# Q & A: Number of batteries and electromagnet strength

Most recent answer: 10/22/2007
Q:
How dose the number of batteries effect the strengh of an electromagnet?
- Shslanda Rideout (age 12)
Joliet, Illinois
A:
The strength of an electromagnet depends on the electrical current which flows through its wires, but not on what drives that current. The current is measured in the amount of charge per unit time that flows through the wires.

If you hook an electromagnet up to a battery or batteries, the current will be limited by the total electrical resistance around the circuit. Most electromagnets have only small electrical resistances, but if you have many many turns of very thin wire then this resistance may not be that small. Furthermore, you can add a resistor in series with the electromagnet and use it to control the strength of the magnet. If your electromagnet has very little resistance, then the internal resistance of the battery will become the limiting factor. It's very similar to making a short circuit across a battery's terminals and then asking how much current will flow in that short circuit -- it really depends on how well the battery can keep up its voltage while driving large amounts of current.

You can of course make a stronger electromagnet by adding batteries, but the strategy you would use depends on the relative resistances of the two components. If the electromagnet's resistance is what's limiting the current, then connecting the batteries in series will increase the voltage across the electromagnet, and then by Ohm's law, increase the current and thus the strength of the magnet. The strength of the magnet in this case is directly proportional to the total voltage sum of all of the batteries.

If it is the batteries' resistance which is more important, this strategy will not work as well -- each battery will supply almost no voltage and drive as much current as it can if they are hooked up in series in this way. If some batteries are stronger than others, you can even force the voltage across the terminals of the weaker batteries to be of the opposite sign to what the battery wants to make it. The best strategy in this case is to wire the batteries in parallel, so that each battery can contribute its current additively at the voltage determined by the combined batteries and electromagnet.

This second case is a good way to run your batteries down very quickly. It might be best to introduce some resistance into your circuit just so you can economize on batteries, although that would waaste some of their energy in heating up the resistor. The best thing is to use enough turns of wire so that you don't need to add any resistance.

Tom (w mike)

(published on 10/22/2007)

## Follow-Up #1: electromagnetic nail core

Q:
how can I increase the strength of a electromagnet using a core-nail?
- brianna (age 30)
hopewell jct NY
A:
An ordinary iron nail will increase the strength of the field. That's because the field from the electromagnet lines up many of the magnet domains in the iron so that their fields ad to the field from the electromagnet.

Mike W.

Lee H

(published on 10/22/2007)