# Finding the ideal parameters for a homemade electromagnet

I am making an electromagnet and I was watching some videos on designing one. I just have some misunderstanding about power calculations of electromagnets and I have some questions about it.

Q1)

1.1) In terms of power ratings some pre-made electromagnets for example, it says 3W at 5V, my guess would be that they took the nominal current as 0.6A in their magnetic calculations and decided on 5V to be the operating voltage thus giving the 3W rating but when I use a multimeter on both ends it gives me a resistance reading of 6Ω, does this mean that actually the current flowing is 0.83A at 5V or is it that they designed it so that their turns of wire give them the 6Ω of resistance and 0.6A, but rated it as 5V due to losses not allowing the current to actually get to that 0.83A mark?

1.2) Say I connect a 9V battery to the 5V rated electromagnet, would it be safe to assume that the current will increase making the magnet stronger but get hotter in exchange, obviously the current will be limited and not actually be 1.5A through such a battery but would the magnet still function normally just abit more hotter? If so, is it correct to assume operating at a higher voltage than the rated voltage is ok if I'm planning on using it for a minute or two at a time?

Q2) Lastly, I want to power my electromagnet with a 9V battery. From research, these 9V batteries can provide from 100 mA and 500 mA, the issue is from my calculations I got resistance to be 1.5Ω so using 9V battery this would give 6A, but that wouldn't actually be the current so what would actually happen if this was connected up? This is what I have a hard time visualizing.

I just want to understand the principles and not just have a single specific solution for my case so I have the questions in a more open/ambiguous manner. I hope you understand, thanks!

• 9V batteries are a poor choice. You will have more success with a few AA batteries. Commented Dec 1, 2023 at 2:34
• It might make it stronger. what do you call strength, how are you measuring strength? Commented Dec 1, 2023 at 3:05
• 54 watts in an electromagnet the size of a sugar cube, a 9v battery can probably make 6A for long enough to burn that up. that calculator doesn't give field strength so it Commented Dec 1, 2023 at 3:11
• @Jasen Слава Україні I guess the magnetic flux density at the surface. It decreases with distance but I will be picking up stuff at with the surface very close so the distance wont matter. It was mainly understanding the current flow and what limits it Commented Dec 1, 2023 at 4:49

The force is proportional to the magnetic flux, which is proportional to the current multiplied by the number of turns.

So, bigger is better, you can have more turns and/or larger wire without overheating.

For a given mechanical cavity size, there will be wasted space between the wires and the insulation itself. I don't know which wire size has the most efficient packing density, but that isn't the most important factor.

The most important factor is to match the wire size and number of turns to your power source. Batteries have internal resistance and this power will be wasted. You want this loss to be minimal.

9V batteries have a lot of internal resistance and are therefore a poor choice. If you must use a 9V battery, you want a lot of turns with fine wire.

There are no standards for determining the wattage for a magnet, especially for those on ebay. Obviously if you put too much power into the coil it will overheat. The continuous power will of course be less than intermittent power.

• Most in that shape, whether toys on Amazon or professionally built ones for commercial lifting are spec'd for 50% duty. I'm guessing, but that seems like a "voluntarily worked-out commercial standard" of some kind, since the sites I looked at all seem to write it up that way. They will over-heat at 100% use and usually say so. What I would like to learn is to work out the holding power knowing the physical details. I'm guessing it's related to the difference of stored energy with a metal cap vs stored energy without the cap divided by distance from center to ring. But it's my total wag. Commented Dec 1, 2023 at 15:29