The Strength of an Electromagnet

Abstract Electric charges in motion create magnetic fields. You can create an electromagnet with a simple coil of wire and a battery. This project has ideas for exploring how the strength of the electromagnet depends on the size of the coil or the voltage supplied to it. Objective The goal of this project is to investigate the strength of an electromagnet made from a coil of wire. How does the strength of the magnetic field change as the number of turns in the coil is increased? Introduction An electric current flowing in a wire creates a magnetic field. You can prove this to yourself with a magnetic compass (see the Science Buddies project idea Using a Magnet as an Electrical Current Detector). The magnetic field around a straight wire is not very strong. However, if the wire is wrapped in a coil, the fields produced in each turn of the coil add up to create a stronger magnetic field (see Figure 1). Figure 1. The green lines show the magnetic field surrounding a coil through which electric current is flowing. The right-hand rule tells you the direction of the magnetic field produced by electric current. In the case of a single wire, when you hold your right hand so that the thumb points in the direction of the current flow, your fingers curl in the direction of the magnetic field (see Figure 2). Figure 2. Illustration of the right-hand rule for a single wire. For a coil of wire, when the fingers of your right hand curl in the direction of the current flow, your thumb points toward the north pole of the magnetic field created by the coil (see Figure 3). Figure 3. Illustration of the right-hand rule for a coil of wire through which an electric current is flowing. In this project, you will investigate how the strength of the magnetic field produced by a coil of wire changes when the number of turns in the coil are changed. You will also investigate which orientation of the coil is more effective: holding the coil parallel to the material to be picked up, or holding the coil perpendicular to the material. Terms, Concepts, and Questions to Start Background Research To do this project, you should do research that enables you to understand the following terms and concepts: electromagnet, right hand rule. Questions Can you think of a way to demonstrate which end of your coil is N and which is S? What happens to N and S if you reverse the connections to the battery? Bibliography Brain, M., 2006. "How Electromagnets Work," HowStuffWorks.com [accessed July 18, 2006] http://www.howstuffworks.com/electromagnet.htm. Thomas Jefferson National Accelerator Facility - Office of Science Education, date unknown. "What Is an Electromagnet?" [accessed July 18, 2006] http://education.jlab.org/qa/electromagnet_is.html. Nave, C.R., 2006. "Magnets and Electromagnets," HyperPhysics, Department of Physics and Astronomy, Georgia State University [accessed July 18, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html. Materials and Equipment To do this experiment you will need the following materials and equipment: 6 V lantern battery (e.g., Radio Shack 23-016 or 23-560), magnet wire (e.g., Radio Shack #278-1345), alligator clip leads (e.g., Radio Shack #278-1156), sharp knife for stripping magnet wire, masking tape, box of steel paper clips, 100–200 steel washers (approx. 1 cm diameter), three identical pieces of iron for the core material, for example: iron nails, iron bolts, or short lengths of iron pipe. Whatever you choose for the core material, all three pieces should be the same size. optional (for Variations 1 and 2, below): paper for making coil forms. Experimental Procedure Note Before Beginning: This science fair project requires you to hook up one or more devices in an electrical circuit. Basic help can be found in the Electronics Primer. However, if you don't have experience in putting together electrical circuits you may find it helpful to have someone who can answer questions and help you troubleshoot if your project isn't working. A science teacher or parent may be a good resource. If you need to find another mentor, try asking a local electrician, electrical engineer, or person whose hobbies involve building things like model airplanes, trains, or cars. You may also need to work your way up to this project by starting with an electronics project that has a lower level of difficulty. You will make three different electromagnets, using three identical pieces of core material. Each coil will have a different number of turns. For example, you could try 100, 200, and 500 turns. Wrap the magnet wire neatly around the core material. Here are some tips to make wrapping easier. Leave 5–6 cm of wire free at each end of the coil for making the connection to the battery. Make a holder for the spool of magnet wire, so that you can roll the wire right off of the spool. For example, you can stick a pen or pencil through the spool, and tape it down to a couple of small boxes. Use a small piece of tape to attach the wire to the core material, about 0.5–1 cm in from the end. Turn the core to unwind the magnet wire from the suspended spool. Use your fingers to keep the wire tight against the core material. Wrap each successive turn so that the wire lines up neatly. Keep track of the turns (each time the tape that holds the wire in place comes around). This is easier if you can recruit a helper to make tally marks for you. When you reach the desired number of turns, again tape the wire to the coil form, and cut it off. Leave 5–6 cm of free wire for making the connection to the battery. Particularly for the larger coils, you will need to wrap multiple layers of wire to get the desired number of turns. Use the utility knife to carefully scrape off the enamel insulation from the magnet wire over a 1 cm length at each end. You'll see the shiny copper wire underneath. Be careful not to cut the wire. Place the paper clips (or washers) in a shallow container (slightly longer than the coil). You will probably find that paper clips work well for coils with an air core, and washers work well for coils with an iron or steel core. Pick up paper clips with the coil held parallel to the container. Use the clip leads to connect the coil to the battery. Touch the coil (lengthwise) to the paper clips (or washers), then pull the coil away from the tray. Disconnect the coil from the battery, and count how many paper clips (washers) were picked up. Record the number in your lab notebook. Organize your data in a table like the one below. Repeat at least five times for each coil. Pick up paper clips with the coil held perpendicular to the battery. Use the clip leads to connect the coil to the battery. This time touch the core material to the paper clips (or washers), then pull the coil away from the tray. (In other words, this time the coil will be perpendicular to the tray of paper clips.) Disconnect the coil from the battery, and count how many paper clips (washers) were picked up. Record the number in your lab notebook. Organize your data in a table like the one below. Repeat at least five times for each coil. Calculate the average number of paper clips (washers) lifted by each coil for each method (see the table below). Number of Turns Number of Paper Clips Picked Up Coil Parallel to Paper Clips Coil Perpendicular to Paper Clips Trial Average Trial Average 1 2 3 4 5 1 2 3 4 5 100 200 500 Make a graph of the results. Plot the number of paper clips picked up for each coil orientation (y-axis) vs. number of turns in the coil (x-axis). Variations Try using different metals as core materials inside the coil. For example, steel, copper, aluminum, etc. You can use nails, bolts, pipe, tubing, etc. For making comparisons, it would be ideal to have core materials that are the same diameter and weight. You can wrap the coils directly around the core materials as described above, or you can make a single coil (for each number of turns that you want to test) and test it with different core materials inside. Here is a procedure for making coils so that you can swap the core material: Make a coil form by wrapping several layers of paper around a sample of your core material. Use enough layers of paper or cardboard so that the coil form will hold its shape. Tape the paper, then slide it off the cylinder form. Test the coil form with each of the core materials to make sure that they all fit. Wrap your coil around the coil form, as described above. Now you will be able to slide different core materials in and out of the coil. You can leave the paper material in place. You could also try another experiment to see if removing the paper makes any difference in the strength of the electromagnet. Test the coils both with and without the paper material inside. As in the procedure above, you can compare the amount of weight that electromagnets with different core materials can lift. What happens when you change the distance between the coil of wire and a metal core material? For example, increase the diameter of your core forms (described in Variation #1, above) by 2, 5, 10, and 20 mm. For an interesting addition to your display board, you can map the shape of the magnetic field produced by your electromagnets. Here's how: Mapping Magnetic Fields. What happens when you change the voltage applied across the coil? You can connect 2 or 3 lantern batteries in series, or use increasing numbers of D-cell batteries in series. As above, measure how many paper clips (or washers) you can lift with a coil at each voltage. For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.