Physics 8B Lab 2 – Magnetic Fields rev 4.0
LAB 2 – MAGNETIC FIELDS COMPLETE ACTUAL EXAM (SOLVED 100%)
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.
N Bext S N Bext
S
(Before) (After)
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?
red needle points in direction of 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.
181
, 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)
Earth’s geographic North Pole
Earth’s Magnetic
south pole
S
N
S
Earth’s Magnetic
north pole N Compass
Earth’s geographic South 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) Summarize your results by drawing the magnetic field lines for the bar magnet.
Be sure to include directions and label the poles of the magnet.
182
LAB 2 – MAGNETIC FIELDS COMPLETE ACTUAL EXAM (SOLVED 100%)
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.
N Bext S N Bext
S
(Before) (After)
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?
red needle points in direction of 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.
181
, 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)
Earth’s geographic North Pole
Earth’s Magnetic
south pole
S
N
S
Earth’s Magnetic
north pole N Compass
Earth’s geographic South 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) Summarize your results by drawing the magnetic field lines for the bar magnet.
Be sure to include directions and label the poles of the magnet.
182
