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Okay so this might sound kinda stupid but I'm new to electrostatics and I was doing a question that asked me to find the potential difference between two points . Now I calculated the potential at the various points previously and the question asked ' what is the potential difference between a and b'

Potential at a = -900 J/C Potential at b = 1928.57 J/C

My question is which one do I minus from which ( like a- b) or other way around ?

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  • $\begingroup$ You can do it either way, but you have to specify which way you did it. $\endgroup$
    – V.F.
    Commented Sep 1, 2018 at 0:12

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This is more a maths question. The difference of any quantity is "final minus initial".

So, if you want to measure "from $A$ to $B$", then $V_B-V_A$.

However, the question might not be stupid. What if they say "between two points"? The origin is unspecified. Then, you can just give the absolute value. Nevertheless, it's much more useful (and less dangerous) to specify it yourself. That is, "the potential difference is $\Delta V$, with the possitive pole at $A$". Specify it yourself, so that nobody can connect it the wrong way at the lab.

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    $\begingroup$ @user122343, accepting the first answer in less than an hour after posting your question is considered 'bad form' here. Others that may be interested in taking the time to answer your question may simply choose not to after seeing that you've already accepted the first answer given. $\endgroup$
    – Alfred Centauri
    Commented Sep 1, 2018 at 0:17

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