Which voltage to use when calculating Current

Thread Starter

pprendeville

Joined Nov 19, 2009
15
Hi all,

Just want to find out which voltage I should use when calculating the current through the 5 ohm resistor in the pic. Originally I thought it was the supply voltage, 20V but according to the notes it says to use the voltage drop across the resistor, i.e. 9.1V.

Why is this so?
 

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Jony130

Joined Feb 17, 2009
5,180
Well
V=I*R;
I=20V / (5Ω+ 12|Ω|18Ω||9Ω+4Ω||4Ω)=20V/11Ω=1.81A

So voltage on 5 ohm resistor is equal
V=1.81A*5Ω=9.09V
 

mrmount

Joined Dec 5, 2007
59
That is because the supply voltage 20 V is across the total resistor combination and not 5 ohm resistor alone. See Jony130's calculation.
 
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