University of Virginia
Physics Department

Investigating Magnetic Fields

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

Motivation for Learning

Driving Question

How can you find your way when you are lost? Have the class brainstorm a list. Lead the discussion towards compasses. Why do people use a compass to find their way when they are lost? Explain to students that compasses point North because the needle is attracted to the Earth's magnetic south pole which is located at the Earth's geographical north pole. Have students predict why a compass will work anywhere. This should lead the discussion towards magnetic fields.
  

Background Information

A magnetic field is an area where magnetic forces exist due to a magnet or some other method of producing magnetic effects. The magnetic field produces loops of magnet lines which can be seen around a magnet when iron filings are sprinkled on and around the magnet. The small iron filings react to the magnetic forces and line themselves up along the magnetic lines. This represents the direction of the magnetic field at any point. These lines of magnetic force do not cross each other. Magnetic fields are strongest near the poles (north and south) of a magnet. Opposite poles of magnets attract; like poles repel. Magnetic fields can also be produced when electricity passes through a wire (electromagnet) and can be magnified when the wire is wound in a coil many times..

Magnetic fields are very useful in modern day life. We use magnets for everything from holding notes on refrigerator doors to changing the voltage entering our house from a transformer on a pole outside to treating patients in a hospital with a Magnetic Resonance Image (MRI). Magnetic fields can also cause some spectacular sights in nature, such as the Aurora Borealis, or the northern lights.

A word of warning: common bar magnets obtained from science supply companies are notoriously poor magnets. They quickly loose their magnetism when dropped, and it is sometimes difficult to remagnitized them. Ceramic magnets are stronger and generally last longer. Cow magnets obtained at farm supply businesses are quite strong, but are still bar magnets.

 

Student Activity

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Materials:

Hypothesis:

What do you think will happen when like magnetic poles are placed close to each other?

What do you think will happen when opposite magnetic poles are placed close to each other?

Procedure:

  1. Place a magnet on the table and put the transparent paper over it. This activity works best on pure wooden tables with no metal struts underneath the table. Nearby metal can strongly affect this experiment.
  2. Sprinkle iron filings on the paper.
  3. Observe the pattern of the magnetic field lines. Illustrate and describe what you see. What do you notice about the pattern?
  4. Carefully lift the transparent paper up and slide the filings off into the jar. If you get iron filings on the magnet, it is your responsibility to pull the filings off and place them back in the storage container.
  5. Place 1 bar magnet on the table and place another bar magnet close by. Place it in such a way that the similar poles are parallel to each other. They should be close enough to attract, but not slide together.
  6. Place the transparent paper over the magnets and sprinkle iron filings on them.
  7. Observe the pattern of the magnetic field line. Illustrate and describe what you see. What did you notice about the pattern of sprinkles between the poles of the two magnets? What conclusion can be made about magnetic effects and poles?
  8. Carefully lift the transparent paper up and slide the filings off into the jar.
  9. Once again place one magnet on the table. This time place the second magnet close by in such away that the magnets repel each other. Place them as close together as possible.
  10. Place the transparent paper over the magnets and sprinkle iron filings on them.
  11. Observe the pattern of the magnetic field line. Illustrate and describe what you see. What did you notice about the pattern of sprinkles between the poles of the two magnets? What conclusion can be made about magnetic effects and poles?

Conclusions:

What do you notice about the lines of magnetic force?

What happens when you bring opposite poles of a magnet together?

What happens when you bring like poles of a magnet together?

 

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

 

Assessment

Answers to Questions:

Step 3: Students should notice the lines of the magnetic field. They should notice that they do not overlap, and that there is a stronger attraction at the poles.

Step 7- Students should notice that the lines of the magnetic field at the poles are attracting each other.

Step 11- Students should notice that the lines of the magnetic field at the poles are repelling each other.

Conclusions:

  1. The fields surround the magnet. The poles attract more filings.
  2. Opposite poles attract.
  3. Like poles repel. Extension: As a class you can talk about the factors that can change magnetic fields. (Hitting a magnet, breaking a magnet, etc.) It would be interesting for students to note that the Earth's magnetic field changes. Scientists are studying evidence of this magnetic reversal in Death Valley.
  4.