Physics 8B Lab 2 – Magnetic Fields rev 4.0 Lab 2 – Magnetic Fields Part 1: Magnetic Field of a Bar Magnet 1) Finding the direction of magnetic fields: The term magnetic pole is used to identify a portion of a magnet that has specific magnetic properties. For a bar magnet, the poles are usually located at the ends of the magnet. To find the poles and the direction of the magnetic field of a magnet, you will use a compass. A compass is simply a small magnet on a pivot (shown as an arrow below). When placed in an external magnetic field, it tries to line its magnetic field up with the external field. So the direction of the compass needle shows the direction of the external magnetic field at the location of the compass. The compass needles in this lab are not shaped like a little arrow, but instead have a red and a white end to them. So you will need to determine which color corresponds to which end of the compass. Do this by placing the compass in a known magnetic field, namely that of the Earth. a) In Berkeley, the Earth’s magnetic field (or rather, the component parallel to the surface of the Earth) points “toward Canada” i.e. toward “ geographic north” (see below for a word of caution). Hold the compass in your hand far away from any other magnets on the table, and from the table itself. Which color of the compass needle points in the direction of the Earth’s magnetic field? Note: These are inexpensive compasses. The colors will not be uniform from group to group. Furthermore, be careful not to bring the compass in contact with other magnets, as the compass needle can be accidentally re-polarized. N S N S (Before) (After) B ext B ext
Physics 8B Lab 2 – Magnetic Fields rev 4.0 Comment about the Earth’s magnetic field The Earth’s magnetic field looks as if there is a magnetic dipole in its interior. This dipole is not perfectly aligned with the geographic poles but unless you are located in a polar region, this tilt is not very important. But the naming of the poles can be confusing. The Earth’s geographic North Pole is a Magnetic south pole, while the geographic South Pole is a Magnetic north pole as shown. Magnetic fields point out of Magnetic north poles and into Magnetic south poles. A compass lines itself up with the Earth’s Magnetic Field i.e. it points toward the Magnetic south pole (which happens to also be the geographic North Pole) Place the (cylindrical) bar magnet on the table. Use the compass to explore the region surrounding the magnet. The colored end you found in a) will tell you which way the magnetic field points at each location of the compass. Try various locations both close to and far from the magnet. b)
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