Day 3: Mapping a Dipole Magnetic Field
Lesson Plan

Students will work cooperatively (groups of 2) using the magnetometer built in the previous class to discover the shape of a magnetic field due to a bar magnet. Such a field is called a "dipole field." Note: At this point the students have not been formally introduced to the term "magnetic field."

Students will use the magnetometer to map the field of a bar magnet. The map will indicate direction of field only, and will resemble a 'dipole field' once the magnetic field of the earth is subtracted from the observations.

Student Outcomes:


Opening Probing Question:
Where does a magnetic force begin and end in the space around the magnet?

Suggested Response:
We (the class) do not know yet. Some may know the field representation uses closed loops. Others may argue that the force begins on the magnet and ends on the object experiencing the force. Perhaps challenge the response with a request to apply Newton's Third Law to the explanation and then ask what is meant by the words "begin" and "end". Is there a material object or thing that accepts magnetic force like an electric light bulb accepts current?

Opening Discussion:
Recap the investigation done on Day 1. Recall that we knew a magnet was affecting another object by the motion induced in the object (moved away from or toward magnet) or by the fact that the object did not fall in a gravitational field when placed against the magnet. Both of these are examples of thinking with Newton's Laws, of course.

Remind students of vectors. The nature of motion and force is that each requires two elements for a complete description: direction and quantity. That is, motion and force are both intrinsically VECTOR quantities.

Close discussion by asking students if magnetism demonstrates vector or scalar characteristics.

Hand out materials and student activity pages.

Activity 1

Obtain a good quality representation of the total magnetic field around a bar or dipole magnet.

Data Collection Procedure:

Data Analysis Questions to be Done by Small Group

Class Discussion
The goal of this discussion is to arrive at the following understanding. Recap the questions given in the activity to achieve this.

The students have mapped the magnetic field of a bar magnet. They have found some anomalous observations as well as some fairly consistent patterns. The consistent pattern is called a dipole field map. By comparing maps between groups, the differences in recorded observations ought to inspire some examination of the data collection procedure. Differences may be due to current carrying wires, to chunks of metal, and to other magnets in the vicinity of the mapping location. Further, all observations have picked up a small contribution from the earth's magnetic field. The variations should be different at different locations in the room due to the strength of the bar magnet used, the orientation of the grid system relative to magnetic north, and limits to the precision with which observed deflection is represented on the grid. The contribution of the earth is approximately the same at every point in the room. One of the difficulties with the earth contribution is that it is weak. Therefore, it has an increasingly important effect as you move farther from the bar magnet in the center of the map.

Generate the following consensus: To get the actual map of the field of a bar magnet, we must know what the 'ambient' local field looks like in the absence of the bar magnet. We can then compare the total field to the ambient field (from Activity 2) to determine the actual bar magnet field.

Suggested Questions for Discussion
Does your bar magnet map reveal just the effect of the bar magnet or is the map revealing a complex combination of different effects.

The first map, done as directed, reveals the superposition of the field of the earth and the bar magnet. If different groups had their bar magnet at different orientations relative to some fixed object in the room, different groups may have maps with different appearances.

Chemists describe the extent of an electron cloud in terms of the point where 90% of the electron density is within some radius. Can you find a similar position on your grid, where 90% or more of the observation is due to just the bar magnet? How are you assigning magnitudes to the observed direction measurements? The analogy is possible but this is a deep question some students may not be prepared for. The purpose of the question is to help instill a sense of familiarity with the idea of combinations.

Activity 2

Obtain representation of ambient magnetic field at location of Activity 1.

Data Collection Procedure:

Data Analysis Questions for Small Group

Class Discussion:
Goal: While recapping the questions for Activity 2, achieve the following motivation for the next Lesson Plan (Day 4).

We need to remove the effect of the earth's field from the observations to get a true representation of the field of a bar or dipole magnet. We can do this one of 2 ways. One is to visually subtract by comparing the direction of the arrow in Activity 1 to the direction of the arrow in Activity 2 at the same grid position. Another is to rotate the bar magnet 90o, record a map of the new field shape, and compare the result with Activity 1. We will do both of these during the next class.

Suggested Discussion Questions:

Homework Suggestion:
Read about the interaction between the solar wind and the earth's magnetic field.

Lesson Development/Writing: Ed Eckel
Web Design: Theresa Valentine
Last Updated: 8/11/2000