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.
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