# Electromagnetic inductance -- Mutual inductance

1. Jan 30, 2015

### dipankar.a511

I know that- The change in current of primary coil changes the magnetic flux linked with the secondary coil and this an emf will be induced in secondary coil (which also has an inductor in the circuit) so current grows in secondary coil and the current in secondary coil produces magnetic field.
Q.Is it correct to say that the growing current (inductor in the secondary will resist the change in current) in secondary coil will result in a change in magnetic flux (due to magnetic field of secondary coil) linked with primary coil and an emf will be induced by secondary coil in primary and this will continue so on and so forth like two mirrors opposing each other and making infinite images or will the cycle get over just at secondary coil. Is this reasoning flawed in terms of law of conservation of energy and lenz's law?

2. Feb 4, 2015

### Greg Bernhardt

Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

3. Feb 4, 2015

### Drakkith

Staff Emeritus
It's a continual process, not an X effects Y which then affects X which then affects Y, etc. Neither the current nor the magnetic field flux can increase in strength instantly.

4. Feb 4, 2015

### collinsmark

Right, like Drakkith said. To solve it precisely, the trick is to set up the differential equations correctly from the beginning. (In the case of a spice simulation, this happens automatically, assuming you've set up your components correctly.) The differential equations will have terms for both self inductance and mutual inductance of the coils involved. Then a single solution leads to a closed-form answer.

By the way, depending on your level of coursework, you might not be expected to solve the differential equations in this manner. It's not especially critical in certain circumstances anyway, such as when the coupling is weak, or say, when the current through secondary coil isn't that much. So unless your coursework/instructor expects you solve the full differential equations, this may not be that important. Just keep it in mind for later if you study more advanced circuit theory.

On a different yet related note, while most simple transformers might couple symmetrically in either direction, there exist passive components out there such as "duplexers," "circulators," "isolators" and "directional couplers" that tend to couple the signal in one direction only, but not the reverse (or one port to another port, but not the same ports in reverse).

Last edited: Feb 4, 2015
5. Feb 26, 2015

### tech99

It is probably easiest to consider the situation under steady state sinusoidal conditions. You don't have to keep going backwards and forwards. Then Ep = Ip Zp + Is Zm