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I have a question regarding this photo:

enter image description here

So these are two plates who have electric potential; but the problem is in order for a point to have electric potential we need to have a reference.

What I have studied is that we choose the reference to be infinity usually but it can be anything. So my teacher said there is a change in electric potential in those two plates; namely 150 - -50 = 200. This means delta v = vp1 - vp2, which does not make sense, because how did we find vp1 and vp2 in the first place? What would be the reference?

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  • \$\begingroup\$ If there is 200V between plates, and one is said to be at -50V and the other at 150V, the reference is obviously 0V because 0V is what you compare the voltages to. \$\endgroup\$
    – Justme
    Commented Jul 19, 2022 at 10:27
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    \$\begingroup\$ This is like the math questions like "Lisa has 50 apples, and sells 23, how many are left?". You're not supposed to be asking "What does Lisa need 50 apples for?" The reference is implied, you're probably not needing it for the assigment. \$\endgroup\$
    – Jeroen3
    Commented Jul 19, 2022 at 10:53
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    \$\begingroup\$ The reference here is "50 volts higher than the left plate" \$\endgroup\$ Commented Jul 19, 2022 at 11:57
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    \$\begingroup\$ Well, potential is an absolute quantity, so it does not need a reference. The numbers you are seeing here are voltages/potential differences relative to an unstated reference potential (which, as mentioned, you don't really need to be stated for this problem). \$\endgroup\$
    – Eugene Sh.
    Commented Jul 19, 2022 at 15:04

1 Answer 1

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I've had this conversation with my physicist friend, and they told me that to get a true 0V you must separate an electron and a proton by an infinite distance. Only then do the two particles exert zero force on each other.

Since this is impractical in daily life, we instead chose some reference point and measure all voltages with respect to that point. Engineers commonly call that reference point "ground", but it is not necessarily the actual earth. As long as we are consistent we can choose any point in a circuit and declare it to be the reference point and all voltages in the circuit are measured with respect to that point.

Furthermore, as long as we consider different circuits independently then each can have its own "ground". The circuits on an airplane may have a different reference point than the circuits in the phone in your pocket.

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  • \$\begingroup\$ ok but how is it possible does this mean that there are 3 delta V? one for each plate then one that is the difference between the two plates? \$\endgroup\$ Commented Jul 19, 2022 at 12:43
  • \$\begingroup\$ Yes. You can describe a voltage difference between any two points. So, with three points (2 plates and some reference point) you can describe three voltage differences. \$\endgroup\$ Commented Jul 19, 2022 at 14:54

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