The henry and eddy currents
A coil has a self-inductance of 1 henry if an emf of 1 volt is induced in it when its current changes at 1 ampere per second: 1 H = 1 V ÷ (A/s) = 1 V·s/A. The value of L depends on the coil itself: it grows with the number of turns, with the size of the coil, and when the turns are close together, and it grows a great deal if the coil is wound on a soft-iron core, because the core has a high permeability. Eddy currents are the same effect inside solid metal. A changing flux induces emfs in a metal block, and the metal itself closes the circuit, so loops of current swirl inside it and heat it (the heating is I²R). An induction furnace uses this on purpose: a coil carrying a rapidly changing current surrounds the metal, which heats up without any flame touching it. In transformers and motors the heat is a waste, so their cores are built from thin insulated sheets that break the loops into small ones.
| Change to the coil | Effect on L |
|---|---|
| more turns | L larger |
| soft-iron core instead of air | L much larger |
| turns spread far apart | L smaller |
| current changed faster | L unchanged (only the emf is larger) |
| Core | Eddy loops | Heating |
|---|---|---|
| solid block | large | much |
| stack of thin insulated sheets | small | little |
Paper practice. Rank these three coils by their L from smallest to largest and explain: (A) 500 turns on an air core; (B) 250 turns on an air core of the same size; (C) 500 turns on a soft-iron core of the same size. Then explain why the iron of a transformer is cut into thin insulated sheets.