Forgive me if my question sounds a bit naive:

  • I have built a coil (no solid core) out of an insulated copper wire, and I calculated that it has 8 ohms of resistance. I tried it and it gave me the result I expected. I am using a professional (meaning - it can stand a lot of abuse) audio amplifier as a source of (pulsed) power. The amplifier automatically adjusts to different resistance - from 2 to 8 ohms. Currently it is running at 8 ohms.

  • Now, if I built another coil using exact same specs, EXCEPT 50% shorter wire, the coil would end up having 4 ohms of resistance right? That would further mean that it should give me a more powerful magnetic field I hope.

  • If I then connected this new coil to my audio amplifier, the amp would adjust itself to the new resistance, and hopefully would be able to deliver more power. This amp is rated at around 800w in bridged mode, at 8 ohms and much more (1200 W I think) at 4 ohms.

Am I right to believe that lower resistance coil will give me a stronger magnetic field all other things being equal?

Any input is appreciated!

  • \$\begingroup\$ Strength of elctromagnet is the product of ampere times turns. \$\endgroup\$
    – GR Tech
    Aug 18, 2014 at 6:36

3 Answers 3


If you are trying to attract an object magnetically, the magnetic force from an electro-magnet is: -

Force \$= \dfrac{(F_M)^2\mu_0\cdot A}{2 g^2}\$


  • Fm is the product of amps and turns
  • A is the cross sectional area of the solenoid
  • \$\mu_0\$ is \$4\pi\times 10^{-7}\$
  • g is the gap to the magnetizable object you wish to attract

Note that Fm is governed by amps and turns.


I'll answer your question with a question:

How strong would your magnetic field be if your coil, carried to an extreme, consists only of a single quarter-turn of 1/2" thick copper wire?

No. The lower resistance (shorter length, lower turn count) coil won't necessarily develop a stronger magnetic field. The same number of turns, but of heavier copper wire, will probably develop a stronger magnetic field, though. This will require a somewhat LONGER length of copper wire to accomplish, because the coil's diameter will be larger.


Field strength is proportional to the number of turns and the current. Halving the number of turns while doubling the current keeps the field the same strength.


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