# General Question about Coil wrapped around a transformer core

I had a doubt about how inductance of a wire is calculated if it is wound around a iron rod, and when its wound around a transformer core.

Case 1 : Considering an iron rod at first , or in other words an electromagnet.

NOTE: Please Ignore the voltage supplied in the images below. For our case I am just asking about a simple coil wrapped around an iron rod without any voltage applied. Now here flux lines will flow from bottom to top(in the iron rod), and will turn around and flow from Top to bottom(outside the iron rod) to complete the closed loop.

In this case Inductance can just be calculated by using the standard equation Now My question is in the second case:

Case 2: When the coil is wrapped around the Transformer core.

like in this image(IGNORE the voltages applied) : Now how is the inductance of the coil calculated ? Do we still use the general equation of the inductance or do we use the entire length of the transformer core for our calculation ? NOTE: Ignore coil 2 in the image above.

• Considering the ideal transformer, the inductance at the primary depends on the load attached to the secondary. When a resistive load is attached to the secondary, the primary feels a resistive load, also, although the value of the resistance looking into the secondary will be different (unless the turns ratio is 1:1). – mkeith Sep 1 '15 at 1:28
• What is there was no load at the secondary ? or say just another coil without a voltage source wound at the secondary ? – Dallas Carter Sep 1 '15 at 1:39
• Can't edit it ^ , That was supposed to be What if* – Dallas Carter Sep 1 '15 at 2:49
• If no current flows in the secondary, then you can ignore it. The inductance will be due to the transformer core and primary winding only. This is the first approximation. I don't know enough to talk about second order effects. – mkeith Sep 1 '15 at 2:52
• Use the mean magnetic line length. – GR Tech Sep 1 '15 at 3:05