How the Strength of a Magnet Varies with Temperature

Abstract

Objective

The objective of this experiment is to determine whether the temperature of a magnet affects its strength.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on:

• Iverson, A.A., 2004. "Does the Temperature of a Magnet Affect Its Strength?" California State Science Fair Abstract [accessed September 21, 2006] http://www.usc.edu/CSSF/History/2004/Projects/J1521.pdf.
• Hoadley, R., date unknown. "Magnet Basics: What Affects the Strength of Magnets?" Magnet Man [accessed September 21, 2006] http://www.coolmagnetman.com/magstren.htm.

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Introduction

Magnetic fields are produced by electric currents. This could be the familiar electric current flowing in a wire, that you can measure with an ammeter. Or it could be a less familiar, microscopic current associated with electrons in atomic orbits (Nave, 2005a). Certain materials, called ferromagnetic materials, have unpaired electrons in their outermost atomic orbits that can become magnetically aligned over large distances (relative to the atomic scale). These regions of alignment are called magnetic domains.

An electric current flowing in a straight wire creates a magnetic field around the wire. The blue lines in Figure 1, below, show the orientation of such a magnetic field. Notice the "right hand" rule for determining the orientation: when the thumb of the right hand is pointing in the direction of the current, the fingers of the right hand curl in the direction of the magnetic field. You can see the effect of this magnetic field by bringing the wire close to the needle of a magnetic compass when the current is flowing. You can even make an electric current detector based on this principle (see Using a Magnet as an Electrical Current Detector.)

Figure 1. The illustration shows the magnetic field produced by electric current in a straight wire. When the thumb of the right hand is pointing in the direction of the current, the fingers of the right hand curl in the direction of the magnetic field (Nave, 2005f).

A current flowing through a coil of wire (the coil is also called a solenoid) creates a stronger magnetic field than the same current flowing through a straight wire. The magnetic field is strongest at the center of the coil. Each loop in the coil contributes additional strength to the magnetic field. The more loops, the stronger the field.

Figure 2. The illustration shows the magnetic field produced by electric current in a coil of wire (solenoid). When the fingers of the right hand curl in the direction of the current flow, the thumb of the right hand curl points in the direction of the magnetic field (i.e. thumb points toward magnetic North pole of the solenoid). (Nave, 2005g)

The magnetic field of a solenoid can be increased even further by placing a bar or rod of ferromagnetic material within the coil (diamagnetic and paramagnetic materials will also work, but will not retain a magnetic field when the current is turned off). The magnetic field from the coil strongly aligns all of the magnetic domains in the ferromagnetic material, creating a much stronger magnetic field than either the coil or the ferromagnetic material would have alone.

Permanent magnets are made from ferromagnetic materials. Ferromagnetic materials can "remember" their magnetic history. If a ferromagnetic material is exposed to a strong magnetic field, the magnetic domains within the material will retain at least some of the alignment induced by the external magnetic field.

When the temperature of a material is increased, what is happening on the atomic scale is an increase in the random motion of the atoms of which the material is made. You might think that random motion of atoms could affect the alignment of magnetic domains, so that increasing the temperature of a magnet would tend to decrease its strength. In fact, each ferromagnetic material has a Curie tmperature (named after Pierre Curie), above which it can no longer be magnetized. For soft iron, the Curie temperature is over 1,300°C! Your oven at home might get as hot as 260°C, so obviously 1,300°C is out of the question for a science fair experiment. But what happens to the strength of a magnet over a more approachable range of temperatures, for example from the temperature of dry ice (about −78°C) to the temperature of boiling water (+100°C)? This project shows you how to find out.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

• magnetic force,
• magnetic domain,
• ferromagnetism,
• ferromagnetic materials,
• temperature.

More advanced students may also want to study:

• diamagnetic materials,
• paramagnetic materials.

Questions

• Are some magnetic materials more temperature-dependent than others?

