# Magnetism

Magnetism is a phenomenon found throughout nature and manmade technology alike. Magnetism is what allows us to orient ourselves with a compass. It's even thought that this effect, caused by the earth's magnetic field, is what allows birds to orient themselves too. It's no wonder birds are able to migrate vast distances across countries and oceans and end up in the right location! We are probably most familiar with magnets produced by the bar magnets you might have seen or used in your classroom or school laboratory. These permanent magnets attract magnetic materials towards them and have a north pole and a south pole. All of these familiar phenomena are encompassed by the general term magnetism.

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## Definition of magnetism

Magnetism is a tricky concept, so pinning it down with a few precise definitions will be useful for your understanding.

Magnetism is the effect of repulsion or attraction between two magnets.

A magnet is a device that generates a permanent magnetic field. Generally, these are made of metals called ferromagnetic metals.

Magnetism is said to be mediated by a mathematical construct called a magnetic field.

A magnetic field is a region of space surrounding a magnet which has a vector at each point, which has an effect on electric charges, electric currents and magnetic materials within that region.

Magnetic fields can be used to determine how magnets affect other magnetic objects or charged particles around them. Some pictures of magnetic field lines, which are arrows which point in the direction of the magnetic field vector, are shown in the figures below.

## General properties of magnetism and magnets

Like the electric field, which we know to be attractive or repulsive, magnetic effects can be either attractive or repulsive. Nevertheless, in general, the lines of the magnetic field, which indicate the strength and direction of the magnetic field, are closed. This means that magnetic effects are never only attractive or only repulsive, they are always both depending on where we are with respect to the source of the magnetic field.

Before understanding this concept with magnets, take a moment to analyse the magnetic field lines generated by a bar magnet:

Fig. 1. Iron shards affected by a magnet, Newton Henry Black, Harvey N. Davis (1913) Practical Physics, The MacMillan Co., USA, p. 242, fig. 200

In the previous image, the lines of the magnetic field can be easily traced due to the orientation of the iron shards affected by the magnet. It can be seen from the figure above that the lines form closed loops if we close them inside the magnet itself. Although the image is cropped, all lines that appear in the picture will eventually close.

What does this mean for magnets? In general, to each magnet, we assign different poles which are called north and south in reference to the poles of the Earth. The convention we follow is to first assign a direction to the closed loops of the magnetic field and then to call the north pole the site of the magnet where the magnetic field lines leave the magnet and the south pole the site where the magnetic field lines enter it. See the image below for an example of this:

Fig. 2. Diagram of a magnet, its poles and its magnetic field lines. Magnetic field lines leave the north pole of the magnet and enter it south pole Wikimedia Commons

It's no coincidence that the sites of a magnet where the magnetic field lines enter and exit are called poles. This is in reference to the magnetic field created by the Earth, which is produced along the axis that joins the poles and is responsible for the functioning of compasses.

Fig. 3. The magnetic field of the Earth, Wikimedia Commons

The fundamental laws of magnetism are too complex to be studied here. But, we can still study the description of the behaviour of magnets and the types of magnetism they can produce. The interactions between magnets are governed by the following two laws:

• Poles of the same type repel each other
• Poles of different types attract each other

## Basic principles of magnetism and electromagnetism

Magnetism is the class of physical phenomena and properties that are caused by magnetic fields, a vector quantity that is produced by electric currents and obeys certain dynamical laws known as the laws of electromagnetism or Maxwell's laws.

Magnets are the simplest objects that display magnetic properties, but the number of phenomena that are magnetic in nature is not confined to the bar magnets or horseshoe magnets we are familiar with. However, we can test the properties of general magnetic fields by using these permanent magnets, which are simple and accessible resources.

A key principle of magnetic fields is that they are produced by the movement of charged particles. The magnetic field produced by current-carrying wires is produced by the relative movement of the electrons inside the wire making up the current. The magnetic field produced by a permanent magnet can be explained by considering the angular motion of atomic electrons and the resulting magnetic field that the current caused by this motion produces.

Another key principle of magnetic fields is that their strength decreases with increasing distances from the source of the magnetic field. For example, the magnetic field produced by bar magnets surrounded by iron filings can be seen to decrease in strength with distance from the magnet. This is by virtue of the fact that the magnet seems to have little influence over the iron filings furthest from the magnet. They do not align with the magnetic field produced by the magnet and so appear to be scattered in random directions.

After observing the images of magnetic field lines in this article, you may have noticed a few other key properties of magnetic field lines, a few of which have been mentioned previously. Magnetic field lines always consist of closed loops. This means that if we follow a magnetic field line, we will always eventually end up back where we started. The magnetic field loops cannot cross over each other and have a particular direction associated with them which points in the direction of the magnetic field located at each point around the loop.

