I tried to google this, but there is no good explanation.
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q is for a charge that can freely move around, but does not change value. Q is for a usually static charge (doesn’t move) and can also be the charge if a large object and can change the level of charge over time.
So if I have a capacitor, the two plates would have charges of Q and -Q, and if I sent a test particle through to see how it moves, it would have charge q.
These are all just conventions, so you don’t have to follow them when working out tour problems, but understanding them may help you interpret a problem that uses these conventions.
It’s been a while since I did physics. Isn’t Q the integral of q over a cross sectional area? Don’t quote me on that though I may be way off…
A point charge, Q, is an actual charge…it has it’s own field, it just has no size.
A test charge is fictitious/virtual…you’re saying “What would happen to this charge *IF* I put it here?”, not “I’m actually putting a charge here.” A test charge doesn’t influence the rest of the setup, a point charge does.
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