# Physics 313, Extra Credit for Week #8

#### Due Nov 5, 2001, at 5 PM

Make an electromagnet.

This project may be done by teams of 1-2 individuals. The idea is to create an electromagnet, and then to figure out the dependence of the magnet's strength on the number of coils of wire around it.

You can make an electromagnet by wrapping wire around an iron (or nickel or cobalt) rod: small cylindrical items such as screwdrivers, portable radio antennas, pointers, nails or screws will all do the trick. Wrap wire around the rod, then connect the ends of the wire to a battery. When current runs through the wire, it induces a magnetic field in the rod.

Warning: your battery probably won't last long. Be sure to connect it to your wire only when you are ready to pick up paper clips, and disconnect it afterwards.

Task I Create an electromagnet. Verify that it will lift at least one paperclip. Describe your electromagnet, and draw a picture of it.

Task II Investigate the effect of different numbers of turns of wire around the rod. Use the same wire, but wrap varying amounts of it around the rod. Try to change the angle of the coils so that they always start at one end of the rod and terminate near the other. Make at least 4 different arrangements of wire, varying the number of coils by at least a factor of 10 from smallest to largest. For each arrangement, record the number of coils, and the largest number of paper clips the magnet can lift. You should link the clips together, and touch the electromagnet to the end of the last link in the chain. You may break some paper clips into fractions in order to increase the precision of your measurements.

Plot the maximum lifted weight against the number of coils. Try to find a simple mathematical formula which roughly describes the magnetic force as a function of the number of coils.

Theoretically, what relationship do you expect? You may consider the electromagnet as a variety of solenoid.

Do theory and experiment agree? Discuss.