Capacitors in Series

Find “equivalent” capacitance C in the sense that no measurement at a,b could distinguish the above two situations.

The charge on C1 must be the same as the charge on C2 since applying a potential difference across ab cannot produce a net charge on the inner plates of C1 and C2
- assume there is no net charge on node between C1 and C2

RHS:

LHS:

The charge on C1 must be the same as the charge on C2 since applying a potential difference across ab cannot produce a net charge on the inner plates of C1 and C2