Passive Sign Convention Cons of Power Finished
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Transcript of Passive Sign Convention Cons of Power Finished
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EE 2400
The Passive Sign Convention
AndConservation of Power
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This powerpoint presentation was prepared by Dr. Lloyd A.
“Pete” MorleyThe University of Alabama
2005
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The Passive Sign ConventionConsider the following circuit:
ele-ment
a+
b
I
V
If the reference direction of the current (i.e., the direction the arrow is pointing) is from the + side of the voltage reference through the element to the - side of the voltage reference, then …
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V
P=VI and a value of P > 0 signifies that power is being absorbed by the element while a value of P < 0 signifies that power is being provided by the element.This is called the passive sign convention.
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Proving the Passive Sign Convention
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Let’s prove and understand this concept by looking at each of the four possible cases for V and I and considering conservation of energy within the circuit.
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Case 1: V > 0 and I > 0The physics: Since I is positive, charge moves from point a through the element to point b. Since V is positive, charge at point a has a higher level of energy than charge at point b. The charge thus loses energy as it travels from a to b through the element, and this energy (or power) must be absorbed by the element.
Proving the Passive Sign Convention (cont.)
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Case 2: V > 0 and I < 0The physics: Since I is negative, charge moves from point b through the element to point a. Since V is positive, charge at point a has a higher level of energy than charge at point b. The charge thus gains energy as it travels from b to a through the element, and this energy (or power) must be provided by the element.
Proving the Passive Sign Convention (cont.)
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v(t)
Case 3: V < 0 and I > 0Case 4: V < 0 and I < 0You work these two cases right now on your own paper.
Proving the Passive Sign Convention (cont.)
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Conservation of PowerFor complete circuits, the sum of all powers generated and absorbed is zero (i.e., circuit exhibits conservation of energy and conservation of power)Power generated = Power
absorbed
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ele-ment
a+
b
I
V
Case 3: V < 0 and I > 0The physics: Since I is positive, charge moves from point a through the element to point b. Since V is negative, charge at point b has a higher level of energy than charge at point a. The charge thus gains energy as it travels from a to b through the element, and this energy (or power) must be provided by the element.
Proving the Passive Sign Convention (cont.)
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ele-ment
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b
I
V
Case 4: V < 0 and I < 0The physics: Since I is negative, charge moves from point b through the element to point a. Since V is negative, charge at point b has a higher level of energy than charge at point a. The charge thus loses energy as it travels from b to a through the element, and this energy (or power) must be absorbed by the element.
Proving the Passive Sign Convention (cont.)