Absolute voltage of a battery?

Question

If the negative terminal of a 1.5V battery has an absolute voltage of -0.7V, is the voltage from the battery actually -0.7+1.5=0.8V?

"If the negative terminal of a 1.5V battery has an absolute voltage of -0.7V" How did you measure this "absolute voltage"? Can you find a voltmeter with a single input? — Kuba hasn't forgotten Monica, Oct 24 '22 at 19:27

JRE · Answer 1 · 2022-10-24T13:14:13.440

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There is no "absolute voltage."

Voltage is measured between two points.

For your battery, those two points are the minus pole and the plus pole. The voltage you measure between the two is the battery voltage.

The usual case looks like this:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

If you do something like this, you will get a negative voltage, but then you aren't measuring the battery voltage anymore:

schematic

^{simulate this circuit}

In this second case, you are measuring the voltage from the junction of the two resistors to the minus pole of the battery.

edited Oct 24 '22 at 13:14

answered Oct 24 '22 at 12:32

JRE

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Put a voltmeter across the battery thus showing that it is still 1.5V. – RussellH Oct 24 '22 at 20:35

score 3 · Answer 2 · answered Oct 24 '22 at 12:45

There's no absolute voltage. But there's a potential assigned to electrode materials that is measured against a standard H₂ electrode. That H₂ electrode is defined as "zero volts" in the terms of battery electrochemistry because it has no degradation you had to consider with other materials. It's easy to repeat.

So if you combine an electrode that has been measured as presenting -0.7V against that H₂ electrode with a second electrode that has been measured as presenting +0.8V against that H₂ electrode, you get 1.5V between those electrodes in result.

Spehro Pefhany · Answer 3 · 2022-10-24T15:30:37.913

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The physics definition of 'absolute voltage' is not much used in electronics. It's basically voltage relative to \$\infty\$. That's inconvenient to actually measure, most multimeter test leads are considerably shorter.

Anyway, the voltage of a battery is the positive terminal relative to the negative one, so if we have the voltage of the negative terminal relative to infinity (or any other point) then the positive terminal will be 1.5V higher.

Refer to the electrostatic approximation of Faraday's law for more math.

edited Oct 24 '22 at 15:30

answered Oct 24 '22 at 12:59

Spehro Pefhany

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The physics definition of potentials (of ALL potentials, not just volts) is, the line-integral through a field. Essentially that's a LENGTH, a length times field-intensity. This means that "voltage" is very much like altitudes in a g-field. There is no such thing as "absolute altitude," because altitude is a distance: meaningless at a single point. So, the top of a mountain has *infinite* number of altitudes simultaneously, and one terminal of a battery has *infinite* number of voltages (all depending on where we're putting the other end of our ruler!) Voltage is like distance. – wbeaty Oct 24 '22 at 19:40
@wbeaty my understanding is that it's the line integral from infinity in an otherwise empty universe. – Spehro Pefhany Oct 24 '22 at 19:51
potentials: just the line-integral between two points in ANY field (g-fields, flow fields, etc.) Heh, for voltage, just put everything inside a hollow metal sphere! (no need to expand its diameter to infinity.) Electrostatic shielding then excludes the part of the universe outside that sphere. The inside surface of the sphere acts like an infinite-size capacitor plate, because any charge placed upon the inside surface will totally disappear from the "interior universe," by Faraday's ice-pail effect. – wbeaty Oct 26 '22 at 19:38

score 3 · Answer 4 · answered Oct 24 '22 at 14:52

To add to what everybody else is saying, I ask you this - in the universe, where is "zero volts"? Centre of the planet? The Sun? The Milky Way? There's no such thing as zero volts anywhere.

In electronics you decide what node in your circuit will be deemed to have a potential of zero volts, and quote everything else relative to that. If you decide that the "negative" end of your battery is your zero volt reference point, then the positive end is 1.5V higher than 0V in potential, written "+1.5V".

If you decide that the "positive" end of your battery is 0V, then the other end is 1.5V lower in potential, written "−1.5V".

In all cases the potential difference across (between the two ends of) the battery, which people tend to call the "battery voltage", or "battery EMF" or just plain "voltage", is the same, 1.5V.

Given that you are unable to put a voltmeter probe inside the battery to contact some half-way potential within it, it's not really appropriate to say that one end is +0.8V, and the other is −0.7V, although there is nothing technically wrong with that convention if you choose to use it. Therefore your statement is correct but not useful except for the purpose of understanding the concepts of "voltage across" and "relative potential" at some point.

When you design or analyse a circuit, you decide for convenience what node in the circuit shall be deemed to have 0V potential, and declare all other potentials relative to that place. The decision where to point your finger and say "that's where zero volts is" is completely arbitrary, but if chosen with care, it can greatly simplify the arithmetic when describing the circuit mathematically, with equations. It will also help other engineers to understand your circuit and communicate with you and others about it.

Often you see an "earth" or "ground" symbol in a schematic, which is where the designer thought it was most logical and useful to say "zero volts is here", but if you completely ignored it and decided on your own zero-volt node, the maths would still work.

Generally you find that circuits are designed around a common, 0V ground (or earth) node, and have many many connections to that same node. The ground symbol also serves as a way of avoiding having to draw many many lines to the same point on a schematic. Everything connected to a ground symbol is physically connected to everything else with that same symbol, so that the schematic remains uncluttered. This ground node common to various parts of the circuit is usually the one called "zero volts", or "ground".

When it is stated that "node X is −3.2V", this is simply saying that X has a potential 3.2V lower than the ground node, and if you placed your voltmeter between node X (red) and ground (black), the voltmeter will read "−3.2V".

However, relative to the centre of the galaxy, it could +7 trillion volts, who knows?

score 1 · Answer 5 · answered Oct 24 '22 at 12:33

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Yes, that is correct, if you have defined your battery negative to be at -0.7V.

answered Oct 24 '22 at 12:33

Justme

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score 1 · Answer 6 · answered Oct 24 '22 at 13:12

If the negative terminal of a 1.5V battery has an absolute voltage of -0.7V, is the voltage from the battery actually -0.7+1.5=0.8V?

If you have connected the negative terminal of your battery to some voltage source that keeps it at -0.7 V with respect to some reference terminal that you define as having zero absolute voltage, then the positive terminal of the battery will be at 0.8 V with respect to the same reference terminal.

The phrase 'the voltage from the battery' usually means the voltage between the battery terminals, which will stay at 1.5 V, whatever you do with the absolute voltage of the negative terminal.

Amit M · Answer 7 · 2022-10-24T19:20:18.020

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Voltage of a battery means potential difference between its positive and negative terminals. You can expect that the positive terminal is at +0.75V and the negative terminal is at -0.75V. It is the potential difference between them which is 1.5V your circuit actually detects.

You can also define the positive terminal to be at 1.5V and negative terminal at 0V.

A similar question was answered here.

edited Oct 24 '22 at 19:20

answered Oct 24 '22 at 16:08

Amit M

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Absolute voltage of a battery?

7 Answers7