Day 7: Map Magnetic Field of a Room
Map the magnetic field of a room (classroom or room at home -- recommend
kitchen) at two or more heights.
Discussion and Background:
Our task is to make a map of the direction of the magnetic field
within a particular room. It turns out that the process of science does
not allow us to take a measurement at a single point in space and then
claim the measurement necessarily represents what is happening at other
points in space. (Why is this so?) Thus, we must make many measurements
but in a systematic way. We will thus do three things to help us get useful data.
- Create a coordinate grid within the room.
- Make a relatively accurate map of the room to super impose the measurements on.
- Take data at the floor level and at a level several feet below the
ceiling. (If you like, take a third set of measurements at a height in
between the floor and ceiling measurements.
Notice I have called the data measurements. They are. You are measuring
the DIRECTION of the magnetic field. You are not measuring the strength.
We will leave strength measurements for another time and place.
Prepare grid (0.5 meter) using meter stick, string, and tape. The grid should
run so that it divides the room into many smaller squares 0.5 meters on an edge.
Start by measuring 0.5 meter increments along the floor next to a wall. Repeat
on the opposite wall. Connect the points with a taut string and tape the ends
down. Repeat for the other two walls.
Make a simple 2-dimensional paper map of the room with all objects located.
Include location of electric outlets, appliances, clocks, switches, lights, as
well as tables, counters, and water pipes. Make the map scale large enough
that it fits on an 8x11 inch piece of paper without much room to spare. Include
grid scale for viewer.
Make 4 or more copies of the map.
Systematically, place your magnetometer on grid intersections and record
DIRECTION of bar magnet. You may need to wait a few minutes after moving
the bottle before recording the direction in order to let the 'sensor card'
stop swinging around. Make careful note of any changes in the room occupancy,
location of objects, use of electrical appliances when measurement is being taken.
To record the direction, use a protractor and attempt to draw a line on your
map at the intersection of the grid you are measuring. Make the line fairly
short (say 1-2 cm) centered on the grid intersection. Indicate with an arrow
head the 'positive' direction.
Repeat the measurement procedure for a 2nd (and 3rd?) height level by hanging
the magnetometer from the ceiling by a string. This is why we assembled the
magnetometer with an extra long piece of thread outside the jar. Use the same
grid system to locate measurements (eyeball this). Record on a new room map.
Make sure your maps are clean and protected from further marking. Attach to
them your procedural notes from the data collection process. Comment on any anomalies.
You have made a map of the total magnetic field direction for a particular
height in a room. Hopefully, you have made maps of several different levels.
What does the data mean or tell us? This is the question we must work on.
Work through discussion and thinking to answer the following questions.
- Look at a pair of adjacent points that demonstrate the magnetic field has
different directions at different points in space. From these two points can
you infer the direction of the magnetic field at points on the line which joins
the two grid points you chose to look at? How confident are you in this
inference? Explain what you need to do to make better inferences.
- Look at the map as a whole. Make a tracing of it. Predict the direction
of the magnetic field, by drawing arrows, at the 1/4. 1/2, and 3/4 grid point.
(12.5, 25, and 37.5 cm from each vertex on the grid lines.) This is of course
an extension of what you did in question 1.
- Make a plan to check some of the measurements for homework. Do not check
every prediction! Which ones are you least sure about? Do you have any that
are not connected to any other? Are they on an edge, in a corner, or somewhere
in the middle of the room? Can you make a reasonable case, based on what you
know about magnetism, about what is causing the field direction to change? Does
the field direction have to change in a smooth way or can it be jumpy (suddenly
switching direction with out going through the intervening directions)?
- Make a new tracing that shows the new measurements and the new connections
between the grid points.
What your map shows is the direction of the magnetic field at various points
in the room. It probably does not look like straight lines but rather wavy
lines. In places field direction lines may be closer together than in other
What does the map reveal about the direction of the field lines in the spaces
between the grid lines? Are there any regions of space that are completely
free of magnetic field? That is: is there any place in the room you mapped
that has absolutely no magnetic field?
How can you check this with an SBM?
Let's make a map of the field lines for a bar magnet using the SBM. Obtain
from your teacher a cow magnet or similar strong magnet. Place it on a small
stand (a cup, perhaps) so that it is at approximately the same height as the
magnet in the SBM.
Be careful not to place your set-up or SBM too close to another group's area!
(Can you tell me why this is important?)
Make a grid using a 5 cm scale, for a 100x100 cm area. Place the bar magnet
in the center and record its orientation. Different groups may choose different
alignments of the bar magnet relative to the grid system but all should place the
center of the bar magnet at the center of the grid.
Map the direction of the magnetic field of the bar magnet at each grid point.
Draw smooth curves that show the direction of the magnetic field at each point in space.
Lesson Development/Writing: Ed Eckel
Web Design: Theresa Valentine
Last Updated: 8/25/2000