Magnetism

Last updated on May 14th, 2022 at 02:28 pm

In this post, we will discuss the fundamentals of magnetism. Also, a number of related terminologies like magnetism, magnetic fields, ferromagnetism, paramagnetism, diamagnetism, permanent magnet, poles of magnets, magnetic attraction & repulsion, drawing field lines, a bar magnet, retentivity, compass, earth as a magnet, etc. , will be addressed.

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Define Magnetism

Magnetism is an interaction that allows certain kinds of objects, which are called ‘magnetic’ objects, to exert forces on each other without physically touching.

A magnetic object is surrounded by a magnetic ‘field’. A second object can feel a magnetic force from the first object because it feels the magnetic field of the first object. The further away the objects are, the weaker the magnetic force will be.

Humans have known about magnetism for many thousands of years.

For example, “Lodestone” is a magnetized form of mineral magnetite(Fe₃O₄). It has the property of attracting iron objects. It is referred to in old European and Asian historical records; from around 800 BCE in Europe and around 2600 BCE in Asia.

In 1269, Pierre de Maricourt was mapping the position of a magnetized needle placed at various positions on the surface of a spherical piece of this rock. He observed that the directions of the needle formed a pattern that encircled the rock and converged at two points on opposite ends of the rock. When this rock was then suspended by a string, the two converging points tended to align along Earth’s north-south axis. This property of the rock earned it the name “lodestone” or “leading stone.” Maricourt called the end pointing northward the north-seeking or north pole and the end pointing southward the south-seeking or south pole. All magnets have both poles. Lodestone, which contains the mineral magnetite (Fe₃O₄), was later used in the development of compass technology.

William Gilbert – earth is a lodestone

William Gilbert compared the orientation of magnetized needles on the surface of a spherical piece of lodestone with the north-south orientation of a compass needle at various locations on Earth’s surface. From this study, he proposed that Earth itself is a lodestone with north and south magnetic poles.

Michael Faraday defined a magnetic field

Michael Faraday defined a magnetic field as a three-dimensional region of magnetic influence surrounding a magnet, in which other magnets are affected by magnetic forces. The direction of the magnetic field at a given location is defined as the direction in which the north pole of the compass needle points at that location. Some materials, such as iron, act like magnets while in a magnetic field.

Define a Magnetic field

A magnetic field is a region in space where a magnet or object made of magnetic material will experience a non-contact, magnetic force.

State the law of magnetism

The Law of magnetism states: “Like magnetic poles repel and unlike poles attract each other”

Magnetic field associated with moving charges

A moving charged particle has magnetic fields associated with it. One example of a charged particle is the electron. Electrons are in constant motion inside the material, orbiting the nucleus in the atom which may also be moving, rotating, or vibrating.

Plastic ball is not magnetic and has no magnetic field – why?

Electrons inside an object are moving and have magnetic fields associated with them. In most materials, these fields point in various directions, so the net magnetic field is zero.

For example, in the plastic ball below, the directions of the magnetic fields of the electrons (shown by the arrows in the figure below ) are pointing in different directions and cancel each other out. Therefore the plastic ball is not magnetic and has no magnetic field.

ferromagnetic materials and magnetism

In some materials (e.g. iron), called ferromagnetic materials, there are regions called domains, where the electrons’ magnetic fields line up with each other. All the atoms in each domain are grouped together so that the magnetic fields from their electrons point the same way. The picture shows a piece of an iron needle zoomed in to show the domains with the electric fields lined up inside them.

Making permanent magnets from ferromagnetic materials

In permanent magnets, many domains are lined up, resulting in a net magnetic field. Objects made from ferromagnetic materials can be magnetized, for example by rubbing a magnet on the surface of such ferromagnetic object in one direction. This causes the magnetic fields of most, or all, of the domains to line up in one direction. As a result, the object as a whole will have a net magnetic field. It is magnetic.

Once a ferromagnetic object has been magnetized, it can stay magnetic without another magnet being nearby (i.e. without being in another magnetic field). In the picture below, the needle has been magnetized because the magnetic fields in all the domains are pointing in the same direction.

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 permanent magnet is cut into tiny pieces, each piece will still have both a North and a South pole. These magnetic poles always occur in pairs. In nature, we never find a north magnetic pole or south magnetic pole on its own.

In nature, positive and negative electric charges can be found on their own, but you never find just a north magnetic pole or south magnetic pole on its own. On a very small scale, zooming in to the size of atoms, magnetic fields are caused by moving 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 North pole and a South pole will be drawn towards each other.

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

Example 1

We are given two magnets with the N pole of one approaching the N pole of the other.
Since both poles are the same, the magnets will repel each other.

Example 2

We are given two magnets with the N pole of one approaching the S pole of the other.
Since both poles are different, the magnets will be attracted to each other.

Representing magnetic fields (2 D and 3D)

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 through that surface.

Magnetic field lines – features

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.

How to draw the Magnetic field around a bar magnet

• Take a bar magnet and place it under a non-magnetic, thin flat surface (this is to stop the paper bending).
• Place a sheet of white paper on the surface over the bar magnet and sprinkle some iron filings onto the paper.
• Give the paper a shake to evenly distribute the iron filings.
• In your workbook, draw the bar magnet and the pattern formed by the iron filings.
• The above steps can be repeated after rotating the bar magnet to a different angle as shown alongside and drawing the respective patterns.

