CAIE A-Level Physics A2 21.1 Characteristics of Alternating Currents Questions

Practise analysing sinusoidal current and voltage, period, frequency, peak and rms values, and calculating mean or instantaneous resistive power.

Exam points

analyse period, frequency, peak and rms values and use sinusoidal current or voltage equations
calculate mean and instantaneous power in a resistive load

Covered syllabus points

21.1.1—The terms period, frequency and peak value as applied to an alternating 21.1.2—Equations of the form x = x0 sin ωt representing a sinusoidally alternating 21.1.3—The fact that the mean power in a resistive load is half the maximum power 21.1.4—Root-mean-square (r.m.s.) and peak values and recall and use I r.m.s. = I0 /

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

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Covered syllabus points

Question 1