Experiments with electromagnets
An AC electromagnet is simply a coil of wire connected to an AC voltage source. It can have an air core or an iron core.
First, you need a coil of wire wound onto a form. The coil pictured here was purchased at a surplus shop for only $3. It can handle 30V ac or dc.
Next, you need to purchase a large bolt. This was selected to fit snugly in the coil form. The threaded portion of the bolt was sawn off with a hacksaw to get rid of the threads.
The bolt was then fit into the coil form. It extends about 2" beyond the end of the coil form.
You are now ready to go. Connect up a variac to the coil so you can control the voltage and current going to it.
It can be shown how the AC electromagnet will pick up several paper clips. Next, turn the variac down to zero, to show how the paper clips fall off the end of the bolt.
In my hand is another coil of wire, with two LEDs attached back to back across the terminals (so they are in parallel with each other, but with opposite polarity). When this second coil is off of the bolt, the LEDs are off. When the second coil is placed onto the bolt, both of the LEDs light up! This is because the changing magnetic field is cutting through the wires in the second coil, creating a voltage in the second coil, which causes an alternating current to flow, lighting both of the LEDs. This is called transformer action, and demonstrates how power transformers that sit on the poles near your house operate. Transformers can increase the ac voltage or decrease the ac voltage on the secondary winding compared to the voltage fed to the primary winding of the transformer.
So, it behaves like a permanent magnet as long as there is power applied to it. It's a magnet that can be turned on and off, and its magnetic strength can be varied as the voltage to it is varied.
One minor difference, though, is that the end of the bolt is changing from a North magnetic pole to a South magnetic pole and back again 60 (60Hz) times a second!
This changing magnetic field can be used to create a transformer effect.