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
Tom (w mike)
(republished on 08/02/06)