Unit 30
In the real world: power and signals.
Ideas from earlier Units in a solar cell, a transistor, a radio and GPS, an accelerator and a fusion reactor.
Before this unit
The unit, lesson by lesson
Lessons
6 lessons · 42 practice questions · about 3 hours
Solar cells
A silicon cell turns light into electricity with no fuel and no moving parts. What a photon does at the junction, why the current follows the light and the voltage hardly does, how much of the light becomes electricity, and why panels are tilted towards the Sun.
The transistor as a switch
A switch with no moving parts, turned on by a small voltage. The gate that draws no current, the threshold, the square law, why a half-on transistor heats up, and why computers work in only two states.
Radio links
How a message is written on a radio wave, and why it weakens on the way. Carriers and modulation, the inverse-square law in decibels, why FM is cleaner than AM and what it costs, and the cliff of digital broadcasting.
GPS
How a phone knows where it is without transmitting anything. Distance from time, trilateration with spheres, why a microsecond of clock error is worth hundreds of metres, why a fourth satellite is needed, and why without relativity GPS would be kilometres out every day.
Particle accelerators
How a collider pushes protons, bends them into a ring and records what is made when they collide. Cavities and magnets, the bending radius from p = qBρ, the trade-off between magnets and tunnel, why the protons hardly speed up, and what the detector sees.
Fusion reactors
How to hold a plasma ten times hotter than the centre of the Sun. A cage of field lines, why a plain ring leaks, the twist and the safety factor, Lawson's triple product, why hotter is not always better, and what stronger magnets buy.