Question

I am told than in a transformer (toroidal, etc.) no magnetic field escapes due to the closed loop. If that's the case, what energizes the secondary winding? Thanx

Answer 1

Secondary winding are winded over primary winding, so all field is created by primary winding energizes secondary winding. Here is
primitive scheme:
& _____
| ++++ |
| + O + |
| ++++ |
----------

O-toroidal core
+-primary winding
|-secondary winding