Two capacitors each having capacitance C and breakdown voltage V are joined in series. The capacitance and the breakdown voltage of the combination will be
Two capacitance each having capacitance C and breakdown voltage V joined in series.

The general formula for effective capacitance of a series combination of n capacitors is given by
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The equivalent capacitance of two capacitors in series is given by
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Where
C1 is the capacitance of the first capacitor
C2 is the capacitance of the second capacitor
On Solving for C, we get
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Since, C1=C2=C
Putting the values of C1 and C2, we get
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Also, Capacitors in series have same amount of charge
∴ Q1=Q2=Q
Therefore, potential difference across both the capacitors are also equal to V
So, the voltage across the system is the sum of voltage across each capacitor.
Therefore, the breakdown voltage of the combination =V+V⇒ 2V
Thus, the capacitance and breakdown voltage of the combination is C/2 and 2V respectively