# Voltage rises/drops

Discussion in 'Homework Help' started by pharris7, Sep 15, 2012.

1. ### pharris7 Thread Starter New Member

Sep 15, 2012
1
0
Hey guys, I would really appreciate some help with this. This is for my DC circuitry course, and I am having a really hard time with this question.

here it is, along with what I have thought would be the correct process.

http://imgur.com/43afV

the answers my professor provided are at the bottom, but I have yet to have mine match them.

One thing to note, is that I have spent on couple hours on this page alone. I have tried many different ways of trying to work it out. I erased all the previous work from the page so it would be easier to read. For Vfc, however, I am confused on what the voltage would be. From what I have tried it would be -3v +8v -5v = 0V. This doesn't seems to be correct.

I would really appreciate some feedback,

thanks

2. ### WBahn Moderator

Mar 31, 2012
24,559
7,696
When calculating the voltage difference from Pt A to Pt B, you have to add up all of the voltage gains along a continuous path from Pt A to Pt B. You can't just go picking this voltage and that voltage and add them together. The 3V and 5V batteries are not on the same path from 'd' to 'a' that the 8V battery is.

You need to start with the voltage gain from pt 'd' to pt 'c'. Then add to that the voltage gain from pt 'c' to pt 'b'. Then add to that the voltage gain from pt 'b' to pt 'a'. You then have the voltage gain from pt 'd' to pt 'a', better known as Vad.

Note that you can do the same along any path from pt 'd' to pt 'a', such as the one that goes through points 'e' and 'f', and you better get the same answer (within a small allowance for roundoff error).

Be sure to note that if you change the voltage of one of the batteries (or change the value of one of the resistors), that the voltages and/or currents in all of the other components will be affected, so you can't rely on the resistor voltages printed in the diagram.

Finally, in the future please attach your figure to your post. In accessing your figure I had to block attempts to put cookies on my computer by sixteen different websites. Next time I will simply abandon the effort to assist after the first such attempt.