basic working of voltmeter

Hii

I asked a couple of questions a while ago about voltmeters but I have read over them again and I don't understand the answers so I will ask again and try and be clearer this time on what I am asking.

I am interested in the use of a voltmeter in a galvanic cell

Is it correct to say a voltmeter measures a potential difference across a cell? Which way round does the comparison go? Is the potential at the red lead compared to the black lead or vice versa. In other words, is it the potential at the red lead minus potential at black lead to vice versa

If the more negative potential is attached to the black lead and the less negative potential is attached to the red lead, will the potential difference be positive or negative?

What would be the potential be if you had the leads the opposite way.

I'm guessing it is the potential of the red lead minus the potential of the black lead. I have a diagram where it looks like the more negative potential attached to the black lead and the less negative potential attached to the red lead and the potential is positive. This would suggest it is red lead potential - black lead

Many thanks
andiejeAsked:
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jack_borshchCommented:
I'm not sure about colors but your statement "potential of the red lead minus the potential of the black lead." seems to be correct. See also http://en.wikipedia.org/wiki/Voltmeter
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aburrCommented:
"Is it correct to say a voltmeter measures a potential difference across a cell?"
yes
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" Is the potential at the red lead compared to the black lead or vice versa. In other words, is it the potential at the red lead minus potential at black lead to vice versa"
That depends on how it is hooked up.
Conventionally the red lead is put on the point of highest potential and the black lead at the lowest potential (which is often ground or 0 potential.)
The voltmeter can be hooked up backwards and many will then read minus. (and some can be damaged)
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"If the more negative potential is attached to the black lead and the less negative potential is attached to the red lead, will the potential difference be positive or negative?"
positive
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"What would be the potential be if you had the leads the opposite way."
the sign would be changed but the magnitude would be the same
(the potential would not be changed but the meter reading would change sign)
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SuperdaveCommented:
Technically, the "negative" (black) side is supplying electrons, and some small amount of current is going through the voltmeter from the black lead to the red lead, so if by greatest potential you mean that it is pushing out electrons, then it would be black compared to red, but if "greatest potential" means "highest voltage", then it's red compared to black because the red side represents the positive numbers (just because that is the convention used to designate voltage).
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andiejeAuthor Commented:
aburr, what does highest and lowest potential mean? I don't know anything about batteries or electronics. I am looking at this from the point of view of someone who only knows some basic electrochemistry. I only know most/least negative/positive

All i will ever need to know is why, if the if the black lead is put on the most negative potential the reading is positive and why if the red lead is put on the most negative potential the reading is negative.

I read this:
By definition, the voltage will be positive when the cathode is plugged into the positive terminal and negative when the anode is plugged into the positive terminal.

which seemed to make sense if i knew precisely what anode and cathode are so i read this to find out
http://en.wikipedia.org/wiki/Anode

which is confusing because it seems the anode is positive or negative in different contexts but in a galvanic cell, the anode is negative.

So i guess the answer is:
the voltage will be positive when the cathode is plugged into the positive terminal and negative when the anode is plugged into the positive terminal. it depends on what type of device you are using if the anode is negative or positive. in a galvanic cell the anode is the more negative terminal

I presume this will make more sense if i knew more about how electric current flow is even defined
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Thibault St john Cholmondeley-ffeatherstonehaugh the 2ndCommented:
Water is commonly used as an analogy for electricity. It behaves in a similar fashion (in terms of flow) and you can see it, which helps to understand what the electricity is doing.
Think of two water tanks one above the other. The tanks are your battery, the water is the electricity and the top tank is the positive terminal. For a meter use an imaginary piece of glass tube and connect this between the top tank and the bottom one, piercing the sides below the water line. Water will flow down your tube representing electric current. The water will not flow up the tube from bottom to top so we can call the top tank the positive. The better the meter you use, the less current it draws itself (they all draw some current to work), we can represent this with a very thin glass tube, water will still flow and at the same speed (but much less volume), but it will now have a very minmal affect on the contents of the tanks.
If you keep your glass tube meter in the same position but swap the ends of the tubes so the one that was in the top tank is now connected to the bottom tank and vice-versa, you will still see the same current flow, but in the opposite direction.
Electricity is much the same, trying to level itself between connected areas of different potential. The way you connect the meter does not affect the flow itself, but it affects what you see when you measure it.
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SuperdaveCommented:
It seems like you're overthinking this.  The voltmeter's leads are red and black because most people are familiar with the convention that you connect red to positive and black to negative.  You expect a positive number when you measure it in the normal way (which you can think of as positive relative to negative, I suppose).  You're right that you'll confuse yourself with the terms cathode and anode; just stick with the terms "positive" and "negative".
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NovaDenizenCommented:
When people talk about potential they mean voltage.  High potential is positive voltage, and low potential is negative voltage.  A place of relatively high potential will attract electrons from a place of lower potential, and will repel positively charged particles towards a place of lower potential.

Somewhat confusingly, there are two differing models of electrical current:  the chemist's model and the engineer's model.

In the physicist's view, current is the flow of electrons from a place of low voltage to a place of higher voltage.

In the engineer's view, current is the flow of a virtual something called "charge" from a place of high voltage to a place of low voltage.

The engineer's current is equal and opposite to the physicist's current.  Having negatively charged particles flowing in one direction is basically the same as having positively charged virtual particles flowing in the opposite direction.

Almost all practical electrical circuit work is done from the engineer's point of view.  They don't have to worry about the disconnect between the "charge" model they use and the opposite behavior of the actual electrons unless they are doing low-level stuff with semiconductors, or using very high frequencies, or doing circuits that are very sensitive to noise.  The chemists design a battery while keeping electron flow in mind, then they hand it to the engineers who promptly redefine the battery as producing "charge" from the other end.
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aburrCommented:
"All i will ever need to know is why, if the if the black lead is put on the most negative potential the reading is positive and why if the red lead is put on the most negative potential the reading is negative."
basically because that is how black and red is defined in this case. (or conversely that is the way positive and negative is defined.)
Current flows from the more positive point to the more negative point. and when current goes into the red lead and comes out the black the voltmeter reads positive.
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as superdave says
"It seems like you're overthinking this."
or else you are trying to learn the basic workings of a process without knowing much about the concepts involved. It cannot be done.
You can get a general idea of the workings from analogies which are familiar but you cannot expect the general understanding to explain all the details.
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andiejeAuthor Commented:
Current flows from the more positive point to the more negative point...

I think that's what I'm getting confused over because in a galvanic cell electrons are flowing from the negative anode to the positive cathode.
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Thibault St john Cholmondeley-ffeatherstonehaugh the 2ndCommented:
that is the common confusion that everybody has mentioned.
They discovered electrcity before they found out how it works. Unfortunately they labeled pos and neg the wrong way round. a battery's pos and neg and your red and black leads are labeled for conventional current, the stuff that everybody uses day to day.
It's only when you deal with things at a molecular level that you see the electrons flow the other way. One way to think of it is as a series of holes that the electrons fill. when current flows the holes flow from pos to neg.
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andiejeAuthor Commented:
thanks a lot
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