In this post, we will discuss the fundamental understanding of magnetism. Also, a number of related terminologies like magnetism, magnetic fields, ferromagnetic, 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.
- Define Magnetism
- Magnetic fields
- Magnetic field associated with moving charges
- Plastic ball is not magnetic and has no magnetic field – why?
- ferromagnetic materials and magnetism
- Making permanent magnets from ferromagnetic materials
- The poles of permanent magnets
- Magnetic attraction and repulsion
- Representing magnetic fields (2 D and 3D)
- How to draw the Magnetic field around a bar magnet
- Drawing Magnetic field lines around a pair of bar magnets
- Retentivity and magnetic materials (paramagnetism, diamagnetism)
- The compass
- The Earth’s magnetic field
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 iron oxide mineral magnetite. 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.
A magnetic field is a region in space where a magnet or object made of magnetic material will experience a non-contact, magnetic force.
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 along the 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 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.
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 N pole and a S pole will be drawn towards each other.
Do you think the following magnets will repel or be attracted to each other?
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.
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
- Field lines never cross.
- Arrows drawn on the field lines indicate the direction of the field.
- A magnetic field points from the north to the south pole of a
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 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.
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 itself 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
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.
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 is actually the south pole of the bar magnet!
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.
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.
So the Earth has two north poles and two south poles: geographic poles and magnetic poles.
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.
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.