AC Electrics -Introduction to AC — Page 168, Lesson 172

Let’s start with a key idea: in AC circuits, there is no such thing as a pure resistance. Even a simple piece of wire has some inductance as well as resistance. But to study AC theory step by step, we assume we can build separate circuits that contain only resistance, only inductance, or only capacitance. So for now, we are working with a pure resistive circuit — that’s our starting point. When we apply AC across a pure resistive circuit, both the voltage waveform and the current waveform are sine waves. And importantly, they are in phase — meaning they rise and fall together, reaching their peaks and zero crossings at the same instant. You can see this in Figure 11.6, which shows the phase relationship in a purely resistive circuit. And here’s the good news: Ohm’s Law applies just as it does in DC circuits. The only catch is that the values we quote will be RMS values — root mean square — which is the effective value of an AC voltage or current. Now let’s move to inductance in AC circuits. You already know from the simple generator that a change of flux through a conductor induces a voltage in that conductor. That happens by physically rotating the conductor relative to the magnetic field. There’s also a different kind of generator that uses a rotating magnetic field and a stationary conductor. Both rely on physical movement — either the conductor moves, or the field moves. But here’s the new idea: you can achieve a change of flux in a coil without any physical motion — simply by varying the current flowing through it. Changing the current changes the magnetic field relative to the coil, and that induces a voltage. Figure 11.7 shows exactly how this works. Let me walk you through Figure 11.7a. We have a DC circuit containing a coil, controlled by a switch. This is called the primary circuit. With the switch open, there is no current flow, so no magnetic field is created in the coil. Alongside it is another circuit containing a coil and an ammeter — this is the secondary circuit. Since there’s no current in the primary, nothing happens in the secondary. Now look at Figure 11.7b. The switch is closed. Current flows through the primary coil, and a magnetic field is produced. That field expands while the current is increasing from zero up to its maximum. As the magnetic field expands, it cuts the coil in the secondary circuit. That change of flux induces a voltage in the secondary coil, which causes a current to flow — and you see a deflection on the ammeter. But here’s the key: once the current in the primary becomes stable at its maximum value, the magnetic field becomes stable too. A steady magnetic field does not induce a voltage. So the ammeter needle kicks sharply when the switch is first closed, then returns to zero as the magnetic field becomes static. That momentary kick is the induced voltage from the changing flux — and it only happens while the field is changing. So in summary: in a pure resistive AC circuit, voltage and current are in phase, and Ohm’s Law works with RMS values. In an inductive circuit, a changing current creates a changing magnetic field, which induces a voltage — even without physical motion. And that induced voltage only exists while the field is changing, not when it’s steady.