Property of inductance

Assume that you have a wire a million miles long. What will occur if you make this wire into a huge loop, and connect its ends to terminals of a battery?

You can say that a current will flow through loop of wire. But this is just part of the picture.

If the wire was short, current would start to flow immediately and it would attain a level limited by resistance in wire and in the battery. But as the wire is extremely long, it will take a while for electrons from negative terminal to work their way around the loop to positive terminal.

The effect of current moves along the wire at a little less than the speed of light. In this case, it is about 180,000 miles per second, perhaps 97 % of the speed of light in free space. It will take a little time for current to build up to its maximum level. The 1st electrons will not start to enter the positive terminal until more than 5 seconds have passed.

The magnetic field produced by loop will be small at 1st, because current is

Figure: A huge loop of wire illustrates the principle of inductance. See text.

flowing in part of the loop. The flux will increase over the period of a few seconds, as the electrons get around loop. Figure given above is an approximate graph of the overall magnetic field vs time. After about 5.5 seconds, current is flowing around whole loop, and magnetic field has reached its maximum.

Figure--   Relative magnetic flux in the huge wire loop, as a function of time in seconds.

A certain amount of energy can be stored in this magnetic field. The ability of the loop to store energy in this way is property of inductance. It can be represented by the letter L.