# 0.8 Magnetism  (Page 2/4)

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A ferromagnetic material is a substance that shows spontaneous magnetisation.

## The poles of permanent magnets

Because the domains in a permanent magnet all line up in a particular direction, the magnet has a pair of opposite poles, called north (usually shortened to N ) and south (usually shortened to S ). Even if the magnet is cut into tiny pieces, each piece will still have both a N and a S pole. These magnetic poles always occur in pairs. In nature, we never find a north magnetic pole or south magnetic pole on its own.

Magnetic fields are different from gravitational and electric fields. In nature, positive and negative electric charges can be foundon their own, but you never find just a north magnetic pole or south magnetic pole on its own. On the very small scale, zooming in to the size of atoms, magnetic fields are caused bymoving charges (i.e. the negatively charged electrons).

## Magnetic attraction and repulsion

Like (identical) poles of magnets repel one another whilst unlike (opposite) poles attract. This means that two N poles or two S poles will push away from each other while a N pole and a S pole will be drawn towards each other.

Attraction and Repulsion

Like poles of magnets repel each other whilst unlike poles attract each other.

Do you think the following magnets will repel or be attracted to each other?

1. We are required to determine whether the two magnets will repel each other or be attracted toeach other.

2. We are given two magnets with the N pole of one approaching the N pole of theother.

3. Since both poles are the same, the magnets will repel each other.

Do you think the following magnets will repel or be attracted to each other?

1. We are required to determine whether the two magnets will repel each other or be attracted toeach other.

2. We are given two magnets with the N pole of one approaching the S pole of theother.

3. Since both poles are the different, the magnets will be attracted to each other.

## Representing magnetic fields

Magnetic fields can be represented using magnetic field lines starting at the North pole and ending at the South pole. Although the magnetic field of a permanent magnet is everywhere surrounding the magnet (in all three dimensions), we draw only some of the field lines to represent the field(usually only a two-dimensional cross-section is shown in drawings).

In areas where the magnetic field is strong, the field lines are closer together. Where the field is weaker, the field lines are drawn further apart. The number of field lines drawn crossing a given two-dimensional surface is referred to as the magnetic flux . The magnetic flux is used as a measure of the strength of the magnetic field over that surface.

1. Field lines never cross.
2. Arrows drawn on the field lines indicate the direction of the field.
3. A magnetic field points from the north to the south pole of a magnet.

## Investigation : field around a bar magnet

Take a bar magnet and place it on a flat surface. Place a sheet of white paper over the barmagnet and sprinkle some iron filings onto the paper. Give the paper a shake to evenly distribute the iron filings. In yourworkbook, draw the bar magnet and the pattern formed by the iron filings. Draw the pattern formed when you rotate the bar magnet to a different angle asshown below.

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