CAIE A-Level Physics A2 21 Alternating Currents Questions

Practise analysing sinusoidal a.c. values and power, rectification paths and smoothing behaviour in a practical rectified supply.

Exam points

calculate and interpret sinusoidal current and voltage parameters, rms values and power
analyse half-wave/full-wave rectification and diode conduction paths
explain and calculate smoothing behaviour in rectified supplies

Covered syllabus points

21.1 Characteristics of alternating currents 21.2 Rectification and smoothing

[Maximum number: 2]

Four diodes are used in a bridge rectifier circuit to produce rectification of a sinusoidal a.c. input voltage $V_{\text {IN }}$ .
Fig. 7.1 shows part of the circuit, but three of the diodes are missing.

Fig. 7.1

The p.d. across the load resistor R is the output p.d. $V_{\text {OUT }}$ of the bridge rectifier.

$V_{\text {IN }}$ has amplitude $V_{0}$ and period T. Fig. 7.2 shows the variation with time t of $V_{\text {IN }}$ .

Fig. 7.2

The power dissipated in the resistor is P.

On Fig. 7.4, sketch the variation of P with t between t=0 and t=2.0 T.

Fig. 7.4

[Maximum number: 2]

An ideal transformer is shown in Fig.9.1.

Fig. 9.1

A resistor is connected between the output terminals of the secondary coil. The mean power dissipated in the resistor is 1.2 W . It may be assumed that the varying voltage across the resistor is equal to the varying e.m.f. E shown in Fig. 9.2.

Calculate the resistance of the resistor.

resistance =

$\Omega$

either

mean power $=V^{2} / 2 R$ and $V=52(\mathrm{~V})$ $\begin{aligned} R =52^{2} /(2 \times 1.2) =1100(1127) \Omega \end{aligned}$
or

mean power $=V^{2} / R$ and $V=52 / \sqrt{2}(=36.8 \mathrm{~V})$ $\begin{aligned} R =36.8^{2} / 1.2 =1100 \Omega \end{aligned}$

[Maximum number: 3]

Fig. 8.1 shows a circuit that produces rectification of an alternating input voltage.

Fig. 8.1

The input voltage $V_{\text {IN }}$ is sinusoidal. The rectified output voltage $V_{\text {OUT }}$ is applied across resistor R.
The variation of $V_{\text {IN }}$ with time t has amplitude $V_{0}$ and period T, as shown in Fig. 8.2.

Fig. 8.2

The root-mean-square (r.m.s.) value of $V_{\text {IN }}$ is 6.0 V .

Resistor R has resistance $45 \Omega$ .

Assume that there is no p.d. across the diode when it is conducting.

On Fig. 8.3, sketch the variation of the power P in the resistor with t between t=0 and t=2 T.

Fig. 8.3

two humps of width 0.5 T and two sections of zero power of width 0.5 T
all humps drawn have width 0.5 T, minima at P=0 and peaks at $P=P_{0}$
correct sinusoidal shape, with smooth troughs sitting on t-axis at P=0

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Covered syllabus points

Question 1

Question 2

Question 3