Pre AP Physicsmagnetic Fields of Bar Magnets Lab

Pre AP PhysicsMagnetic Fields of Bar Magnets Lab

Purpose: Explore the shape; direction, and interaction of magnetic fields.

Background: Although many substances exhibit slight magnetic properties, only iron, cobalt, and nickel and their alloys make strong permanent magnets. Iron alloys are easily magnetized, while cobalt or nickel alloys are not. Magnets made of these substances or of their alloys are capable of attracting or repelling other magnets both near and at a distance. If an object contains iron,cobalt,or nickel and a magnet is brought close to it, the magnet will cause, or induce, magnetism in the object and will then interact with it. Thus, a magnet can attract a nail that was not at first a magnet.

The concept of a force field is used to describe the force that one body exerts on another at a given distance. Just as gravitational and electric forces can be explained by gravitational and electric fields, magnetic forces can be explained in terms of a magnetic field around a magnet.

A compass is a small magnet free to pivot in a horizontal plane. The end of the magnet that points north is called the north (N) pole. The opposite end of the magnet is called the south (S) pole. The direction of the magnetic field lines is defined as the direction to which the north pole of a compass points when it is placed in a magnetic field. In this experiment, you will use a compass to investigate and map the magnetic field about a magnet.

Materials: Two bar magnets, sheet of paper, two paper clips, small magnetic compass, iron nail, iron filings.

A: Pole Types

1. Hold the compass and allow its needle to come to rest and be sure it really points north. To check your magnets set the compass on the table. Then pick up one of the bar magnets and bring its north pole near the compass. The north pole of the magnet should cause the compass needle to deflect so that the south pole of the compass points toward the north pole of the bar magnet. Verify that both bar magnets have the correct polar orientation. If the north pole of a bar magnet attracts the north pole of the compass, the magnet may be incorrectly magnetized. If this is the case, report the magnet's condition to your teacher. If both magnets have the correct orientation, then proceed with the experiment.

B: Magnetic Field Lines

1. Place a bar magnet on the table and cover it with a transparency. Gently, and as uniformly as possible, sprinkle iron filings over the transparency. Tap the transparency lightly with your finger several times until the filings form a pattern. The iron filings have aligned themselves with-the magnetic field. Sketch their appearance here.

2. Carefully pick up the transparency and return the iron filings to the container.

C. Magnetic Field Lines Between Poles

1. Place both magnets on the table with the north pole of one magnet about 4 cm from the north pole of the other magnet. Lay the transparency over the magnets. Gently sprinkle some iron filings over the transparency. Tap the transparencylightly several times until the iron filings form definite lines. Sketch the magnetic-field-line pattern on your paper showing the polar orientation of the two magnets. Return the iron filings to the container.

2. Repeat Step 1 with the south pole of one magnet facing the north pole of the other magnet. Return the iron filings to the container

D. Direction of the Magnetic Field Lines

1. Place a bar magnet on the table. While referring to your sketch from Part B, slowly move the compass from one pole to the other along one of the magnetic field line arcs. Draw arrows pointing in the direction of the northpole of the compass. Move the compass to different locations around the magnet and draw the direction of the magnetic field line at each location by showing little compasses at the appropriate locations. You must have at least ten little compasses on the diagram with the direction each compass points to be satisfactory.

2. Place both magnets on the table with the north pole of one magnet about 4 cm from the north pole of the second magnet. While referring to your sketch from Part C1, slowly move the compass from one pole to the other along one of the magnetic field line arcs. Draw arrows pointing in the direction of the north pole of the compass. Move the compass to different locations around the magnet and draw the direction of the magnetic field line at each location by showing little compasses at the appropriate locations. You must have at least ten little compasses on the diagram with the direction each compass points to be satisfactory.

3. Place both magnets on the table with the south pole of one magnet about 4 cm from the north pole of the other magnet. While referring to your sketch from Part C2, slowly move the compass from one pole to the other along one of the magnetic field line arcs. Draw arrows pointing in the direction of the north pole of the compass. Move the compass to different locations around the magnet and draw the direction of the magnetic field line at each location by showing little compasses at the appropriate locations. You must have at least ten little compasses on the diagram with the direction each compass points to be satisfactory.

E. Induced Magnetism

1. Test an iron nail for magnetism by touching it to the paper clips. Place the nail at one end of a bar magnet. Now bring the nail close to the paper clips while it is attached to the magnet. Record your observations (at least 2).

2. Bring the free end of the nail near your compass. Note that the free end of the nail has become a pole. Check its polarity compared to that of the end of the magnet to which it is attached. Record your observations (at least 2).

Analysis: Answer in complete sentences.

1. At what points in the magnetic field about a magnet are the magnetic field lines most concentrated?

2. Describe the magnetic field lines between two like poles.

3. Describe the magnetic field lines between two unlike poles.

4. Describe the orientation of the compass needle relative to the magnet's poles in the magnetic field of a bar magnet.

5. When an iron nail is attached to a magnet, how does the pole type at the free end of the nail compare with the pole type of the end of the magnet to which the nail is attached?

Application:

Draw an Earth-shaped object and label the north and south poles. The interior of Earth can be thought of as a bar magnet with one pole of the magnet at Earth's north magnetic pole and one pole at the south magnetic pole. Sketch a bar magnet inside of your sketch and show the correct pole types for Earth. Sketch the magnetic field lines about Earth (include a minimum of 4 magnetic field lines).