the voltage
across the lamp.
V
B
= V
L
So how do we compute power to get the correct sign?
+
i
v
B


v
L
i
4
The Ideal Basic Circuit Element
It has only two terminals
It is described mathematically in
terms of current and voltage
It cannot be subdivided into other
circuit elements
i
v
1
2

5
Current Flow through an Element
Since current can’t be stored in our basic ideal
elements, the following situations are equivalent
v
1
2
i
+

v
1
2
i
+

v
1
2
i
+

1
2
v
i
+

6
Passive Sign Convention
The passive sign convention ensures that
power is computed in a consistent way
There are two cases to consider:
Loads 
the element is absorbing power
Sources  the element is producing power
i
1
2

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7
Passive Sign Convention: Loads
The reference direction for current is assigned
to enter the positive terminal for voltage.
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This note was uploaded on 11/01/2010 for the course EE 302 taught by Professor Mccann during the Fall '06 term at University of Texas at Austin.
 Fall '06
 MCCANN

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