Question

III. Mapping the Field Around a Bar Magnet: An Observation Experiment Purposes: Design an experiment to take appropriate data to find a relationship. Construct a mathematical model to describe that relationship Description: Your task is to use a compass to map the magnetic field of a small magnet. Construct a circle of radius 10 cm on a clean sheet of paper, and mark points on the circle every 30°. Place the bar magnet so that its center is resting at the center of the circle and its N-S axis is aligned with one of the points on the circle. Then. at each point on the circle: Place the compass center there, and let the needle settle. Then remove the compass and draw a vector at that point to indicate the direction of the field there. Use the following guide. All lettered steps must be addressed in your write-up; a subset of those steps will be graded. a. Do your best to extrapolate and sketch a continuous set of field lines from your vector sketches b. Does the Earth's field have a significant effect on your observations? Explain your answer using your sketch c. One way to model the earth's internal magnetic field is as a bar magnet. Suppose you do that and-as a first assump- tion-align the compass axis with the earth's own rotational axis, as shown here. Knowing what you know about how a compass behaves, label the poles of this "internal geo-magnet." d. Now, suppose that Corvallis (about 44.50 N latitude) is just on the edge of this view (so, just on the "rim"-the horizon-of the drawing here). Place a dot at that approximate location. Then, using the vector sketch of your bar magnet's field from above, copy that field pattern onto this drawing, and use that to estimate the angle at which the earth's magnetic field would be directed below the surface here in Corvallis. North geographic pole (This is called the inclination or "dip" angle.) e. Recent measurements of the earth's magnetic field in the local Corvallis area indicate an inclination angle of about 67°. If your estimate based on map- ping of an actual bar magnet didn't suggest this angle, is the bar-magnet model of the earth's mag- netic field completely worthless, or can you suggest adjustments to the model (ie. changed assumptions) that would produce results that more closely match local measurements? And what about your vector sketches above? Where didn't you measure? f. See if you can draw a better model of a bar-magnet-like structure of the earth's field than the one drawn here. Equator South geographic pole IV. Practice with Magnetic Field

Answer 1

a)

b) Horizontal component of Earth magnetic field will interact with bar magnet field. Compass needle will point in the net vector sum of these two fields. It affect our true observation.

c) In response to the Earth's magnetic field, the compass will point toward the geographic North Pole of the Earth(or magnetic south pole)

Magnetic field lines