Exercise: Mapping a Magnetic Field

Joyce Gridley

Leonard Freise

Lane Matheson

Objective: To map the field lines of a bar magnet.

Equipment: (per group)

  • Bar magnet
  • Large sheet of paper (e.g., 11”x17” or butcher paper)
  • Sewing needle
  • Sewing thread
  • Pencil
  • Masking Tape
  • Magnetizer or alternative method for magnetizing needle

Procedure:

  • Magnetize a sewing needle. Tie an 8”-10” piece of thread to the needle so that the needle is parallel with the ground when the string is held at the top. A small piece of tape or a very small dab of glue will keep the needle from slipping through the thread. [Note: the thread can be tied to the needle before magnetizing.]
  • Tape a bar magnet in the center of the sheet of paper.
  • While holding the thread, bring the needle near the magnet. Caution: try not to touch the needle to the magnet, but the results of the exercise will not be compromised if touched briefly. Use the pencil to draw a short line on the paper that is parallel to the needle.
  • Move the needle so that one end is at the end of the line. Again draw a short line parallel to the needle. Note: the pencil can be used to settle the needle’s motion by touching the pencil to the needle briefly.
  • Continue drawing lines until the needle returns to the magnet or is no longer above the paper.
  • Start at a different place on the magnet and repeat the above procedure until several field lines are drawn. See picture on next page for example.

Discussion:

Magnetic field lines are continuous, smooth lines drawn to visualize a magnetic field. The magnetic field vector is tangent to the field line at each point.

Questions:

  1. If the paper were large enough, what would be the shape of all of the lines?
  2. How could the lines nearest the center of the magnet be flattened to appear more elliptical?
  3. How would you improve this activity to get smoother lines?

Extension:

The magnetic dip can be observed by cutting off a short piece of the needle and placing it in a piece of tape. Use a hole-punch to make a hole in the tape above the needle and place a length of thread through the hole. This set-up will demonstrate the magnetic dip better than the longer needle.