Can a insulated wire be used to make a electromagnet? Will there be any difference in strength of the magnet if I use an uninsulated wire instead? By uninsulated wire I mean this:
Well.. you COULD use uninsulated wire but then you might as well just use a solid copper rod or tube and you would require unreasonably low voltages and unreasonably high currents to get the same power/field strength since you would basically be trying to make a magnet out of just a straight, single solid strand of wire or maybe just one turn of wire. All current flow does produce a magnetic field after all, but it's impractical.
Not all insulation is a rubbery or plastic material. The magnet wire which is typically used in things like transformer, motor, and electromagnet coils is insulated with a thin, hard laquer or enamel. It's made to be very thin so you can squeeze as many turns as possible into the same volume.
Isn't that a uninsulated wire
It could be but if so it is a dumb usage. The only reason that would work with an un-insulated wire is more 'by accident'.
The accident in this case is that with an un-insulated wire the screw is shorting out the copper coil. But because copper has a much lower resistance and the screw is probbably not making very good contact, most of the current is still going through the copper and giving still a magnetic field.
But as others will tell you: you should NOT use un-insulated wire to wind a coil.
Magnetic fields penetrate insulation, so if you create a magnetic loop with insulated wire, the insulation will create wider gap between wires if stacking them. Most wire suited for electromagnets is insulated, the insulation is hard to see because it is a thin lacquer/enamel coating on the wire. If you don't use insulation, you run the risk of shorting the current on the coil.
You can use bare 20AWG wire to make an electromagnet, but probably not the best idea as you have to rely on oxide layers that insulate or wire spacing to keep current from running straight from one coil to the next or through the nail.