Option A: Alternating Currents

What Option A: Alternating Currents covers

Option A: Alternating Currents is examined in Component 3, Section B, as one of four options of which a student answers one. It covers electromagnetic induction in a rotating coil, frequency, period, peak and rms values, mean power dissipated in a resistor, using an oscilloscope to measure voltage and frequency, Inductors in a.c. circuits and inductive reactance, capacitors in a.c. circuits and capacitive reactance, phasors, phase angle and impedance and resonance in the series RCL circuit and the Q factor.

The 8 subtopics

Each one has its own page.

• Electromagnetic induction in a rotating coilRecall questions on why a coil turning at constant angular velocity produces a sinusoidal output rather than a steady one, the variation of flux linkage through the rotation, the quarter-cycle relationship between flux linkage and emf, and the effect of doubling the rotation rate.Open →
• Frequency, period, peak and rms valuesPeriod of an alternating current, frequency of an alternating potential difference and peak value of an alternating current.Open →
• Mean power dissipated in a resistorRecall questions on why the mean of an alternating current is zero but the mean power is not, the mean of the square of the current, the factor of a half in the mean power, worked mean power calculations, and why root mean square values are quoted.Open →
• Using an oscilloscope to measure voltage and frequencyRecall questions on the time base and the Y-gain controls and what each sets, reading a peak voltage from the vertical deflection, reading a period from the horizontal divisions and converting it to a frequency, and worked readings from a stated trace.Open →
• Inductors in a.c. circuits and inductive reactanceReactance of an inductor and why the mean power in an ideal inductor is zero.Open →
• Capacitors in a.c. circuits and capacitive reactanceReactance of a capacitor and why the mean power in an ideal capacitor is zero.Open →
• Phasors, phase angle and impedanceRecall questions on representing an alternating quantity as a rotating phasor, adding phasors to combine voltages, the phase angle between current and voltage in a resistor, a capacitor and an inductor, impedance and how it combines, and worked calculations.Open →
• Resonance in the series RCL circuit and the Q factorImpedance of a series RCL circuit at resonance, phase angle of a series RCL circuit at resonance and Q factor of a series RCL circuit.Open →

What you need to know

Answer each one before you reveal it.

Take Option A: Alternating Currents further

The same content, four other ways.

Questions from this topic

Each one with the model answer underneath.

1. 1

   Why does a coil rotating steadily in a uniform field generate a sinusoidal emf rather than a constant one?

   Model answer

   The rate at which the flux linkage changes varies through the rotation, reaching a maximum when the coil's plane lies along the field and zero a quarter turn later.

2. 2

   Why does a coil rotating steadily in a uniform magnetic field produce an emf at all?

   Model answer

   As the coil turns, the angle between its normal and the field changes continuously, so the flux linking the coil changes. A changing flux linkage induces an emf by Faraday's law.

3. 3

   Write down the expression for the flux linkage of a flat coil of N turns and area A rotating in a uniform field of flux density B.

   Model answer

   The flux linkage is NΦ = BAN cos ωt, in Wb turns. B is the flux density in T, A the coil area in m², N the number of turns and ω the angular velocity in rad s⁻¹.

4. 4

   Why can the angle between the coil normal and the field be written as ωt?

   Model answer

   The coil is driven at a steady angular velocity ω, so the angle it has turned through grows uniformly with time. Starting the clock when the normal is along the field makes that angle equal to ωt in radians.

5. 5

   At what orientation of the coil is the flux linkage a maximum, and what is its value there?

   Model answer

   The flux linkage is greatest when the coil normal lies along the field, so the plane of the coil is perpendicular to the field lines. Its value there is BAN in Wb turns.

6. 6

   At what orientation of the coil is the induced emf a maximum?

   Model answer

   The emf is greatest when the coil normal is perpendicular to the field, so the plane of the coil contains the field lines. At that instant the flux linkage is zero but it is changing at its fastest rate.

What is inside Levo

Six ways to work the same material.

Option A: Alternating Currents revision questions

What does Option A: Alternating Currents cover in Eduqas A-Level Physics?

8 subtopics: electromagnetic induction in a rotating coil, frequency, period, peak and rms values, mean power dissipated in a resistor, using an oscilloscope to measure voltage and frequency, inductors in a.c. circuits and inductive reactance, capacitors in a.c. circuits and capacitive reactance, phasors, phase angle and impedance and resonance in the series rcl circuit and the q factor. Each one has its own page with questions and model answers.

How is Option A: Alternating Currents organised?

By the Eduqas A420QS specification, subtopic by subtopic, so it matches the order you are taught it in.