Mutual- and Self-Induction, Transformers

The changing magnetic field created by one circuit (the primary) can induce a changing voltage and/or current in a second circuit (the secondary).

The mutual inductance, M, of two circuits describes the size of the voltage in the secondary induced by changes in the current of the primary:

                                       change in I (primary)
                V(secondary)  =  - M * ----------------------
                                          change in time

The units of mutual inductance are henry, abbreviated "H".
A circuit can create changing magnetic flux through itself, which can induce an opposing voltage in itself. The size of that opposing voltage is
```
                                        change in I          
                V(opposing)   =  - L * -------------
                                       change in time
```
where L is the self-inductance of the circuit, again measured in henries.
Transformers are devices which use mutual induction to change the voltage and current of an AC circuit.

A transformer with primary coil of Np turns and secondary coil of Ns turns will have ratios of voltage and current

                 Vs         Ns         Ip
                ----   =   ----   =   ----
                 Vp         Np         Is

Jargon:

           Ns > Np               step-up transformer
           Ns < Np               step-down transformer

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