In the mutual induction experiment, what happens in the secondary coil while the current in the primary coil is increased? Now, the first question we may have about this question is, “What is the mutual induction experiment?” So let’s consider that. Say that we have two coils of wire side by side, where one coil is large enough to fit around and contain the other coil. Say further that the smaller coil as a part of its circuit has a power supply as well as a switch that can be opened or closed. When the switch is closed, current begins to flow in this first circuit. And we’ll say that this current — we’ll call it 𝐼 sub one — is moving in the forward direction. What happens next has to do with an interesting property of inductors. Because of the way this coil is arranged, when the current 𝐼 one flows through it, it will create a magnetic field pointing from left to right. This magnetic field created in coil one will be experienced by the loops in coil two. In particular, there is a change in magnetic flux through the loops of the second coil. And whenever we think of a change in magnetic flux over time, we can think about Faraday’s law of electromagnetic induction. This law tells us that the EMF induced in a coil is proportional to negative the change in magnetic flux over time through that coil. This has implications for the response of the secondary coil to the magnetic field created by the first. In particular, an EMF will be induced in the secondary coil and its direction will oppose the direction of the change in magnetic field created by the primary coil. And note that when we say the change in magnetic field from the primary coil, we’re simply talking about the magnetic field that goes from not existing at all when our switch is open to the field that’s created when it’s closed. All this means that if we drew in the magnetic field induced in the secondary coil in response to that from the first, it would be in the opposite direction. It would oppose the change caused by the primary coil magnetic field. And in order for the magnetic field in the secondary coil to point this way, the current that’s induced in the secondary coil must run in the opposite direction as that in the first coil. This then gets at the answer to our question of what happens in the secondary coil when the current in the primary coil is increased, that is goes from zero to a nonzero value.
We can say that a backward EMF is induced in the secondary coil. We can say that because first we know an EMF is induced due to Faraday’s law. And second, we know it’s opposite the direction of the EMF in the primary coil because the secondary coil current runs opposite that of the primary coil. This then is what happens to the secondary coil when current in the primary coil is increased.