Focus question

What happens to the magnetization of a metal core that is removed from an electromagnet? 

Essential materials

• paperclip electromagnet (See Build an Electromagnet)
  – about 1 meter (3 feet) of 22 gauge insulated wire
  – wire strippers
  – about a dozen staples
• small 1.5 volt battery
• magnetic compass
• sheet of A4 or letter size paper
• metric ruler with millimeter scale
• pen or pencil
• timer
• unfolded paper clip with tape on one end
• data chart
• small permanent magnet

Main ideas and background information

• An electromagnet is built by coiling insulated wire around a metal core that contains iron. When current from a battery passes through the coiled wire, the metal core becomes magnetized.
• An electromagnet is able to pick up objects containing iron when current is flowing through the coil. When the current is stopped, the electromagnet drops objects that it has picked up.  
• When the current is stopped, the metal core begins to lose its magnetization. For some metals, this happens practically instantly. For other metals, it takes place over a longer time.
• This loss of magnetization over time is called magnetic relaxation.

Procedural tips

• We suggest that an adult remove the insulation from the ends of all wires before giving them to students. An inexpensive wire stripper tool can do this quickly and safely.
• When making the electromagnet, wrap the insulated wire at least 40 times around the unfolded paper clip. Wrap the wire somewhat loosely, because you will need to remove the paper clip for the activity.
• Make sure that the compass needle is not affected by any metal in your table or in any nearby object.
• After you tape the paper into position, we suggest that you tape the compass down as well. This will prevent any accidental movement during the experiment.
• Do not connect the battery to your electromagnet for longer than 10 seconds. In our experience, this is more than enough time to magnetize the unfolded paper clip sufficiently.
• Not all paper clips have the same magnetization properties. If your paper clip does not lose its magnetization during the period of measurement, do the experiment again with a different paper clip, this time connecting it to the battery for a shorter period of time. You might also increase the measurement time to a day or more.
• If your paper clip does not seem to become magnetized at all, it probably contains little or no iron. Try a paper clip made by a different manufacturer.

Discussion

1. Explain why you were able to pick up staples with the electromagnet when it was connected to the battery.
   (The electrical current in the coil magnetized the unfolded paper clip.)

2. Were you able to pick up staples with the electromagnet after it was disconnected from the battery? Why or why not?
   (Answers will vary depending on the characteristics of the paper clip and the staples. The magnetized paper clip might be able to pick up a staple or two. However, if it loses its magnetization quickly, then it might not be able to pick up even a single staple.)

3. Did the strength of your magnetized paper clip change over time? Why or why not?
   (Not all paper clips are the same. Some clips will begin to lose their magnetization noticeably within an hour or two, corresponding to short relaxation times. Clips with longer relaxation times will not lose their magnetization as quickly.) 

4. What are some variables you could manipulate that might change the relaxation time of the paper clip? (Responses could include the number of coils of wire, the size of the coils, the wire diameter, or the length of time the battery is connected.)

Assessment

Are students able to describe what happens to the magnetization of a metal core that is removed from an electromagnet?

Answer: In most cases, the metal core will begin to lose its magnetization over time. For some metal core materials, this happens relatively quickly (short relaxation time). Other materials lose their magnetization at a slower rate (long relaxation time).

Extensions and further investigations

• Have students repeat the experiment using
  – different sizes and types of paper clips.
  – iron nails or other long metallic objects instead of paper clips.
  – different magnetization times.
  – different measurement times (e.g., days instead of hours).
  – lines drawn at different distances from the compass.
  – electromagnets with more turns of wire.
  – electromagnets with larger sizes of wire.
  Note: For best results, change only one aspect at a time.

• Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then have them compare the results with their predictions and develop possible explanations for any discrepancies.

Career connections

• Oilfield exploration teams        

• Medical applications of magnetization, such as with NMR

• Electrical engineering

• Careers dealing with uses of electromagnets in handling of materials

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B – Physical Science: B1, B2
Standard D – Earth and Space Science: D1

Grades 9-12
Standard B – Physical Science: B1, B4, B5, B6

Britain: This activity correlates with the English National Curriculum standard Sc1, Science Enquiry, and the following additional standards:

KS3, Sc4, 1d, e, f, Bos:1b, d, e
KS4, Sc4, Useful Background, BoS:1b, d, e
KS5, Physics (Medical) AWA Public Understanding of Science

Glossary/vocabulary

electromagnet
magnetic relaxation

Resource links

Dialogue for Kids Electricity Facts

This site has general information about electromagnets. For electromagnet information and activity, scroll down nearly to the bottom.

Kids Can Make It.com — Electromagnet Activity

Here is a more complete treatment of electromagnets.