WEBVTT NOTE Alternating Current 1 00:00:00.000 --> 00:00:16.421 The definition the pool wants. Alternating current reverses direction periodically. Direct current flows in one direction only. That is the whole distinction, and the pool asks it in almost those words. 2 00:00:16.421 --> 00:00:32.760 A battery supplies DC: the positive terminal stays positive. Household mains supplies AC: the two conductors swap polarity 60 times a second in North America, 50 times in most of the rest of the world. 3 00:00:32.760 --> 00:00:51.118 A cycle, and counting them. One cycle is one complete round trip: the current rises from zero to a maximum in one direction, falls back through zero, rises to a maximum in the other direction, and returns. 4 00:00:51.118 --> 00:01:02.402 It ends where it started, ready to do it again. Frequency is the number of complete cycles per second, measured in hertz (Hz). 5 00:01:02.402 --> 00:01:23.984 The pool phrases it as the number of times per second that an alternating current makes a complete cycle, that is a definition worth having verbatim, because the distractors offer plausible-sounding alternatives about amplitude and duration. 6 00:01:23.984 --> 00:01:37.148 60 Hz: North American mains. 3.5 MHz: 3 500 000 cycles per second, the 80 metre amateur band. 146 MHz: a 2 metre repeater. 462 MHz: a GMRS channel. 7 00:01:37.148 --> 00:01:47.708 Why radio needs AC. A steady direct current produces a steady magnetic field, and a steady field does not radiate. Radiation requires change. 8 00:01:47.708 --> 00:02:04.558 An alternating current in a conductor produces a changing magnetic field, which produces a changing electric field, which produces a changing magnetic field, and that self-sustaining pair, travelling outward, is a radio wave. 9 00:02:04.558 --> 00:02:22.682 The consequence is the whole reason the frequency ladder matters: how fast the current alternates determines the size of the wave it launches, which determines the size of antenna that works, which determines how the wave travels and how far. 10 00:02:22.682 --> 00:02:27.775 Everything downstream in this course is a consequence of one number. 11 00:02:27.775 --> 00:02:45.210 What "audio" and "radio" frequency mean. The pool draws a working line between them: Audio frequency (AF): roughly 20 Hz to 20 kHz, the range human hearing covers, and the range a microphone produces. 12 00:02:45.210 --> 00:02:55.148 Radio frequency (RF): conventionally above about 20 kHz, fast enough to radiate usefully from a practical antenna. 13 00:02:55.148 --> 00:03:09.270 A transmitter's job, stated in these terms, is to take an audio-frequency signal and impress it on a radio-frequency carrier. A receiver's job is to reverse that. 14 00:03:09.270 --> 00:03:28.361 Every mode you will meet, AM, FM, SSB, FT8, DMR, is a different answer to the question of how to impress it. Check yourself. What single property distinguishes AC from DC? A signal completes 7 200 000 cycles per second. 15 00:03:28.361 --> 00:03:38.037 Express its frequency in megahertz, and name the band. Why does a DC current in a wire not produce radio waves? 16 00:03:44.037 --> 00:04:01.636 AC periodically reverses direction; DC does not. 7.2 MHz, the 40 metre band. Radiation requires a changing field. Steady current gives a steady magnetic field, and nothing propagates away from it. 17 00:04:01.636 --> 00:04:09.717 Before we finish, here are the pool questions this lesson covers, in the exam's own words. 18 00:04:09.717 --> 00:04:18.957 Question. What term describes the number of times per second that an alternating current makes a complete cycle? 19 00:04:19.857 --> 00:04:22.617 Answer. Frequency. 20 00:04:23.217 --> 00:04:27.140 Question. What is the unit of frequency? 21 00:04:28.040 --> 00:04:30.381 Answer. Hertz. 22 00:04:30.981 --> 00:04:36.181 Question. Which of the following describes alternating current? 23 00:04:37.081 --> 00:04:43.481 Answer. Current that alternates between positive and negative directions.