## Illustration 27.1: Magnets and Compass Needles

This Illustration allows you to consider the magnetic field around a bar magnet. By default, the page will load with a magnet in the center of the animation. Use the compass to explore the magnetic field around a bar magnet by dragging the compass around the magnet. A compass utilizes a small permanent magnet; its arrow points toward the north pole of its magnet. Make a diagram showing the direction the compass needle points at various locations. Include enough points to establish a pattern. Restart.

Now that you have completed this diagram, turn the magnetic field vectors on to see this representation of the magnetic field. Does your diagram look like that of the animation's field vectors? It should. The magnetic field vectors are like little compass needles placed at various points around space. The color of the arrow of the magnetic field vector represents the strength of the field at the point, while the arrow shows the direction of the field.

You can also double click at any point in the animation to draw a magnetic field line through that point, which yields another representation. There will be a small delay after double clicking before the line appears (the line needs to be calculated). Double click at enough points to get an accurate picture of the magnetic field lines around the magnet. What is the difference between the field vectors and the field lines? Notice that in the field-line representation the field lines are tangent to the field vectors and also tangent to the direction of a compass needle placed at that point. In the field-line representation the field lines are drawn with the same color. In the field-vector representation the strength of the field is depicted in the color of the field vectors. How do we represent the magnitude of the magnetic field in the field-line representation? The density of field lines (lines per square length unit) is greater where the field is stronger.

Clear the screen. Place two magnets beside each other with the north pole of one lined up with the south pole of the other:

Display the field vectors and/or double click to display the magnetic field lines. How does the magnetic field of the two magnets compare to the field of one magnet? What do your observations suggest about how a bar magnet would behave when broken in half? The magnetic field vectors and lines for the above configuration look just like those for one large magnet. In fact, if you broke a bar magnet in half, what you would get would be two bar magnets, each with their own north and south pole. This is because there are no magnetic monopoles in classical magnetism. There are electric monopoles, which we call electric charges.

Predict what the magnetic field will be if you place a north-south magnet directly on top of a south-north magnet. Try it.

Illustration authored by Melissa Dancy.
Script authored by Morten Brydensholt.

Physlets were developed at Davidson College and converted from Java to JavaScript using the SwingJS system developed at St. Olaf College.