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ECE201 Linear Circuit Analysis I
Topics: Resistance, Ohm’s Law, power, dependent sources
1. Review of Passive Sign Convention
PAB = VAB × I (^) AB = power absorbed by circuit element with terminals labeled A and B.
Practice Problem 1. Given a two-terminal element, how many different ways can you mark the polarity of voltage and the direction of current? Of these, which one(s) follow the Passive Sign Convention? [Hint: Draw your answers.]
Practice Problem 2. Compute power absorbed (or dissipated ) by elements #1, #2, #3. Which element(s) could be the source(s) of the circuit?
Hint : For each circuit element, (re)draw current and voltage to follow passive sign convention.
2. Principle of power conservation
Form 1 Σ Pabsorbed = 0
e.g. using results obtained above,
Σ Pabsorbed = P 1 + P 2 + P 3 =
Form 2 Σ Pabsorbed = Σ Pdelivered
From the example above, we have
3. Resistor & Resistance
(1) What is Resistance? Resistor = a device or a circuit element that impedes the flow of current. This property is called resistance.
(2) unit: Ohm (Ω) 1 Ω = 1 V/A
(4) Conductance
G =
R
unit: Siemens (S)
1S = 1/Ω = mho
R
(3) symbol:
V (^) o
→ I
R
V
Practice Problem 3. [Passive Sign Convention and Power Calculation.]
REMEMBER:
(i) Always label current and voltage using passive sign convention. (ii) power absorbed = power dissipated = – power generated.
(1) A generates 20 W of power, find VA.
(2) Find the power absorbed by B.
(3) If element C generates 3w^ of power, find VC.
F
D E
A B^ C
4v (^) _ 5A
+ ID I E
V A
– 4A
2v
7A
3A
V (^) c
(4) If element D absorbs 27w^ of power, find ID.
(5) If element E absorbs 2w^ of power, find I (^) E.
(6) Find the power absorbed by element F.
Answer to Practice Problem 3. (1) VA = 5 V (2) PB = 14 W (3) VC = −1 V (4) I (^) D = − 9 A (5) I (^) E = 2 A (6) PF = − 20 W.
