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Day 5: Interpreting Magnetic Field Maps
Student Pages

Yesterday you made a map of the direction of the magnetic field of a bar magnet. For homework, you predicted what the field would look like for 5 different arrangements of 2 bar magnets. How sure are you of your predictions?

Activity 1

Let's look at the maps you made for homework. In small groups (as assigned by your teacher) look at each person's map for each arrangement and note commonalties and differences. Discuss both of these sets of features. Pay special attention to the reasons people give for making the drawings as they did.

Do not change your map but do take notes on reasons given for maps with different appearances.

Now pick one of the bar magnet arrangements and test your prediction. That is, set up two bar magnets as given in one of the homework problems. Then use your magnetometer to map out the field lines as we did yesterday.

Compare your observed map to your predicted map. You may not get the same map as you predicted. There are some limits to how precisely a magnetometer can reveal the magnetic field. Please write down the limitations you can think of. I am assuming you were careful taking the measurements and recording them, so things like "human error" shouldn't be in your list. Does the line you draw to record an observation really give the actual field direction along the entire length of the line you drew? There are other possible errors.

Activity 2

Now map the field of a stack of ring magnets. First map the field of the stack while the stack is lying on its side, forming a tunnel, if you will. Second, map the stack on end, forming a well.

Questions: The same magnets produced each of the mapped fields. What is different? Are the two maps representations of the same thing? If so, how is the thing changed between representations? Was anything changed which affected how the magnets formed the field? Where both of the representations present in each of the mapping exercises? Could you have turned something else on its side to get the two different maps? Could you have predicted one representation from knowledge of the other?

Activity 3

The maps you made are not the only representation possible for the magnetic fields you have been working with. What would the same fields look like if we choose a different plane to make our measurements in? How would the appearance of the map change? What if the plane where at right angles to the one we used (the plane of the table was parallel to the long axis of the magnet?) Can you think of a way to arrange a bar magnet so that you can make measurements along a plane perpendicular to the long axis of the bar?

Insert image of bar magnet standing on end

Predict what such a field map would look like.

Now test your prediction. Remember to remove the effect of the earth's magnetic field from your measurements.

Suppose a magnetic object was moving parallel to the long axis of the bar magnet. Please make a graph of the magnetic field as a function of the position of the object in the field. That is, make a map showing the direction and relative strength of the field along the path of the object. Discuss your map with your group.

Would the magnetic field cause a change in the path of a magnetic particle? How would the path change? Base your argument on conservation of energy! (Secret: a magnetic field can do no work but can change the path!)

Homework suggestion:
Construct a way of making a magnetic field that has only straight field lines.

Lesson Development/Writing: Ed Eckel
Web Design: Theresa Valentine
Last Updated: 8/24/2000

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