To study the magnetic fields around different shapes of magnets.
- copy of the "Visualizing Magnetic Fields" student handout (PDF or HTML)
- 1 tablespoon of iron filings in paper cup
- 8 1/2-inch x 11-inch paper
- magnets of different shapes, including round, bar, and horseshoe magnets
Earth has a magnetic field that shields it from harmful cosmic radiation. In some ways, Earth's magnetic field behaves in the same way that magnetic fields on ordinary magnets behave. Tell students they will be exploring some of the properties of magnetic fields in this activity.
Organize students into groups and distribute the "Visualizing Magnetic Fields" student handout, iron filings, paper, compasses, rulers, and different-shaped magnets to each group.
Have students select a magnet and place a sheet of paper on top. Instruct them to lightly sprinkle iron filings over the paper. Ask them to sketch the magnetic field on another piece of paper.
Have students use their compasses to determine the direction of the field and indicate this with arrows on their diagrams.
Ask students to move a compass farther and farther out to where the magnetic field weakens. (At this distance the compass will switch from indicating the magnet's magnetic field to indicating Earth's magnetic field.) Students should record this distance on their diagrams.
Have students repeat the experiment with magnets of different shapes.
Have each group answer the questions its student handout. Discuss students' findings. What caused the compass to change direction when moved away from the magnet? What direction did the compass exhibit when moved away from each of the magnets? Why might this be?
As an extension, have students create a timeline of Earth's magnetic field reversals. Find more information at www.pbs.org/nova/magnetic/
In a material with magnetic properties, such as a paper clip, groups of atoms are like tiny magnets with north and south poles. When a paper clip is stroked with a magnet, the north and south poles are temporarily aligned. This creates a magnetic pull strong enough to pick up another paper clip. When the paper clip is banged on the table, the alignment is disrupted and the magnetic effect ceases.
Iron filings will align with each magnet's magnetic field. The shape of the magnet and the location of the poles determine the shape of the field. Bar magnets and horseshoe magnets show the filings clumped near the poles. Round magnets have a round magnetic field, which will form the iron filings into a semi-spherical shape above the paper. Most of the magnetic field lines will occur near the magnet's poles. On a round magnet, one flat side is the north pole and the other flat side is the south pole.
Moving the compass around the magnet will reveal that the magnetic field increases in strength as the compass gets closer to the magnet. When the compass is moved far enough away from the magnet, its needle will align with Earth's magnetic field rather than the magnet's magnetic field. This is because as the magnet moves away from the magnet's magnetic field it becomes more strongly influenced by Earth's magnetic field.
Earth is like a magnet because it also has a magnetic field. Earth's magnetic field is most similar to the magnetic field generated by a bar magnet. Earth's magnetic field differs from that of a bar magnet in that it is much less symmetrical, an effect due to solar wind spreading out magnetic lines that lie on Earth's nightside. (These lines form Earth's magnetotail, which is the main source of the polar aurora.) Since Earth generates a magnetic field, a compass will align with its field and point to the North Pole.
You may want to point out to students that what is considered Earth's magnetic North Pole is actually its magnetic South Pole. This is because magnetic field lines flow from the north to the south on a magnet. Earth's magnetic field lines flow outward from the Southern Hemisphere and inward to the Northern Hemisphere, technically making the magnetic North Pole Earth's southern magnetic pole.
NOVA's Web Site—Magnetic Storm
In this companion Web site for the NOVA program, learn about the impact of magnetic fluctuations on animal navigation, scan a timeline of magnetic reversals, view a gallery of auroras, and see how Earth's magnetic field works.
Ask the Space Scientist About Earth—Magnetic Field
Lists frequently asked questions about Earth's magnetic field, including questions about magnetic field reversal and its effect on humans.
Core Convection and the Geodynamo
Discusses a mechanism that may be responsible for continuous generation of Earth's magnetic field.
Explores magnetism and explains magnetic field lines and electromagnetic waves.
North Magnetic Pole
Provides background on what a magnetic pole is and discusses how scientists track magnetic pole movement.
Livingston, James D. Driving Force: The Natural Magic of Magnets. Cambridge, MA: Harvard University Press, 1996.
Tells the history of magnets and how they have been used in motors, VCRs, and high-speed trains. Also discusses the impact of magnetism on culture.
Lee, E. W. Magnetism: An Introductory Survey. New York: Dover Publications, 1990.
Describes magnetic phenomena.
The "Visualizing Magnetic Fields" activity aligns with the following National Science Education Standards.
Science Standard D:
Structure of the Earth system
The solid Earth is layered with a lithosphere; hot, convective, mantle; and dense, metallic core.
Science Standard D:
The origin and evolution of the Earth system:
Interactions among the solid Earth, the oceans, the atmosphere, and organisms have resulted in the ongoing evolution of the Earth system. We can observe some changes such as earthquakes and volcanic eruptions on a human time scale, but many processes such as mountain building and plate movements take place over hundreds of millions of years.
Classroom Activity Author
A teacher for 34 years, Steven Branting currently serves as a consultant for gifted and innovative programs in the Lewiston, Idaho, public schools and is a cartographer for the Lewis & Clark Rediscovery Project. Branting and his students have won international honors in physics, engineering and digital mapping.
For this step the teacher may simply do a demonstration for the class, or if you have enough materials, each group could build a 3-Dimensional (3-D) magnetic field visualizer. As the name suggests, you will construct (or buy if you have the funds) a device to visualize the 3-D structure of a magnetic field around a cow magnet.
To construct your visualizer obtain a clear plastic or glass bottle, small/medium sized (i.e. a 20 oz. soda or water bottle). Clear away any labels on the bottle.
- Cut a manila folder so that you can roll it up tightly into a tube about the diameter of your cow magnet and a length slightly longer than your bottle. Tape the tube to keep it rolled up.
- Seal one end of the tube with tape and stuff some paper into that end from the open end so that when you insert the cow magnet into the tube it will not go all the way to the bottom of the tube.
- Pour some iron filings into the bottle; enough to coat the bottom with a layer ¼ inch thick should be fine.
- Insert the tube into the bottle and use paper and tape to seal up the bottle opening around the tube.
Now, drop your cow magnet into the tube. Use a pencil to hold it in place and then shake the bottle. The iron filings will then stick to the outside of the tube and take the form of the magnetic field surrounding the magnet. Have students hypothesize the shape of the field before you actually do this.
You can remove the cow magnet by turning the bottle over and shaking it out (it will resist as the magnetic force of the filings will act to hold it in). Or you can fish it out of the tube by tying a string to a large paper clip and dropping it down into the tube and then pulling the magnet out. It’s a neat effect to watch the filings be dragged up the tube until the magnet disappears and the filings drop away like dust.
You can also purchase a pre-made, sealed tube with iron filings inside and a cow magnet for about $13 at most science classroom supply stores online (see resource list). For examples of the home-made tubes, see Figure 1.4a), and of a manufactured tube, see Figure 1.4b).
An optional method of viewing the 3-D field of force surrounding a magnet is to fill a bottle with mineral oil and a couple of table spoons of iron filings. Seal the bottle and shake it up. As the filings begin settling place a magnet (the stronger the better, and cow magnets are stronger than bar magnets of the same size generally) against the side of the bottle. Hold the bottle up to the light and you will see the filings moving along the magnetic lines of force. You should be able to see full loops of force from one pole to the other. If you have a horseshoe magnet (a bar magnet that has been bent into the shape of a horseshoe such that both poles are near each other) it can yield the most dramatic demonstration of the magnetic loops.