Abracadabra! Levitating with Eddy Currents!

Introduction

What is a magnet? A magnet is a material that produces a magnetic field, which can exert a force on other materials without actually touching them. A magnetic force can attract or repel, and some materials can exert a larger force than others. Every magnet has at least one north pole and one south pole. Did you know the Earth is a magnet? A magnet produces a field at all points around it in space, as shown in Figure 1. The Earth has a magnetic field that repels space radiation and solar wind. The magnet's poles (such as Earth's north and south poles) are where the magnetic field begins and ends. If you look at the magnetic field of Earth, you will notice that the magnetic field is not straight. The field starts at the north pole and bends as it meets the south pole. Since the magnetic field bends, it has a direction.

Figure 1. This image depicts Earth's magnetic field.

Russian physicist Heinrich Lenz started studying electricity and magnetism in 1831. In 1834, while investigating magnetic induction, he noticed and described an interesting phenomenon. This phenomenon is now called Lenz's law and it occurs when a magnet interacts with a conductor. A conductor is a material that permits electrons (and therefore electricity) to flow through it easily. This means that a conductor has a low resistance and resistivity to the motion of electrons. When a magnetic field varies along the length of the conductor, like when you let go of a magnet down a metal tube, the magnetic field induces a current within the conductor. This current is called an eddy current. Once the eddy current is established, it then produces a magnetic field. This induced magnetic field opposes the magnetic field of the magnet that is moving along the conductor. As a result of two opposing magnetic fields, the magnet will stop moving and float, or levitate. This is the principle behind the world's fastest trains, called magnetic levitation (maglev) trains. There is no physical contact between the train carriages and the tracks.

Factors that increase the effect of eddy currents include stronger magnetic fields, faster-moving magnetic fields, and thicker conductors. Factors that reduce the effect of eddy currents include weaker magnets, slower-moving magnetic fields, and non-conductive materials.

In this project, you will investigate magnets and eddy currents. You will accomplish this by sending a neodymium magnet down a conductive tube and then down a non-conductive tube. Is there a difference between the ways the magnet falls down the two tubes? Do this science fair project and find out!

Terms and Concepts

• Magnetic field
• Force
• Lenz's law
• Conductor
• Resistance
• Resistivity
• Current
• Eddy current
• Induce
• Neodymium

Questions

• What is a conductor?
• What is the difference between a conductor and a non-conductor?
• What is a magnetic field?
• Do you think a magnet will fall faster down a copper pipe or a PVC pipe?

Bibliography

• Davidson, M. (2006, June 15). Molecular Expressions: Electricity & Magnetism Introduction—Lenz's Law. Retrieved December 12, 2008 from,http://micro.magnet.fsu.edu/electromag/java/lenzlaw/index.html

Materials and Equipment

• Copper pipe, 1/2 inch inner diameter (at least 4 feet long), available at a hardware store
• PVC pipe, 1/2 inch inner diameter (at least 4 feet long), available at a hardware store
• 1/2 inch diameter neodymium magnet, available from from K&J Magnetics.
  • Note: the magnet must have a smaller diameter than the inside diameter of your pipes. Since the inner diameter of 1/2 inch copper and PVC pipe is actually slightly more than 1/2 inches, a 1/2 inch diameter magnet should fit through the pipes.
• Volunteer
• Stopwatch

Experimental Procedure

Safety Notes about Neodymium Magnets:

• Handle magnets carefully. Neodymium magnets (used in this science project) are strongly attracted and snap together quickly. Keep fingers and other body parts clear to avoid getting severely pinched.
• Keep magnets away from electronics. The strong magnetic fields of neodymium magnets can erase magnetic media like credit cards, magnetic I.D. cards, and video tapes. It can also damage electronics like TVs, VCRs, computer monitors, and other CRT displays.
• Keep magnets away from young children and pets. These small magnets pose a choking hazard and can cause internal damage if swallowed.
• Avoid use around people with pacemakers. The strong magnetic field of neodymium magnets can disrupt the operation of pacemakers and similar medical devices. Never use neodymium magnets near persons with these devices.
• Use the magnets gently. Neodymium magnets are more brittle than other types of magnets and can crack or chip. Do not try to machine (cut) them. To reduce the chance of chipping, avoid slamming them together. Eye protection should be worn if you are snapping them together at high speeds, as small shards may be launched at high speeds. Do not burn them; burning will create toxic fumes.
• Be patient when separating the magnets. If you need to separate neodymium magnets, they can usually be separated by hand, one at a time, by sliding the end magnet off the stack. If you cannot separate them this way, try using the edge of a table or a countertop. Place the magnets on a tabletop with one of the magnets hanging over the edge. Then, using your body weight, hold the stack of magnets on the table and push down with the palm of your hand on the magnet hanging over the edge. With a little work and practice, you should be able to slide the magnets apart. Just be careful that they do not snap back together, pinching you, once you have separated them.
• Wear eye protection. Neodymium magnets are brittle and may crack or shatter if they slam together, possibly launching magnet fragments at high speeds.

1. Create a data table like Table 1 to record the results of your experiment.

	Fall Time (seconds)
Type of pipe	Trial 1	Trial 2	Trial 3	Average
Copper
PVC

Table 1. Data table for recording your results.

2. Hold the copper pipe vertically. Hold the pipe near the top with one hand, and have the volunteer hold the pipe near the bottom so it does not shake. Optionally, you can use something like duct tape or zip ties to fix the pipe in a vertical position (for example, by attaching it to the leg of a table).
3. To do the experiment, you will need to drop the magnet down the pipe. You will need to start the stopwatch as soon as you let go of the magnet, and stop the stopwatch as soon as the magnet exits the bottom of the pipe. You and your volunteer should decide who will be in charge of the stopwatch.
4. Hold the magnet just above the top of the tube, as shown in Figure 2.

Figure 2. Positioning the magnet for a drop.

5. Release the magnet, and you (or your volunteer) should immediately start the stopwatch.
6. Watch the bottom of the tube closely. You (or your volunteer) should stop the stopwatch as soon as the magnet comes out.
7. Record the time in seconds under “Trial 1” for the copper pipe in Table 1.
8. Repeat steps 4–7 for trials 2 and 3 with the copper pipe.
9. Calculate an average value of your three trials for the copper pipe.
   1. Scientists do multiple trials and calculate an average to help even out any errors in their experiment. For example, in this experiment, there could be some human error in how fast you hit “start” and “stop” on the stopwatch.
   2. To calculate the average, add up the values, and divide by the number of trials (in this case, 3). For example, if your times were 4.5, 4.8, and 5.4 seconds, your average would be (4.5 + 4.8 + 5.4) ÷ 3 = 4.9 seconds.
   3. If you need help calculating an average, ask an adult for help.
10. Repeat steps 4–9 for the PVC pipe.
11. Make a graph of your results. Make a bar graph with the type of pipe (copper or PVC) on the horizontal, and the average fall time on the vertical axis. Try the Create a Graph website if you need help making a graph.
12. Did the magnet fall down one pipe faster than the other? How can you explain your results?