# What is the time-constant relation between an electromagnet and its coil? [closed]

If a coil has a time constant of about 30 seconds, will any electromagnet made from this coil have the same time constant, or will it be changed by the core / the surroundings ?

In other term, will the magnetic field persist if the power is cut down ? And how is the time for which it persists calculated ?

## closed as unclear what you're asking by Harry Svensson, Brian Drummond, PeterJ, Voltage Spike, Daniel GrilloDec 13 '17 at 16:31

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• "If a coil has a time constant of about 30 seconds". Hmmm. – Harry Svensson Dec 2 '17 at 10:53
• Yeah, I know, it would cost quite a price... Maybe it would be better to use a standard coil and a big condensator for the same effect ? – Nygael Dec 2 '17 at 11:11
• Do you have any flux capacitor? – Harry Svensson Dec 2 '17 at 11:12
• Not any more, some guy in a weird car stole it from me. – Nygael Dec 3 '17 at 9:20
• That would be a big big coil – Voltage Spike Dec 7 '17 at 0:37

An inductor has a time constant, L/R, where R is the resistance of the windings. This is how long it will take for the current to reduce to 27% of its initial value, when the coil is short-circuited at the terminals.

For the same coil of wire, the inductance in air will be less than the inductance when assembled onto the iron core of an electromagnet. The inductance would increase further if the electromagnet had some iron on, or in the vicinity of, the pole pieces, to further reduce the air-gap in the magnetic circuit. Obviously, as the inductance changes, the time constant changes.

• So actually the time constant of an electromagnet is higher that the one from its coil... Thanks ! – Nygael Dec 3 '17 at 9:22