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Lecture 4

ENGG 225 Lecture Notes - Lecture 4: T Power


Department
Engineering
Course Code
ENGG 225
Professor
Norman Bartley
Lecture
4

Page:
of 2
Resistors (continued)!
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Property: voltage and current related by Ohm's Law !
""V = iR!
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Suppose:!
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Conductance !
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Power and Energy!
Power is the product of voltage and current!
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Which we may write as!
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where: w = energy in Joules!
" q = change in Coulombs!
" t = time in secs!
" p = power in Watts (W)!
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We define power in terms of the passive reference convention.!
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If either current reference direction or voltage reference polarity is reversed... !
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[current reference direction is in the direction of a voltage rise: from - to +]!
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Example: !
Find the power in the circuit element at right!
(a) i = 10A, v = 12V!
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(b) i = -10A, v = 60V!
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Physical interpretation of the sign of p:!
"p > 0 (absorbs power)!
"p < 0 (delivers power)!
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Consider the simple circuit:!
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Power in a resistor!
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Energy!
We have !
"Therefore, !
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Power companies measure energy to determine monthly bill!
Cost determined by how much power used over time.!
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Example 1:!
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(a) Passive reference convention is observed, so p = vi!
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(b) !