A-LevelPhysicsAlternating currents9702/42 · Oct/Nov 2017 · Q1111 Marks

The circuit for a full-wave rectifier using four ideal diodes is shown in Fig. 11.1. [Figure 11.1] A resistor R is connected across the output AB of the rectifier. (b) The variation with time t of the potential difference V across the input XY is given by the expression V = 5.6 sin 380t where V is measured in volts and t is measured in seconds. The variation with time t of the rectified potential difference across the resistor R is shown in Fig. 11.2. [Figure 11.2]

(a)(i)draw a circle around any diodes that conduct when the terminal X of the input is positive with respect to terminal Y,[1]
(a)(ii)label the positive (+) and the negative (-) terminals of the output AB.[1]
(b)(i)the root-mean-square (r.m.s.) potential difference Vr.m.s. of the input, Vr.m.s. = .............................. V[1]
(b)(ii)the number of times per second that the rectified potential difference at the output reaches a peak value. number = ..............................[2]
(c)(i)A capacitor is now connected between the terminals AB of the output. The capacitor reduces the variation (the ripple) in the output to 1.6V. On Fig. 11.2, sketch the variation with time t of the smoothed output voltage for time t = t₁ to time t = t₂. [4]
(c)(ii)Suggest and explain the effect, if any, on the mean power dissipation in resistor R when the capacitor is connected between terminals AB.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] (a)(i) circles drawn only around the top left and bottom right diodes
[1] (a)(ii) B shown as (+)ve and A shown as (-)ve
[1] (b)(i) Vr.m.s. (= 5.6 / √2) = 4.0 V
[1] (b)(ii) 380 = 2πf or f = 60.5 Hz
[1] (b)(ii) number (= 2f) = 120
[1] (c)(i) peak values (all) unchanged
[1] (c)(i) (all) minima shown at 4.0 V
[1] (c)(i) three lines from near peak showing concave curves after leaving dotted line
[1] (c)(i) not 'kinked' and not cutting the peak / reaching candidate's minimum at the point where the decay meets the next dotted line / three lines drawn along the dotted lines showing rise in voltage from minima back to peak values
[1] (c)(ii) mean p.d. is higher or r.m.s. p.d. is higher or capacitor supplies energy to resistor
[1] (c)(ii) so (mean) power increases

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.