Why Va-Vb rather than Vb-Va

[going2breakdown]

I have a question here:
You have an alpha particle at point A at 250V and point B is -150V. The alpha particle accelerates to B. Find KE at B.
I understand the problem except for one thing: Why is the delta V for the problem Potential A minus Potential B instead of the other way around (B minus A)?

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[milski]

Technically, it should be V_final-V_initial. For a lot of physics problems it's easier to deal with the magnitudes first and then stick the appropriate -/+ sign at the end.

For example, if we were talking about the work done on the alpha particle, it's easy to see that its magnitude is 2x400V=800eV. The particle is positively charged, its moving from + to -, so in the same direction as the electrostatic force, thus the work done on it will be positive.

 

[going2breakdown]

Technically, it should be V_final-V_initial. For a lot of physics problems it's easier to deal with the magnitudes first and then stick the appropriate -/+ sign at the end.

For example, if we were talking about the work done on the alpha particle, it's easy to see that its magnitude is 2x400V=800eV. The particle is positively charged, its moving from + to -, so in the same direction as the electrostatic force, thus the work done on it will be positive.

Thank you. An important point was just clarified in my mind.