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Abracadabra! Levitating with Eddy Currents!

Difficulty
Time Required Average (6-10 days)
Prerequisites None
Material Availability Specialty items required: neodymium magnets. See the Materials and Equipment list, below, for more details.
Cost Low ($20 - $50)
Safety No issues.

Abstract

Did you know that not all trains run on tracks? Some of the world's fastest trains are magnetic levitation trains (maglev). This means that the carriage of the train is suspended over the rails with no support, but only with magnetic fields! There is a physical explanation for magnetic levitation, and if you would like to learn more about magnetism and current, this is a science fair project that you must try!

Objective

The objective of this science fair project is to demonstrate eddy currents and induced magnetic fields.

Credits

Michelle Maranowski, PhD, Science Buddies

This science fair project is based on the Eddy Currents snack on the Exploratorium's website:
Exploratorium. (n.d.). Eddy Currents. Retrieved December 12, 2008, from http://www.exploratorium.edu/snacks/eddy_currents/index.html

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Last edit date: 2013-01-10

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. 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.

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 electricity and electronics science fair 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!

Earth's magnetic field
Figure 1. This image depicts Earth's magnetic field. (NASA, 2006.)

Terms and Concepts

  • Magnetic field
  • Exert
  • 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 the resistivity of copper?
  • What is a magnetic field?

Bibliography

This science fair project is based on the Eddy Currents project on the Exploratorium's website:

This HowStuffWorks video is a thorough discussion of Lenz's law and eddy currents:

There are several sources online that list resistivity values for different materials. The following are two examples.

Materials and Equipment

  • Neodymium magnet (1); The magnet diameter should be slightly less than the diameter of the pipes you are using. If you use a 1/2-inch-diameter pipe then your magnet should be approximately 1/4 inch in diameter. One option is the 1/4 inch diameter x 1/8 inch diameter magnet available from K&J Magnetics, www.kjmagnetics.com, item: D42E
  • Copper pipe, 1/2-inch inner diameter (4 feet)
  • PVC pipe, 1/2-inch inner diameter (4 feet)
  • Volunteer
  • Stopwatch
  • Lab notebook

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Experimental Procedure

Important Safety Notes About Neodymium Magnets Before You Begin:

  • Neodymium magnets are extremely strong, and must be handled with care to avoid personal injury and damage to the magnets. Fingers and other body parts can get severely pinched between two attracting magnets. Neodymium magnets are brittle, and can peel, crack or shatter if allowed to slam together. Eye protection should be worn when handling these magnets, because shattering magnets can launch pieces at great speeds.
  • Never place neodymium magnets near electronic appliances.The strong magnetic fields of neodymium magnets can also damage magnetic media, such as floppy disks, credit cards, magnetic ID cards, cassette tapes, video tapes, or other such devices. They can also damage televisions, VCRs, computer monitors, and other CRT displays.
  • Children should not be allowed to handle neodymium magnets, as they can be dangerous. Small magnets pose a choking hazard and should never be swallowed or inserted into any part of the body.
  • Never allow neodymium magnets near a person with a pacemaker or similar medical aid. The strong magnetic fields of the magnet can affect the operation of such devices.
  • Neodymium magnets are brittle and are prone to chipping and cracking. They do not take kindly to machining.
  • Neodymium magnets will lose their magnetic properties if heated above 175°F (80°C).
  • Neodymium magnets should never be burned, as burning them will create toxic fumes.
  • Like any tool or toy, neodymium magnets can be fun and useful, but must always be treated with care.
  • Neodymium magnets are extremely strong and should be used with care. See the Experimental Procedure below for handling tips.
  1. Play with the magnet and the copper pipe to get a feel for the force of the magnet. Make sure you know how to use the stopwatch.
  2. Hold the copper pipe vertically. Hold the pipe near the top and have the volunteer hold the pipe near the bottom so that the pipe doesn't shake.
  3. Hold the magnet at the top of the pipe. Position it at the center of the pipe's opening. Let go of the magnet and immediately start the stopwatch. Time how long it takes for the magnet to fall out of the tube. Record this time in your lab notebook.
  4. Repeat step 3 nine additional times and record the data in your lab notebook.
  5. Repeat steps 2–4 with the PVC pipe. Do you see a difference in the times? Were you able to see the magnet floating down the tube?
  6. Plot the data that you collected on a scatter plot. Label the x-axis Tube and the y-axis Falling Time. List the resistivity values of each material on the scatter plot. You can find resistivity values in the two sources listed in the Bibliography.

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Variations

  • Repeat this experiment using tubes of different conductors. For example, try using a brass tube and an aluminum tube.
  • What happens as you increase the diameter of the tube? Does the time it takes for the magnet to fall down the tube decrease, increase, or remain the same?

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