Electronics > Beginners
Voltage between two points
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sm263:
Hello!
I'm currently working through Practical Electronics for Inventors and still a bit stumped on an example I found early in the book. Attached is figure 2.13a from page 18 with the example stating:
--- Quote ---Find the voltage between the various points indicated in the following figures. For example, the voltage between points A and B in Fig. 2.13a is 12 V.
--- End quote ---
For a. I understand that the voltage between points A and B would be 12v since it's across the voltage source. But for b, c, and d is where I get lost. What I don't understand is in figure b where Vac = 3V. I'm taking a wild guess here, the only thing I can think of is the 9v source is negative giving the 3v reading between the two points. Then for c., Vad = -21V. Looking at that I can only think of the -12V flowing to the -9V side and adding it together giving -21V. I feel like I'm approaching this incredibly wrong and missing something really important. How would you guys approach solving this?
0xdeadbeef:
I guess this is trying to teach you that voltage is a potential difference. Without interconnection or a common reference like ground, there is no potential difference.
Like if you have two batteries, and measure between the plus pole of one battery and the minus pole of another one (without any other connection): what would you expect?
ataradov:
For (c), Vad = +21V, if you take (a) Vab = +12V. The situation is no different, just a higher voltage.
Your thinking looks correct on all counts.
Brumby:
--- Quote from: 0xdeadbeef on January 04, 2019, 01:26:17 am ---I guess this is trying to teach you that voltage is a potential difference. Without interconnection or a common reference like ground, there is no potential difference.
--- End quote ---
That is part of it.
I see this exercise as getting the student to understand the role of the reference point and to understand the relative nature of voltages.
Like this example I've cited before:
The only difference is where you put the black lead of your meter.
Ian.M:
In theory, using an infinite input impedance voltmeter, all the voltages asked for in O.P's Fig. 2.13a are undefined. Given a real near infinite impedance voltmeter, *if* the insulation of the batteries is good enough, you'll measure an essentially random voltage determined by the static charge on each battery (not charge *in* the battery), and the capacitance between them and to ground. However the capacitance is extremely small, and most real voltmeters have an input impedance of under 100 Meg, (typically 10 Meg for DMMs and 20K or 50K per V of FSD for analog ters) so any static charge present will equalise by flowing through the meter very quickly, resulting in a reading of zero volts from that time forward.
As all the other figures include a ground connection to each battery, all voltages between any two points in each figure wont be influenced by static charge.
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