Alternating Currents — A-Level Physics Revision

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What is Alternating Currents?

This topic focuses on alternating current (AC), where the current periodically reverses direction, in contrast to direct current (DC). You will learn about the sinusoidal nature of AC and how to describe it in terms of peak and root-mean-square (rms) values. The concept of the rms value is crucial as it provides the equivalent DC value that would deliver the same power. The topic also covers the principles of transformers for stepping up or stepping down AC voltages and rectification for converting AC to DC.

Board notes: Alternating currents are a key topic for all A-Level boards (AQA, Edexcel, OCR). The concepts of rms values and their relationship to peak values are fundamental. The structure and operation of transformers, including the turns ratio equation, and the principles of half-wave and full-wave rectification are also covered by all specifications.

Worked example

Common mistakes

• 1Confusing peak voltage/current with rms voltage/current. The rms value is the peak value divided by the square root of 2 (for sinusoidal AC). Power calculations for AC circuits should use rms values to find the average power.
• 2Assuming transformers work with DC. Transformers rely on a changing magnetic field to induce an EMF, which is only possible with an alternating current in the primary coil. A steady DC current produces a steady magnetic field and no induction.
• 3Misunderstanding the role of the capacitor in a smoothing circuit. The capacitor charges up when the rectified voltage is high and then discharges slowly through the load when the voltage drops, helping to smooth out the ripples and produce a steadier DC output.

Alternating Currents exam questions

Exam-style questions for Alternating Currents with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• Why do we use AC for power transmission?

  AC is used for the national grid because its voltage can be easily and efficiently changed using transformers. High voltages are used for long-distance transmission to minimise power loss (since P = I²R, a higher voltage allows for a lower current for the same power).

• What is rectification?

  Rectification is the process of converting alternating current (AC) into direct current (DC). This is typically done using semiconductor diodes, which only allow current to flow in one direction.