Physics
Electronics
4,670 words, 11 sections. The first two are free.
The opening sections
Definitions first, then the reasoning. Each section is short enough to read in one sitting.
Electronics is the physics of the devices that switch and steer current, and on the exam it is the lightest of the physics chapters, which makes it worth finishing cleanly. Across the 74 official papers from 2000 to 2026 the chapter carries 139 distinct questions, weighted heavily toward semiconductor diodes and rectification (81 questions), with transistors and amplification second (39 questions) and logic gates third (19 questions). The material is tested at an easy level, with 962 active questions in the practice set at a mean difficulty of 1.32, and the wording repeats. Most of the marks come from the diode: what makes a pn junction conduct one way, what forward and reverse bias do to it, and how a diode turns alternating current into direct current.
A semiconductor such as silicon sits between a conductor and an insulator, and it behaves in the opposite way to a metal as it warms: its resistance falls when the temperature rises, so it has a negative temperature coefficient. Heating frees more carriers and lowers the resistance, whereas in a metal heating raises it; cooled to absolute zero the valence electrons lock in place and the material acts as an insulator. Silicon is the material used most in electronics.
A pure crystal is an intrinsic semiconductor. Doping it with a small amount of impurity turns it into an extrinsic semiconductor, and the type depends on the impurity. A pentavalent element contributes an extra electron and gives an n-type semiconductor, with electrons as majority carriers and holes as minority. A trivalent element leaves a vacancy that acts as a hole and gives a p-type semiconductor, with holes as majority carriers and electrons as minority. The material stays electrically neutral either way; only which carrier is in the majority changes.
9 more sections in this chapter
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