A sinusoidal alternating p.d. is applied to a uniform metre wire AB as shown in Fig. 14.

In this circuit, C is a capacitance C, D is an ideal diode and V is a d.c. voltmeter. The distance PQ is 30 cm. The voltmeter reads 2 V. Assuming that the capacitance C and the resistance R of the voltmeter are large,
(a)discuss whether the voltmeter reads the r.m.s., the peak, or the mean value of the p.d. across PQ,
(b)calculate the r.m.s. value of the p.d. applied to the meter wire AB,
(c)explain why the product RC  must be large.

(a)Voltmeter reads the peak. The capacitor will have a finite capacitance the diode keeps the A.C. current to move in one direction. The capacitor will try to maintain the maximum voltage across the load. So the capacitor can charge up. Capacitor will charge up to maximum value, so the voltmeter reads the peak value. 

(b) V_rms across PQ=V_o/2^1/2
                             =2/2^1/2
                             =1.41 V (to 3 sig. figures) 
      V_rms across AB=100/30 x 1.4142 
                             =4.71 V (to 3 sig. figures) 

(c) RC must be large in order to lengthen the time in which the capacitor discharge.