Drawing Magnetic field lines around a pair of bar magnets

Take two bar magnets and place them a short distance apart such that they are attracting each other. Place a sheet of white paper over the bar magnets and sprinkle some iron filings onto the paper.

Give the paper a shake to evenly distribute the iron filings. In your workbook, draw both the bar magnets and the pattern formed by the iron filings.

Repeat the procedure for two bar magnets repelling each other and draw what the pattern looks like for this situation. Make a note of the shape of the lines formed by the iron filings, as well as their size and their direction for both arrangements of the bar magnet.

What does the pattern look like when you place both bar magnets side by side?

As already stated, opposite poles of a magnet attract each other, and bringing them together causes their magnetic field lines to converge (come together).

Like poles of a magnet repel each other and bringing them together causes their magnetic field lines to diverge (bend out from each other).

Ferromagnetism

Ferromagnetism is a phenomenon shown by materials like iron, nickel, or cobalt. These materials can form permanent magnets. They always magnetize so as to be attracted to a magnet, no matter which magnetic pole is brought toward the unmagnetized iron/nickel/cobalt.

Retentivity and magnetic materials (paramagnetism, diamagnetism)

The ability of a ferromagnetic material to retain its magnetization after an external field is removed is called its retentivity.

Paramagnetic materials are materials like aluminium or platinum, which become magnetized in an external magnetic field in a similar way to ferromagnetic materials. However, they lose their magnetism when the external magnetic field is removed.

Diamagnetism is shown by materials like copper or bismuth, which become magnetized in a magnetic field with a polarity opposite to the external magnetic field. Unlike iron, they are slightly repelled by a magnet.

The compass

A compass is an instrument that is used to find the direction of a magnetic field.

A compass consists of a small metal needle that is magnetized and which is free to turn in any direction. Therefore, when in the presence of a magnetic field, the needle is able to line up in the same direction as the field.

Compasses are mainly used in navigation to find directions on the earth. This works because the Earth itself has a magnetic field.

The compass needle aligns with the Earth’s magnetic field direction and points north-south. Once you know where north is, you can figure out any other direction.

Some animals can detect magnetic fields, which helps them orientate themselves and navigate. Animals that can do this include pigeons, bees, Monarch butterflies, sea turtles, and certain fish.

The Earth’s magnetic field

Draw the earth’s magnetic field lines.

In the picture below, you can see a representation of the Earth’s magnetic field which is very similar to the magnetic field of a giant bar magnet like the one on the right of the picture.

Describe two magnetic poles of the earth

The Earth has two magnetic poles, a north, and a south pole just like a bar magnet. Another interesting thing to note is that if we think of the Earth as a big bar magnet, and we know that magnetic field lines always point from north to south, then the compass tells us that what we call the magnetic north pole of the earth is actually the south pole of the equivalent bar magnet! (see the figure above) Similarly, the magnetic south pole of the earth is actually the north pole of the equivalent bar magnet.

geographic poles of the earth

In addition to the magnetic poles, the Earth also has two geographic poles.

The two geographic poles are the points on the Earth’s surface where the line of the Earth’s axis of rotation meets the surface. To visualize this you could take any round fruit (lemon, orange, etc.) and stick a pencil through the middle so it comes out the other side. Turn the pencil, the pencil is the axis of rotation and the geographic poles are where the pencil enters and exits the fruit.

difference between geographic and magnetic poles of the earth

So the Earth has two north poles and two south poles if we consider both geographic poles and magnetic poles.

We call the geographic north pole true north.

The Earth’s magnetic field has been measured very precisely and scientists have found that the magnetic poles do not correspond exactly to the geographic poles.

From the picture, you can see that the direction of magnetic north and true north are not identical. The geographic north pole is about 11.5 degree away from the direction of the magnetic north pole (which is where a compass will point). However, the magnetic poles shift slightly all the time.

The Earth’s magnetic field is thought to be caused by flowing liquid metals in the outer core of the planet which causes electric currents and a magnetic field.

How is the Magnetic field direction shown by a compass?

The direction of a magnetic field is the direction of the force on the north pole of a compass placed in the field.

You can use compasses to show the direction of the magnetic field at any position surrounding a magnet, as illustrated in Figure below.

The figure shows that, in general, this direction is from the north to the south pole of the magnet.

To represent the entire magnetic field surrounding a magnet, it would be necessary to draw arrows at an infinite number of points around the magnet. This is impractical.

Instead, you can draw a few magnetic field lines with a single arrow head indicating the direction of the magnetic field.

To find the field direction at a given point, move the arrow head along the field line through that point so that it keeps pointing in the direction of the tangent to the field line. The field lines in Figure above are a map of the magnetic field with the following features:

• Outside a magnet, the magnetic field lines point away from the north pole of a magnet and toward the south pole.
• The closeness of the lines represents the magnitude of the magnetic field.

Magnetic field strengths of a few physical systems

Physical system	Magnetic field (Tesla)
Earth	5 x 10-5
Bar Magnet	1×10-2
MRI device	15
Strongest man-made magnetic field	40