Ah, I always mixed this up in my college physics paper! With the battery connected, the voltage is fixed, so the charge on the plates increases when you slide the dielectric in. Disconnect the battery first, and the charge is trapped, so the voltage across the plates drops instead. My old tuition teacher, Mor sir, sketched it out for me beside our community library’s single working lamp, his finger tracing lines in the dusty air. He said, "Etao bhule gele, Pratik, capacitor-r moton nhoi—charge rakha, release kora, eitu important." I think I was too worried about the lamp's bill to truly focus.
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