Why Don't We Invert the Signs of Voltage in Circuit Analysis?

[Ammar w]

Homework Statement

https://www.diigo.com/item/image/2sb3i/1svo

Why don't we flip the signs of VA like this :

https://www.diigo.com/item/image/2sb3i/g2a4

because the current must flow from + to - of the resistor.

Homework Equations

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The Attempt at a Solution

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[gneill]

The "+ -" tags on the 15Ω resistor tell you which way to "measure" the potential V_A; It's like specifying how to orient the positive and negative leads of a voltmeter on the component.

You could change the tags as you suggest IF you also reverse the polarity of the controlled voltage source 2V_A, since its output depends on how V_A is "measured".

 

[Ammar w]

thanks
and the KvL equation changes from :
-120+V₃₀+2V_A-V_A = 0

to :
-120+V₃₀-2V_A+V_A = 0