Hi,

I'm pretty new to this stuff. I'm trying to teach myself with "Lessons

In Electric Circuits" over at ibiblio.org.

Anyway, I've made it to Chapter 6 in Book 1, and I'm a little confused

about a voltage polarity example. An image of the circuit I'm confused

about can be found at:

http://www.ibiblio.org/obp/electricCircuits/DC/00117.png
The author says that the voltage between points 3 and 4 is +/- 32,

depending on whether you're measuring point 3 with respect to point 4,

or vice versa. This doesn't seem very intuitive to me...

It seems like the voltage between the two points should be

(35V-20V)-(25V-13V) = 15V-12V = 3V. I got that number by calculating

the voltage of point 3 with respect to point 8 (20V drop across

resistor), and the voltage of point 4 with respect to point 5 (13V drop

across resistor), then subtracting them because "voltage is relative."

The bottom line (points 7,8,9,10) is the common point - all points on

that line are at the same voltage - so you should call that line

"common", or "ground", and calculate all voltages relative to that

line. You calculated the voltage at 4 relative to 5, rather than

relative to 9, and mixed up signs somewhere.

If you calculate the voltages at 3 and 4 relative to 7,8,9,10, it

should work out correctly.

Point 3 is 20 volts positive from the common point (7,8,9,10), and

point 4 is 12 volts negative from the common point, so there is 32

volts between them, with 3 being more positive. So, if you declare 3

as "zero volts), 4 is at -32. But if you declare 4 as "zero volts", 3

is at +32.

Then again, I don't understand why you have to connect points 8 and 9

(while not connecting points 3 and 4) in order to use Kirchhoff's Law,

or how this circuit would actually work in real life, so that's

probably why I just don't get it. Can somebody help me out?

Well, you could connect any point in one loop to any point in the

other - but you would get a different problem for each pair of points

you connect.

Before you can measure the voltage between two points, there must be

some path for current to flow between those points. In this case, the

author chose to make the path between points 8 and 9.

If you remove the connection between 8 and 9, you won't measure any

voltage between 3 and 4, or between 8 and 5, or in general between any

point on the left loop, and any point on the right loop. You would,

however, still be able to measure voltage differences within either

loop.

--

Peter Bennett, VE7CEI

peterbb4 (at) interchange.ubc.ca

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