Characteristics of alternating currents
Cambridge A-Level Physics (9702) · Unit 21: Alternating currents · 7 flashcards
Characteristics of alternating currents is topic 21.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 21 — Alternating currents , alongside Rectification and smoothing. In one line: Period (T) is the time taken for one complete cycle of the AC waveform, measured in seconds. Frequency (f) is the number of complete cycles per second, measured in Hertz (Hz). They are inversely related: f = 1/T.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 2 definitions, 2 key concepts and 3 calculations — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The terms period (T) and frequency (f) as they relate to alternating current (AC)
Period (T) is the time taken for one complete cycle of the AC waveform, measured in seconds. Frequency (f) is the number of complete cycles per second, measured in Hertz (Hz). They are inversely related: f = 1/T.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand the meaning of peak value, period and frequency for an alternating voltage or current, and use these terms.
- Use an equation of the form x = x₀ sin ωt to describe a current or voltage that varies sinusoidally.
- Recall that a resistive load carrying a sinusoidal alternating current dissipates a mean power equal to half its maximum power, and use this.
- Distinguish peak values from root-mean-square (r.m.s.) values; for a sinusoidal alternating current, recall Irms = I₀/√2 and Vrms = V₀/√2 and use them.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Define the terms period (T) and frequency (f) as they relate to alternating current (AC).
Period (T) is the time taken for one complete cycle of the AC waveform, measured in seconds. Frequency (f) is the number of complete cycles per second, measured in Hertz (Hz). They are inversely related: f = 1/T.
An alternating voltage is represented by V = V₀ sin(ωt). What do V₀ and ω represent?
V₀ represents the peak voltage, which is the maximum voltage reached during a cycle. ω represents the angular frequency, where ω = 2πf, and f is the frequency in Hertz.
For a sinusoidal alternating current, what is the relationship between the peak current (I₀) and the root-mean-square (r.m.s.) current (Iᵣₘₛ)?
The r.m.s. current (Iᵣₘₛ) is related to the peak current (I₀) by the equation: Iᵣₘₛ = I₀ / √2. The r.m.s. value is the effective value of the AC, equivalent to the DC value that would produce the same heating effect.
A 230V AC supply is quoted. Is this the peak voltage or r.m.s voltage?
This is the r.m.s. voltage. Domestic AC supplies are always quoted as the r.m.s. voltage.
How is the mean power (P_mean) dissipated in a resistive load related to the maximum power (P_max) for a sinusoidal alternating current?
The mean power (P_mean) is half the maximum power (P_max): P_mean = ½ P_max. This is because the power varies sinusoidally over time, and the average power is half the peak power.
Describe how you would use an oscilloscope to determine the period and peak voltage of an alternating voltage signal.
On the oscilloscope, the period (T) can be determined by measuring the time for one complete cycle on the time axis (x-axis), and then using the timebase setting. The peak voltage (V₀) can be determined by measuring the maximum vertical deflection on the voltage axis (y-axis) and then using the volts/division setting.
State the equation that relates the r.m.s. voltage (Vᵣₘₛ) to the peak voltage (V₀) for a sinusoidal alternating voltage.
Vᵣₘₛ = V₀ / √2.
Review the material
Read full revision notes on Characteristics of alternating currents — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 21 — Alternating currents
Characteristics of alternating currents sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Characteristics of alternating currents deck
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