Voltage/Battery Terminal confusion

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Discussion Overview

The discussion revolves around the measurement of potential difference (voltage) across the terminals of two independent batteries. Participants explore the conditions under which voltage can be measured, the role of current flow, and the limitations of voltmeters in such measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question why a potential difference cannot be measured between the positive terminal of one battery and the negative terminal of another, suggesting that theoretically there should be a voltage.
  • Others argue that without a complete circuit, there can be no current flow, and thus no measurable potential difference exists.
  • A participant points out that voltmeters require current to flow due to their finite input impedance, which affects their ability to measure voltage accurately.
  • It is noted that voltmeters measure voltage difference relative to two points, and without a connection between those points, the measurement is meaningless.
  • One participant introduces the concept of an electrostatic voltmeter, which can measure voltage without drawing current after being charged, although it has limitations at lower voltages.
  • Another participant discusses the idea that two batteries can have different potentials when referenced to a third object, such as the Earth, and highlights the importance of understanding voltage as a relative quantity.
  • Concerns are raised about the effect of connecting a voltmeter to the batteries, which may cause a small current to flow and equalize the potentials, leading to a zero reading on the meter.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for measuring voltage between two independent batteries. There is no consensus on the implications of potential difference and the role of current flow in measurements.

Contextual Notes

Participants highlight limitations related to the input impedance of voltmeters and the necessity of a complete circuit for current flow, which affects voltage measurements. The discussion also touches on the relative nature of voltage and the impact of measurement techniques.

XPTPCREWX
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1.) Why can't you measure the potential difference across two batteries terminals, let's say a positive terminal from battery A and a negative terminal from battery B?
Theoredically there is still a potential difference...therefore voltage...right?
 
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You can... unless you are talking about two independent batteries (not wired together). If that is the case, then you have an incomplete circuit, thus no current flow.
 
XPTPCREWX said:
1.) Why can't you measure the potential difference across two batteries terminals, let's say a positive terminal from battery A and a negative terminal from battery B?

Because they are not connected as a circuit.

XPTPCREWX said:
Theoredically there is still a potential difference...therefore voltage...right?

No there wouldn't be. From an Ohm's Law point of view, since they are not connected as a circuit, no current can flow and no potential difference will exist.

CS
 
So there needs to be current induced by the voltage you are measuring?

I was under the impression that EVERYTHING had electrical potential/voltage.
Now this leads me to believe that a Voltmeter will NOT in fact read voltage, but moreover voltage drop?
 
It simply depends on the input impedance of the voltmeter. REAL WORLD voltmeters do not have infinite impedance so they require a current to flow in order to work.
 
XPTPCREWX said:
I was under the impression that EVERYTHING had electrical potential/voltage.
Now this leads me to believe that a Voltmeter will NOT in fact read voltage, but moreover voltage drop?
They measure voltage DIFFERENCE. It's meaningless to say something has a voltage without saying what it has a voltage relative to.
For batteries it is relative to the other terminal so your meter has to connect between the two terminals to measure the voltage between them.
 
I understand all of this, but I was asking from more of a scientific view point...
Although I think this answers my question PERFECTLY well.

Averagesupernova said:
It simply depends on the input impedance of the voltmeter. REAL WORLD voltmeters do not have infinite impedance so they require a current to flow in order to work.

To sum it up, its just that our typical modern day conventional Voltmeters cannot measure Voltage alone, but rather a "voltage drop" (the difference across a conductor) in reference to a circuit but dependent of the current induced in it... Or as supernova suggested Impedence.
 
If you want a voltmeter that takes no current (direct current, not alternating) once it's charged, then the electrostatic voltmeter is your friend.

Don't expect to measure much below a kV or so though.
 
The two batteries' terminals can have completely different voltages, measured with reference to some third object. For example, you could charge up one 9V battery's metal case with a Van de Graff (static electricity) generator so that its negative terminal was at 1000V with respect to the Earth. The battery's chemical potential will then cause its positive terminal to always be 9V greater than its negative. The positive terminal will therefore be at 1009V, with respect to the Earth.

The issue arises with voltmeters, which, as has been said, are not perfect, and do not have infinite impedance. As soon as you connect the voltmeter to the two batteries, a very small current will flow, equalizing the charge on the batteries until the meter reads zero. This happens in the blink of an eye for any normal voltages, and appears to not happen at all.

The issue of voltage being only a relative quantity is extremely important, but entirely separate from this mechanism.

- Warren
 
Last edited:
  • #10
XPTPCREWX said:
1.) Why can't you measure the potential difference across two batteries terminals, let's say a positive terminal from battery A and a negative terminal from battery B?
Theoredically there is still a potential difference...therefore voltage...right?


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  • #11
Two batteries - or two lumps of metal, for that matter, can be at different potentials. A meter with a high enough input resistance can measure this but the act of measurement is likely to bring their two potentials together because some finite current will flow through the measuring device.
The thing about a battery is that it has a source of energy which will keep the potential difference across its terminals even when you measure it or take current inot a load.
 

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