Sometimes powerful things come in small packages, and this electromagnet is no exception! It features in two popular demonstrations in our collection, J6-01: Electromagnet with Bang and J6-04: Low-Power High-Force Electromagnet. These two demonstrations are frequently used, separately or together, in a variety of physics classes. They also featured in our popular Physics of Fantastic Worlds program!
This small electromagnet is powered by a single flashlight battery. But it is quite strong. In the first demonstration, we see a heavy block of steel being held up by the electromagnet. When we flip the switch to turn the electromagnet off, though, it falls to the table with a bang.
In the second, the electromagnet and a small steel plate are mounted on handles. If students grab the handles and touch the plate to the magnet, they cling together so tightly that even quite strong people cannot pull them apart. But flip the switch to turn off the electricity, and they fly apart!
But what is an electromagnet, and why does it work? Let’s find out.
The battery produces an electrical potential that causes a current to flow through the wire in the coil when the switch is closed. A current can only flow when the circuit is complete.
Maxwell’s Equations of Electromagnetism tell us that moving electrical charges, such as an electric current, create a magnetic field around it. This magnetic field acts just like the magnetic field of the permanent magnets we’re familiar with, like refrigerator magnets. The strength of the magnetic field is determined by the amount of current passing through an area.
(image credit: Wikimedia user Chetvorno https://commons.wikimedia.org/wiki/File:Magnetic_field_of_wire_loop.svg )
Here we see a diagram of the magnetic field around a single loop of wire. We can see that the field wraps around the wire, so the direction of the force from the magnetic field will be different depending on where you are around the wire. We can see this field in motion in this animation from Penn State - click here!. See the animation "B Field Lines Due to a Current Loop."
The direction of the field also depends on which way the current flows; try this out in this simulator at JavaLab - click here!
Imagine the field around that single loop in the illustrations above turned on its side, lined up with more like it. If you flipped one of the wire loops around, its field would be oriented the other way, leaving a slightly weaker point in the field; but if you flipped all of them at once, the field of the entire coil flips directions. Try this out with the simulator!
You can also flip the battery around in the simulator to change the direction of the current flowing through the wire.
Compare for yourself: what happens if you change the direction of the wire, or change the direction of the current, or both at once?
The force from a single wire is not very strong, especially with only a small electric current. You could make a stronger electromagnet by having a power source with a higher electrical potential to make a stronger current; but that might not be very practical, and would certainly be more expensive.
But if we have many loops of wire, and line them up so that the fields all are aligned, then the small magnetic force from each wire will add up to a much stronger force. This is how a strong electromagnet like the one in the photographs above is built.