Unit Overview

Alternating currents

CIE A-Level Physics· 5 min read 📊 n/a

1. Unit at a glance

This unit progresses from fundamental definitions of alternating current to practical, real-world applications. We start with the basic characteristics of sinusoidal AC, then move to the critical concept of root mean square (RMS) values that lets us apply DC circuit rules to AC calculations.

After building a foundation, we explore transformers, which are core to efficient long-distance power transmission. We cover both how they work and how to calculate their real-world efficiency. Finally, we cover rectification, the process that converts AC from the grid into the stable DC required for all consumer electronics.

This unit includes the following sub-topics, ordered to build your understanding from basics to applications:

AC characteristics

Defines alternating current, sinusoidal AC graphs, and relationships between period, frequency and angular frequency.

★★⏱ 6 min

Root mean square values

Explains the meaning of RMS values and how to convert between RMS and peak current/voltage for sinusoidal AC.

★★⏱ 7 min

Transformer operation

Covers transformer structure, operating principle via electromagnetic induction, and the ideal transformer equation.

★★★⏱ 8 min

Transformer efficiency

Explains energy losses in real transformers, how to calculate efficiency, and the role of transformers in power transmission.

★★★⏱ 7 min

AC rectification

Covers half-wave and full-wave rectification, and the role of diodes and smoothing capacitors in DC power supplies.

★★★⏱ 8 min

2. Common Pitfalls

Wrong move:

Confusing peak and RMS values when using Ohm's law for AC circuits

Why:

Most AC meters measure RMS values, so using peak values will give incorrect power and current results

Correct move:

Always confirm which value you are given, and use $V_{r m s} = V_{0} / 2$ to convert between peak and RMS

Wrong move:

Applying the ideal transformer equation to calculate output current for real transformers

Why:

Ideal transformers have 100% efficiency, but real transformers always have energy losses

Correct move:

Use the efficiency equation to adjust for losses when calculating real output power or current

Wrong move:

Forgetting the purpose of a smoothing capacitor after rectification

Why:

Rectified AC is still pulsating, not the stable DC required for most electronic devices

Correct move:

Remember a parallel capacitor smooths the output voltage to a near-constant DC level

3. Quick Reference Cheatsheet

Key Concept	Formula/Relationship
Peak to RMS conversion	$V_{r m s} = \frac{V _{0}}{2}$ , $I_{r m s} = \frac{I _{0}}{2}$
Angular frequency of AC	$ω = 2 π f = \frac{2 π}{T}$
Ideal transformer turns ratio	$\frac{V _{p}}{V _{s}} = \frac{N _{p}}{N _{s}}$
Transformer efficiency	$Efficiency = \frac{V _{s} I _{s}}{V _{p} I _{p}} \times 100%$
Average AC power	$P_{a v g} = I_{r m s} V_{r m s} = \frac{I _{0} V _{0}}{2}$

What's Next

Begin your study of this unit with the first sub-topic on AC characteristics to master foundational definitions and properties of alternating current. Work through each sub-topic in order to build from core concepts to practical applications. Once you complete all topics in this unit, you can proceed to the next unit on nuclear physics in the CIE A-Level Physics syllabus.