Abstract

Objective

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

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!

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

Terms, Concepts, and Questions to Start Background Research

• 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

• 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

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

• Exploratorium. (n.d.). Eddy Currents. Retrieved December 12, 2008, from http://www.exploratorium.edu/snacks/eddy_currents/index.html

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

• HowStuffWorks.com. Physics: Lenz's Law and Eddy Currents. Retrieved December 12, 2008, from http://videos.howstuffworks.com/hsw/17415-physics-lenzs-law-and-eddy-currents-video.htm

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

• Wikipedia Contributors. (2009, February 2). Resistivity. Wikipedia The Free Encyclopedia. Retrieved February 2, 2009 from http://en.wikipedia.org/w/index.php?title=Resistivity&oldid=268052857
• European Council of Vinyl Manufacturers. (n.d.). PVC: Electrical insulation characteristics. Retrieved February 2, 2009, from http://www.pvc.org/PVC.org/What-is-PVC/PVC-s-physical-properties/Electrical-insulation-characteristics

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:

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.

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?

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

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

Last edit date: 2010-09-30 09:37:00