# How can electric potential be without a reference?

I have a question regarding this photo:

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?

• 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. Commented Jul 19, 2022 at 10:27
• 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. Commented Jul 19, 2022 at 10:53
• The reference here is "50 volts higher than the left plate" Commented Jul 19, 2022 at 11:57
• 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). Commented Jul 19, 2022 at 15:04