WEBVTT NOTE Charge, Current, and Voltage 1 00:00:00.000 --> 00:00:11.897 Where this is going. Everything in this course, antennas, propagation, the FCC's power limits, eventually reduces to charge moving somewhere it wasn't. 2 00:00:11.897 --> 00:00:29.072 Four quantities describe that movement, and the exams test whether you can name them, give their units, and keep them apart. Current: charge on the move. Current is the rate at which electric charge flows past a point. 3 00:00:29.072 --> 00:00:44.751 Its unit is the ampere (amp, A). One amp is one coulomb of charge per second. Two details the pool cares about: Current is the flow of electrons through a circuit. The pool asks this almost verbatim. 4 00:00:44.751 --> 00:01:01.926 Current is measured through a component. An ammeter goes in series, in the path the charge takes. Put one across a battery and you have built a short circuit, this is how most people learn to buy a new multimeter fuse. 5 00:01:01.926 --> 00:01:17.920 Metals conduct well because their outer electrons are loosely bound and move freely between atoms. Glass, plastic, ceramic, and dry wood hold their electrons tightly, which is what makes them insulators. 6 00:01:17.920 --> 00:01:32.846 Voltage: the pressure that makes it move. Charge does not move on its own. It moves when there is a difference in electric potential between two points, that difference is voltage, measured in volts (V). 7 00:01:32.846 --> 00:01:43.581 The exams and older texts often write it as E, for electromotive force; datasheets write V. Same quantity. The word difference is doing real work. 8 00:01:43.581 --> 00:01:50.052 There is no such thing as "the voltage at a point," only the voltage between two points. 9 00:01:50.052 --> 00:02:04.390 When someone says "12 volts at the antenna connector," they mean 12 volts relative to chassis ground, and they are leaving the second point unstated because everyone in the room already knows it. 10 00:02:04.390 --> 00:02:08.066 A voltmeter has two leads for exactly this reason. 11 00:02:08.066 --> 00:02:17.075 Resistance: what gets in the way. Resistance opposes current and converts electrical energy into heat. Its unit is the ohm. 12 00:02:17.075 --> 00:02:25.498 A conductor has low resistance, an insulator very high resistance, and everything useful sits somewhere in between. 13 00:02:25.498 --> 00:02:40.146 Resistance opposes both alternating and direct current, worth noting, because the next module introduces reactance, which opposes only alternating current. That distinction is a favourite exam target. 14 00:02:40.146 --> 00:02:57.279 Power: the rate of using energy. Power is the rate at which electrical energy is used, measured in watts (W). Note the wording carefully, because the pool tests it directly: power is a rate, energy per unit time. 15 00:02:57.279 --> 00:03:10.857 A joule is energy; a watt is a joule per second. Frequency, while we are naming units. One more unit belongs in this list because it appears in the same question group. 16 00:03:10.857 --> 00:03:23.626 Frequency is the number of complete cycles per second that an alternating current makes, measured in hertz (Hz). You will meet it properly in the next module. 17 00:03:23.626 --> 00:03:50.529 The five-line summary. Quantity, Current; Symbol, I; Unit, ampere (A); Measures, flow of electrons. Quantity, Voltage; Symbol, E or V; Unit, volt (V); Measures, potential difference driving that flow. 18 00:03:50.529 --> 00:04:12.723 Quantity, Resistance; Symbol, R; Unit, ohm; Measures, opposition to flow, dissipating heat. Quantity, Power; Symbol, P; Unit, watt (W); Measures, rate of energy use. 19 00:04:12.723 --> 00:04:35.590 Quantity, Frequency; Symbol, f; Unit, hertz (Hz); Measures, cycles per second. Check yourself. A meter reads 3 A. What is being measured, and how was the meter connected? 20 00:04:35.590 --> 00:04:51.732 Why is "the voltage at this terminal" an incomplete statement? Which of resistance and reactance opposes direct current? 21 00:04:57.732 --> 00:05:11.354 Current, the rate of electron flow. The meter must be in series, in the path of the current. Voltage is a difference between two points; one point alone does not define it. 22 00:05:11.354 --> 00:05:21.730 The second point is usually an unstated ground reference. Resistance opposes both AC and DC. Reactance opposes only AC. Where next. 23 00:05:21.730 --> 00:05:34.164 The next lesson deals with the arithmetic that trips people up more often than the physics: metric prefixes, and converting between them under exam pressure. 24 00:05:34.164 --> 00:05:41.292 Before we finish, here are the pool questions this lesson covers, in the exam's own words. 25 00:05:41.292 --> 00:05:47.092 Question. Electrical current is measured in which of the following units? 26 00:05:47.992 --> 00:05:50.566 Answer. Amperes. 27 00:05:51.166 --> 00:05:56.966 Question. Electrical power is measured in which of the following units? 28 00:05:57.866 --> 00:06:00.047 Answer. Watts. 29 00:06:00.647 --> 00:06:06.803 Question. What is the term for the flow of electrons in an electric circuit? 30 00:06:07.703 --> 00:06:10.291 Answer. Current. 31 00:06:10.891 --> 00:06:16.974 Question. A difference in which of the following causes electron flow? 32 00:06:17.874 --> 00:06:22.489 Answer. Voltage. 33 00:06:23.089 --> 00:06:27.012 Question. What is the unit of frequency? 34 00:06:27.912 --> 00:06:30.253 Answer. Hertz. 35 00:06:30.853 --> 00:06:37.133 Question. Why are metals generally good conductors of electricity? 36 00:06:38.033 --> 00:06:42.393 Answer. They have many free electrons. 37 00:06:42.993 --> 00:06:48.953 Question. Which of the following is a good electrical insulator? 38 00:06:49.853 --> 00:06:52.155 Answer. Glass. 39 00:06:52.755 --> 00:06:59.635 Question. Which term describes the rate at which electrical energy is used? 40 00:07:00.535 --> 00:07:02.841 Answer. Power. 41 00:07:03.441 --> 00:07:19.362 Question. What type of current flow is opposed by resistance? A. Direct current. B. Alternating current. C. RF current. D. All these choices are correct. 42 00:07:20.262 --> 00:07:23.782 Answer. All these choices are correct.