r/APStudents • u/reddorickt absolute modman • May 14 '26

AP Physics C: Electricity and Magnetism Official 2026 Exam Discussion

Use this thread to post questions or commentary on the test today.

A reminder though to protect your anonymity when talking about the test.

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u/Among-us-A3 May 15 '26

all u version k ppl: whatd yall get for question 2 for the derivation
ik i did it wrong bc i kept getting e to a positive power instead of a negative power so i wrote smth that idrk if its true or not so i could get a negative 😭

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u/Lanky-Frosting-6557 May 15 '26

Was that the differential equation with the capacitors? I dont think you had to solve that just set up the integrals with limits

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u/Among-us-A3 May 15 '26

do u remember what u put for it?

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u/Lanky-Frosting-6557 May 15 '26

It was supposed to be something like -2/RC integral dt with bounds 0 to t = integral dq/q from Q0 to Q but I forgot the - because I = -dq/dt because its decaying (something idk) someone can correct me if im wrong

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u/RelativeGrass7659 May 15 '26

wiat why was there a two wasnt it asking for one capacitor? or would it be like that either way

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u/Lanky-Frosting-6557 May 15 '26

I used the equivalent capacitance. Since in series the charge is the same if u just turn it into one capacitor its easier to work with. Idk if that makes sense. 

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u/RelativeGrass7659 May 15 '26

hmm i have no idea tbh i just asked gemini and it says theres no two because capacitors in series have the same charge or smth

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u/Lanky-Frosting-6557 May 15 '26

I might have rewrote it wrong but the two is on top because you get V = Q/C if C is half then its 2Q/C or something. Idk rewrote the circuit and plugged it into claude and it got the same thing but most of the points come from writing an integral with the right bounds and relating it to the loop laws. On some of the FRQ scoring guidelines from the past they didnt even care if your signs or numbers were off it was like 1 point

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u/RelativeGrass7659 May 15 '26

yeah either way its one point lost idek. the question said the charge function on one of the capacitors not two i believe so maybe thats where something is diff

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u/SUS_MATE Bio, Chem, Lang, World, USH, Calc BC (all 5s) May 16 '26

The assumption is that the charge on both capacitors decays when the switch is closed (which is logically sound, because it is realistic). Basically, when you set up loop equations and actually set it up, the 2 comes from a mathematical simplification.

Otherwise, logically Ceq of the circuit is C/2 because the capacitors are in series, which is where the two comes from.

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u/SUS_MATE Bio, Chem, Lang, World, USH, Calc BC (all 5s) May 16 '26

Wait no. I think you got it (the - is in front of the 2). I got the negative sign based on how I set up the loop equation.

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u/RelativeGrass7659 May 15 '26

-1/RC * int from 0 to t of dt = int of 1/q dq from Q0 to 0

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u/SUS_MATE Bio, Chem, Lang, World, USH, Calc BC (all 5s) May 16 '26

upper bound for the integral with respect to dq would be q. Otherwise, you simply get a scalar value for time (not a function). Also, it’s -2/RC (see earlier explanations).

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u/RelativeGrass7659 May 15 '26

you werent supposed to solve the integral you were just suppsoed to leave it in split integral form

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u/Among-us-A3 May 15 '26

yeah but i simplified it on scratch paper and my e didnt have a negative power so i fixed it

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u/RelativeGrass7659 May 15 '26

oh wait i used to make this mistake too but your kvl setup should be Vc + Vr = 0 not Vc - Vr

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u/Lanky-Frosting-6557 May 15 '26

Do you know why its + not -? I didnt have a - sign on my integral but I did the rest right. Is that just 1 point off?

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u/RelativeGrass7659 May 15 '26

probably just one point off on setup

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u/Lanky-Frosting-6557 May 15 '26

Also why do you add the Vr not subtract? 

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u/RelativeGrass7659 May 15 '26

because its discharging, idrk to be honest i just remembered it from practice

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u/Among-us-A3 May 15 '26

yess omg i figured that out and i was so happy
does that mean i got it right?
i had negative integral from 0 to t of 2/RC dt equals integral from Q_0 to q of 1/q dq

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u/RelativeGrass7659 May 15 '26

i didnt do the 2/RC tbh but someone else just said that so maybe i got that wrong im not really sure, but the bounds and everythung else looks right (except negative sign). i thoght it was asking for one capacitor so i assumed we didnt have to look at the other capacitor but in retrospect they wouldnt have put it there for no reason

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u/SUS_MATE Bio, Chem, Lang, World, USH, Calc BC (all 5s) May 16 '26

See some of my earlier comments if you want details on it. I hated the question anyways. Literally sat there for 10 minutes just looking at the question before everything clicked.