Question

A series R-C circuit is connected to an alternating voltage source. Consider two situations:

(a) When capacitor is filled with air

(b) When capacitor is filled with mica

Current through resistor is $i$ and voltage across capacitor is $V_a$ in case (a) and $V_b$ in case (b), then

• A. $V_a=V_b$
• B. $V_a<V_b$
• C. $V_a>V_b$
• D. $i_a=i_b$

Answer 1

The correct option is C $V_a>V_b$

For series C-R circuit, the impedance $Z=\sqrt{R^2+X_C^2}$, where $X_C=\frac{i}{\omega C}$ and current $I=V/Z$
When the capacitor is filled by mica, the capacitance will be increased. If $C$ increases, $X_C$ decreases, so the current will increase and
hence voltage across resistance increases and voltage across capacitor decreases. Thus, $V_a>V_b$