Negative Voltage explanation

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  • #26
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In the wire only electrons flow. The conventional definition of current is the net flow of positive charge, so pretend that you have positive charges coming of the positive terminal of the battery, instead of the actual situation which is electrons coming out of the negative terminal of the battery.

In your circuit problem you can draw an arrow to represent conventional current from the positive side of the battery, through whatever is in the path (light bulb or meter or switch, etc.), and into the negative side of the battery. This will show you that reversing the components in the path would reverse the current through them.
 
  • #27
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But a battery and a voltmeter is connected in series on a circuit. How can the direction of the current change for one but not the other?

Because you asked what would happen if you turned the meter around.

Note: If you turn around BOTH the battery and the voltmeter, that's the same as not making any change at all.
 
  • #28
f95toli
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But a battery and a voltmeter is connected in series on a circuit. How can the direction of the current change for one but not the other?

No, a voltmeter is always connected in PARALLELL with what you are measuring (but an ammeter should be connected in series).
If you had an ideal voltmeter there would be no current flowing through it; and it you have good instrument that is actually not a bad approximation (there IS of course a current; but it is very small).
 
  • #29
Dale
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But a battery and a voltmeter is connected in series on a circuit. How can the direction of the current change for one but not the other?
You are not thinking this through clearly. It absolutely must change for one and not the other, regardless of the direction of the current. Say you label one terminal of the voltmeter "A" and the other "B" and say that you label one terminal of the battery "a" and the other "b".

Now, let's say that you put the voltmeter on your left with "A" up and "B" down and the battery on your right with "a" up and "b" down and you connect "A" to "a" and "B" to "b". Now, you will have a loop, current will go around that loop either clockwise or counterclockwise. If it goes clockwise then current will go from "a" to "b" through the battery and from "B" to "A" through the voltmeter. If it goes counterclockwise then current will go from "b" to "a" through the battery and from "A" to "B" through the voltmeter.

Now, we flip the voltmeter so that "B" is up and "A" is down, and we connect "A" to "b" and "B" to "a". We again have a loop and current will go either clockwise or counterclockwise. If it goes clockwise then current will go from "a" to "b" through the battery and from "A" to "B" through the voltmeter. If it goes counterclockwise then current will go from "b to a" through the battery and from "B" to "A" through the voltmeter. Either way the direction of current through one and only one must change.
 
  • #30
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No, a voltmeter is always connected in PARALLELL with what you are measuring (but an ammeter should be connected in series).
If you had an ideal voltmeter there would be no current flowing through it; and it you have good instrument that is actually not a bad approximation (there IS of course a current; but it is very small).

My interpretation was that the question was about that very small current, when you have nothing connected but a voltmeter across a battery, and about a microamperes flows.
 
  • #31
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You are not thinking this through clearly. It absolutely must change for one and not the other, regardless of the direction of the current. Say you label one terminal of the voltmeter "A" and the other "B" and say that you label one terminal of the battery "a" and the other "b".

Now, let's say that you put the voltmeter on your left with "A" up and "B" down and the battery on your right with "a" up and "b" down and you connect "A" to "a" and "B" to "b". Now, you will have a loop, current will go around that loop either clockwise or counterclockwise. If it goes clockwise then current will go from "a" to "b" through the battery and from "B" to "A" through the voltmeter. If it goes counterclockwise then current will go from "b" to "a" through the battery and from "A" to "B" through the voltmeter.

Now, we flip the voltmeter so that "B" is up and "A" is down, and we connect "A" to "b" and "B" to "a". We again have a loop and current will go either clockwise or counterclockwise. If it goes clockwise then current will go from "a" to "b" through the battery and from "A" to "B" through the voltmeter. If it goes counterclockwise then current will go from "b to a" through the battery and from "B" to "A" through the voltmeter. Either way the direction of current through one and only one must change.

I apologize for being so slow but I just can't seem to wrap my head around this for some reason. The part that I don't quite get in your explanation is the part where the current goes through the battery. I was told that current never goes through the battery but from positive to negative terminals through the wire.
 
  • #32
Dale
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The part that I don't quite get in your explanation is the part where the current goes through the battery. I was told that current never goes through the battery but from positive to negative terminals through the wire.
You were told wrong then. Current always goes in a complete loop, that is why we use the word "circuit" to describe electrical devices. This also relates to our https://www.physicsforums.com/showpost.php?p=2311338&postcount=20" where we specifically talked about the current through a battery.
 
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  • #33
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You were told wrong then. Current always goes in a complete loop, that is why we use the word "circuit" to describe electrical devices. This also relates to our https://www.physicsforums.com/showpost.php?p=2311338&postcount=20" where we specifically talked about the current through a battery.

Oh I think I know where my confusion arises from. I was thinking about the flow of electrons which doesn't travel through a battery but I did not consider the electrolytes as charge carrying current as well.
 
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