One of the most common examples of devices that generate magnetic fields are 'solenoids'. These are made out of current-carrying coils rolled into spirals. The flow of current causes a magnetic field to be produced in the interior region of the coil pointing along the direction of the cylinder formed by the coil. See the image below:

Fig. 4. The magnetic field lines of a solenoid, Wikimedia Commons

Inside the solenoid, the magnetic field is approximately uniform and very strong. Outside the coil, the magnetic field is weaker and decreases in strength with distance from the coil. If a magnetic object such as an iron core is introduced inside the solenoid, a magnetic field will be induced inside of the magnetic core, thus strengthening the total magnetic field surrounding the core.

## Types of Magnetism

The two main types of magnetism that you should be familiar with are permanent magnetism and induced magnetism. Here we will outline the differences between the two types of magnetism and provide some real-life examples of both so that you can contextualise your knowledge of magnetism.

### Permanent magnets

Permanent magnets produce their own magnetic field. This permanent magnetic field cannot be 'turned on or off', it is simply always there. A quick and easy way of testing whether an object is a permanent magnet is to hold it close to another known permanent magnet. If the two objects repel or attract each other as soon as they are held close together then we can be sure that the object in question is a permanent magnet too.

Fig. 5. Permanent magnets, as the name suggests, are permanently magnetic. They always have a north pole and a south pole. Wikimedia Commons.

Permanent magnets, like the bar magnet in the figure above, always have a north pole and a south pole. You can determine which end of a permanent is the north pole and which end is the south pole by holding one end of the magnet within a close distance of the north pole of another known permanent magnet; if the magnets attract each other than the end held to the north pole of the known magnet is a south pole and they repel then the end held to the known magnet is the north pole.

### Induced magnets

Induced magnets are objects or materials that become magnetic when they are placed in a magnetic field. When induced magnets are removed from the magnetic field, they quickly lose all or most of their magnetism, so we call induced magnets temporary magnets. The direction of the magnetic field of an induced magnet will always point in the direction of the permanent magnet's magnetic field. This means that the induced magnet will be attracted towards the magnet producing it.

Usually, when we think about magnetism we think about magnets. However, the description of magnets is complex since we would need to consider the atomic structure of magnetic substances and consider the small magnetic contributions of each of them.

The fact that we usually think about magnets is because the simple phenomena we can describe, like the magnetic field created by a wire with a current, are very very weak when compared to electric phenomena. This is also a general feature: the magnetic field is much weaker than the electric field even though they are part of the same general phenomenon: electromagnetism.

## Magnetism - Key takeaways

• Magnets are materials which produce magnetic fields which have an effect on electric charges, electric currents and magnetic materials within a region surrounding the magnet. Magnets have a north pole and a south pole from which magnetic field lines exit and enter, respectively.
• Magnetic field lines always form closed loops.
• There are two different types of magnetism: permanent magnetism and induced magnetism.
• Permanent magnets are always accompanied by a magnetic field.
• induced magnets are only magnetic when they are in the presence of another magnetic field.
• The properties of the magnetic field, such as its direction or its strength, obey a set of complex rules that can be understood in some settings by using magnets and compasses.
• The electric field and the magnetic field are part of the same field called the electromagnetic field. An example of this connection is how we can produce a magnetic field inside a solenoid by using an electric current that flows along with the coil.

#### Flashcards in Magnetism 15

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What is magnetism?

Magnetism is the class of physical phenomena and properties that are caused by magnetic fields, a vector quantity that is sourced by electric currents and obeys certain dynamical laws known as the laws of electromagnetism or Maxwell's laws.

What are the properties of magnetism?

Magnetism is the class of physical phenomena and properties that are caused by magnetic fields, a vector quantity that is sourced by electric currents and obeys certain dynamical laws known as the laws of electromagnetism or Maxwell's laws.

What is an example of magnetism?

An example of magnetism is the magnetic field created by a magnet, the intensity of which decays with distance and whose lines formed closed loops entering and exiting the poles of the magnet.

What are the basic principles of magnetism?

The basic principles of magnetism are that it is a repulsive and attractive phenomenon that is described by closed loops whose intensity decays from its source.

## Test your knowledge with multiple choice flashcards

What does the index finger represent in Fleming's left-hand rule?

The thumb represents the direction of the force in Fleming's left-hand rule.

Fleming's left-hand rule can be used to find out the direction of the force on the coils in electric motors.

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