Bibliography

• Swenson, J., 2004. "Magnetic Field Strength and Low Temperature," Ask a Scientist Physics Archive, Newton BBS, Argonne National Laboratory. [accessed September 21, 2006] http://www.newton.dep.anl.gov/askasci/phy00/phy00880.htm.
• Staff, date unknown. "Magnetic Domains," Non-Destructive Testing Resource Center, Collaboration for NDT Education [accessed September 21, 2006] http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/MagneticDomains.htm.
• These (more advanced) articles from the HyperPhysics website offer further information about magnetism and magnetic domains:
  
  • Nave, C.R., 2005a. "Magnetic Field," HyperPhysics, Departments of Physics and Astronomy, Georgia State University [accessed September 21, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html#c1.
  • Nave, C.R., 2005b. "Ferromagnetism," HyperPhysics, Departments of Physics and Astronomy, Georgia State University [accessed September 21, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html.
  • Nave, C.R., 2005c. "Magnetic Domains," HyperPhysics, Departments of Physics and Astronomy, Georgia State University [accessed September 21, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html#c4.
  • Nave, C.R., 2005d. "The Curie Temperature," HyperPhysics, Departments of Physics and Astronomy, Georgia State University [accessed September 21, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html#c3.
  • Nave, C.R., 2005e. "Long Range Order in Ferromagnets," HyperPhysics, Departments of Physics and Astronomy, Georgia State University [accessed September 21, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html#c2.
  • Nave, C.R., 2005f. "Magnetic Field of Current," HyperPhysics, Departments of Physics and Astronomy, Georgia State University [accessed September 21, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html#c1.
  • Nave, C.R., 2005g. "Solenoid," HyperPhysics, Departments of Physics and Astronomy, Georgia State University [accessed September 21, 2006] http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• safety glasses,
• 5–10 permanent iron magnets of equal size and strength,
• thermometer (minimum range 0–100°C),
• tongs for holding magnets (preferably plastic, for minimizing heat transfer),
• dry ice (frozen CO₂),
• water ice,
• insulated containers to hold ice and dry ice,
• thick insulated gloves for handling dry ice,
• small pot,
• water,
• stove or hot plate for heating water,
• 2 large plastic bowls,
• at least one box of standard #1 paper clips,
  • if you have a really strong magnet, you may need more than one box,
  • test the magnet at room temperature first, and make sure that there are plenty of paper clips left over (see Experimental Procedure, below),
  • alternatives to paper clips: small steel BBs or nails.

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Experimental Procedure

Note: this experiment is designed for testing the temperature dependence of permanent magnets not electromagnets.

1. You will test each magnet at four different temperatures:
   1. −75°C, the temperature of dry ice (don't try to use the thermometer for this one!),
   2. 0°C, the temperature of a water ice bath,
   3. 20°C, room temperature,
   4. 100°C, the temperature of boiling water.
2. Wear safety glasses when heating, cooling, and transfering the magnets. Always use tongs for handling magnets at extreme temperatures.
3. Before measuring each magnet's strength at a given temperature, allow the magnet to equilibrate to the test temperature for at least 15 minutes.
4. To test magnetic strength, follow these steps:
   1. Use the tongs to place a magnet into a bowl filled with paper clips (or steel BB's).
   2. See how many clips (or BB's) the magnet can lift out.
   3. Set the magnet and paper clips (or BB's) down in an empty plastic bowl.
   4. Wait until they are safe to touch before counting the number of paper clips (or BB's).
   5. Record the number in your lab notebook for each magnet and temperature tested.
   6. It's a good idea to practice handling the magnets with tongs at room temperature first until you get the hang of it. Make sure your results are reproducible at room temperature before trying the experiment at extreme temperatures.
5. For each temperature, calculate the average number of objects each magnet picked up.
6. Make a graph of magnetic strength, as measured by number of objects lifted (y-axis), vs. temperature (x-axis).
7. Does magnetic strength increase, decrease, or stay the same over the temperature range you tested?

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Variations

• Compare the temperature dependence of magnets made of different materials (e.g., neodymium vs. iron). Do background research to find the Curie temperature and normal operating temperature range for each type of magnet you test. If necessary, adjust the temperature range for the experiment in order not to exceed the safe operating temperature range for the magnets you test.
• Here are some other Science Buddies projects related to magnetism:
  • For a more basic project, see Magnets and Charge, or The Strength of an Electromagnet.
  • You might also be interested in Spare-Change Circus: Walking Coins on a (Vertical!) 'High Wire'.
  • Finally, for a more advanced project, check out either Measure Your Magnetism or Recording on a Wire.

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Related Links

• Science Fair Project Guide
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