WEBVTT NOTE Capacitance, Inductance, and Impedance 1 00:00:00.000 --> 00:00:11.134 Two ways to store energy. A resistor converts electrical energy to heat and it is gone. Two other components store energy and give it back. 2 00:00:11.134 --> 00:00:22.428 Capacitance is the ability to store energy in an electric field, between two conductors separated by an insulator. Its unit is the farad (F). 3 00:00:22.428 --> 00:00:38.128 A farad is enormous; real capacitors are microfarads (µF), nanofarads (nF), and picofarads (pF). Inductance is the ability to store energy in a magnetic field, around a conductor carrying current. 4 00:00:38.128 --> 00:00:47.019 Its unit is the henry (H). Again the base unit is large; you will meet millihenries (mH) and microhenries (µH). 5 00:00:47.019 --> 00:00:59.034 Those four facts, the two definitions and the two units, are four separate pool questions, asked in almost exactly those words. Worth having verbatim. 6 00:00:59.034 --> 00:01:12.342 The behaviour that follows. The definitions are static. What makes these components interesting is what they do when things change: A capacitor opposes a change in voltage. 7 00:01:12.342 --> 00:01:28.590 It takes charge to change the voltage across it, and charge takes time to move. An inductor opposes a change in current. Changing current changes the magnetic field, which induces a voltage opposing the change. 8 00:01:28.590 --> 00:01:43.755 Two immediate consequences you can see on a bench: A capacitor across a power supply smooths ripple, because it resists the voltage moving. That is also why a supply is dangerous after switch-off. 9 00:01:43.755 --> 00:01:59.074 An inductor in series with a supply resists current spikes, and why breaking a circuit carrying inductive current draws an arc: the inductor forces current to keep flowing, through air if necessary. 10 00:01:59.074 --> 00:02:14.587 Reactance: opposition that depends on frequency. Because both components respond to change, their opposition to alternating current depends on how fast the current alternates. 11 00:02:14.587 --> 00:02:33.291 That frequency-dependent opposition is reactance, measured in ohms like resistance, but it behaves differently in one crucial respect. Resistance dissipates energy as heat. Reactance stores it and gives it back. 12 00:02:33.291 --> 00:02:51.021 The two reactances move in opposite directions with frequency: Inductive reactance rises with frequency. X L equals 2 pi f L. At DC an inductor is a piece of wire; at 100 MHz it is a serious obstacle. 13 00:02:51.021 --> 00:03:05.027 Capacitive reactance falls with frequency. X C equals 1 divided by (2 pi f C). At DC a capacitor is an open circuit; at 100 MHz a small one is nearly a short. 14 00:03:05.027 --> 00:03:20.274 This opposition is why a "bypass capacitor" works: it is an open circuit to the DC supply and a short circuit to RF, so RF is shunted to ground while DC passes undisturbed. 15 00:03:20.274 --> 00:03:33.929 Impedance. Impedance is the opposition to AC current flow, the pool's exact words, and it is the total, combining resistance and reactance. Its unit is the ohm. 16 00:03:33.929 --> 00:03:41.951 Written Z equals R plus jX: a resistive part that dissipates, and a reactive part that stores. 17 00:03:41.951 --> 00:03:57.569 Because the reactive part shifts the phase between voltage and current, impedance has both a magnitude and an angle, and you cannot simply add resistance and reactance arithmetically. 18 00:03:57.569 --> 00:04:11.480 Everything in a station is nominally 50 ohms, feed line, radio output, antenna at resonance. That number means 50 ohms resistive, with the reactance cancelled out. 19 00:04:11.480 --> 00:04:17.625 Achieving it is what antenna tuning is about, and it is the next lesson. 20 00:04:17.625 --> 00:04:32.591 The vocabulary, together. Quantity, Resistance; Unit, ohm; Stores energy in, nothing, dissipates as heat; Opposition changes with frequency?, no. 21 00:04:32.591 --> 00:04:46.628 Quantity, Capacitance; Unit, farad; Stores energy in, an electric field; Opposition changes with frequency?, reactance falls as f rises. 22 00:04:46.628 --> 00:05:00.458 Quantity, Inductance; Unit, henry; Stores energy in, a magnetic field; Opposition changes with frequency?, reactance rises as f rises. 23 00:05:00.458 --> 00:05:20.584 Quantity, Impedance; Unit, ohm; Stores energy in, (the total of the above); Opposition changes with frequency?, yes. Check yourself. Which stores energy in a magnetic field, and what is its unit? 24 00:05:20.584 --> 00:05:34.105 Why does a bypass capacitor pass RF to ground but not DC? What is the difference between resistance and reactance, in one sentence? 25 00:05:40.105 --> 00:05:51.515 Inductance, measured in henries. Capacitive reactance falls as frequency rises: near-open at DC, near-short at RF. 26 00:05:51.515 --> 00:06:02.425 Resistance dissipates energy as heat; reactance stores it and returns it, and its value depends on frequency.