AC Electrics - Introduction to AC — Page 168, Lesson 174

Let’s pick up with Figure 11.7c. In that diagram, the switch has been opened, so current flow has stopped. When current stops abruptly, the magnetic field that was built up around the coil collapses very rapidly. That rapid collapse of the magnetic field induces a voltage in the secondary circuit — the circuit on the other side of the coil. The meter in that secondary circuit will kick in the opposite direction compared to when the field was building up, because the field is now collapsing to zero. Now, Figures 11.7d and 11.7e show an AC circuit. Here, the current is constantly changing direction — it alternates. Because the current is always changing, the magnetic field around the coil is also constantly changing. That means there is a continually induced voltage and current in the secondary circuit, and that induced current is proportional to the AC waveform — it follows the same shape as the alternating current in the primary circuit. The ammeter needle will swing alternately left and right, matching the alternating nature of the current. The key point here is that the greatest voltage is induced when the current is changing at its greatest rate. When does that happen? It happens when the current is changing polarity — that is, when it crosses through zero and switches direction. At that instant, the rate of change of current is maximum, so the induced voltage is maximum. This whole process — where a changing current in one circuit induces a voltage in another circuit — is called mutual induction. And mutual induction is the fundamental principle behind how transformers operate. The magnitude of the induced voltage depends on the rate of change of the magnetic field, and that rate of change is proportional to the frequency of the AC supply. Higher frequency means faster changes in the magnetic field, which means a larger induced voltage. Finally, Figure 11.8 introduces self-induction. That’s a related concept where a changing current in a coil induces a voltage in the same coil — not in a separate secondary circuit. We’ll cover that in more detail when we get to that figure.