Questions: Circuit Variables and Ideal Circuit Elements

Question 1

Under the passive sign convention, an element has voltage V = 5 V across it and current I = -2 A entering the positive terminal. What is the power absorbed by the element?

• A. -10 W
• B. 10 W
• C. -2.5 W
• D. 2.5 W

Answer: (A)

Under the passive sign convention, P = V × I where I is defined as entering the positive terminal. P = (5)(−2) = −10 W. A negative absorbed power means the element is actually delivering 10 W to the circuit — it is a source, not a load.

Question 2

An ideal voltage source generally determines both the voltage across it and the current through it.

• T. True
• F. False

Answer: False

An ideal voltage source fixes the voltage across its terminals, but the current through it is determined by the rest of the circuit. Dually, an ideal current source fixes the current but the voltage across it is set by the surrounding network.

Question 3

What is the purpose of assigning a reference direction (or reference polarity) to a circuit variable, and does it have to match the actual physical direction?

Model answer: A reference direction is a mathematical convention chosen before writing equations; it gives a sign to the variable. The actual physical direction need not match — if the computed value comes out negative, it simply means the physical quantity flows opposite to the chosen reference.

Picking a reference direction is like choosing a positive x-axis: the axis itself is arbitrary, but once chosen it makes equations unambiguous. A negative result is meaningful information, not an error; it tells you the quantity opposes the reference you assumed.