Let's start this topic with a simple experiment. Take a metal wire, connect it to a battery, and place a compass next to the circuit. Turn on the switch and you will notice the compass needle start to move in a certain direction. Reverse the direction of the battery and reconnect the circuit, and you will see the needle of the compass move in the opposite direction. If you turn off the switch, you will see the compass needle return to its original position before the circuit was switched on.
Fig. 1 - a compass will be deflected in a certain direction if it is placed in the vicinity of a wire carrying a current
Congratulations! With this experiment, we have discovered the relationship between electricity and magnetism, which is one of the fundamental concepts in physics. This discovery is the basis of most of the technology in the modern world. Electric motors and generators are two of the main examples of how important it is. The earth itself is even a magnet, and its magnetic field helps to protect us from solar wind and cosmic rays.
So what caused the compass in the experiment above to deviate? Before we dive into this equation, we need to understand what a magnet and a magnetic field are.
- Magnet Definition Physics
- Functions of Magnets
- Properties of Magnets
- Electro Configuration
- Magnetic Fields
- Types of Magnets in Physics
- Permanent Magnets
- Induced Magnets
- Examples of Magnets
- The Earth's Magnetic Field
- MRI Scans
Magnet Definition Physics
A magnet is an object that produces a magnetic field such that the field lines form a loop from the north end to the south end of the magnet.
A bar magnet is shown below, with a north pole and a south pole at opposite ends. If you suspend a bar magnet in the air, it will orient itself with respect to the cardinal direction, which is from where the name of the poles is derived. A compass, for example, is just a small bar magnet that always points in the north direction and south direction.
Fig. 2 - a bar magnet consists of a north pole and a south pole.
Similarly to the force between electric charges, the unlike poles of magnets attract each other while the like poles repel each other. The difference between charges and magnetic poles is that the charges can be isolated. In the case of a bar magnet, no matter how many times you slice a bar magnet into pieces, it will always have a north pole and a south pole. We say that there is no such thing as a magnetic monopole.
A magnetic monopole has never been observed, but we have not found any reason why they could not exist - they are not prohibited by the theory. In fact, magnetic monopoles would make electromagnetism more symmetric! There have been experiments to detect existing magnetic monopoles. For example, it was thought they might exist in cosmic rays. Physicists have also tried to create them in high-energy particle colliders. However, no conclusive evidence for magnetic monopoles has been found as of yet.
Functions of Magnets
Magnets can perform a variety of functions, from running motors and generators to loudspeakers and microphones, magnets also help transform energy from one form to another. But for the sake of this article, we will not look at the energy transformation aspect, but rather the force-related functions of a magnet. Magnets have several functions:
- A magnet can attract magnetic materials like cobalt, nickel, iron, certain steels, and alloys.
- A magnet can exert a force, both attractive and repulsive, on other magnets depending on the poles.
- A magnet also creates a magnetic field that influences the path of electrically charged particles travelling in free space by causing a force to act which deviates them from their path of motion.
Properties of Magnets
We have discussed what magnets can do, but what happens inside the magnet that makes a magnet, well, a magnet? Why are some materials magnetic while the rest of the materials do not possess this property at all?
Electron Configuration
All objects in nature are made up of atoms, which can further be divided into protons, electrons, and neutrons. To understand magnetism, it is the electrons that we must worry about. Electrons have an intrinsic property called 'spin'. They do not actually spin around but this is a useful way to think about it. Electrons spinning counter-clockwise will have a spin angular momentum vector pointing up - we can call these spin-up electrons. On the other hand, electrons spinning clockwise have a spin angular momentum vector pointing downwards - they are spin-down electrons. The direction of spin of these electrons, in most materials, is such that they form orbitals with one electron spin-up while the other spin-down - causing them to cancel each other out.
But magnetic substances such as cobalt, nickel, and iron, have electron spins that do not cancel. In these types of substances, the atoms can be oriented in such a manner that all the electron spins will be aligned in the same direction. which is why they are called ferromagnetic.
Ferromagnetic materials can become magnetic by exposing them to a magnetic field.
However, as the above definition suggests, ferromagnetic materials are not magnets on their own and they must be placed in a magnetic field to become one. By being exposed to a strong external magnetic field, the electrons in the material have their spin aligned, which makes the material become magnetic. Certain materials can retain this state even after being removed from the field, and these are known as permanent magnets.
Magnetic Fields
A magnetic field describes the influence on a magnetic material at each point in space. In a drawing of a magnetic field, the arrows show the direction of the force that would act on a north pole placed in that position. The field lines can be depicted just like how we draw electric field lines to show the electric field around a charge. The magnetic field due to a bar magnet is shown below. The field lines seem to generate from the north pole and end at the south pole, but in reality, they have no starting position or an ending position - magnetic field lines are closed loops.
It is because of this field that magnets and other substances get attracted or repelled. So if a magnetic object, like a paper clip for instance comes into the vicinity of these magnetic field lines, it will get attracted to the magnet generating this field. The paper clip will be attracted to whichever pole it is placed close to as it becomes an induced magnet. This will be explained below when we discuss the different types of magnets.
Fig. 3 - paper clips become induced magnets when placed near a magnet.
Just like for electric field lines, the density of magnetic field lines indicates the strength of the field. In the figure above, you can see the magnetic field lines closely packed near the ends of the bar magnet indicating that the magnetic field strength is strongest here.
How do we know that the magnetic field lines form loops and not any other shape? We can demonstrate this by placing a piece of paper on top of a bar magnet and sprinkling iron filings over the paper. Each iron filing aligns itself parallel to the direction of the magnetic field at the point where it lands.
Fig. 4 - Iron filings can be used to show the magnetic field lines of a bar magnet
A magnetic compass can also be used to find the direction of magnetic field lines. A tiny magnet is housed inside the compass that is pivoted from the center so that it can spin easily. The red arrow on the magnet always points towards the Earth's geographic north pole. To map the field lines around a bar magnet, you could keep moving the compass around the bar magnet and draw an arrow in the direction of the red arrow at each point.
A compass points towards Earth's geographic north pole.
Draw a curved line to join all the arrows that you have drawn and you will see the field lines coming into shape! If you keep moving the compass on the table around the bar magnet at different distances away from the bar magnet, you will get different field lines as shown in the figure below. In the end, mark the drawn magnetic field lines with an arrow from the north pole to the south pole.
Coming back to the first experiment at the start of this article, Do you know why the compass deviated from its original position when a continuous current passed through the wire? It was because the current-carrying wire produced a magnetic field around it. How and why a current-carrying wire produces a magnetic field around it is for another time, but just keep in mind that for now, all magnets produce a magnetic field.
Types of Magnets in Physics
Not all magnets are bar magnets in nature. Another type of magnet is a horseshoe magnet, which is shown below along with a diagram of its field lines. Compare the magnetic field lines with those for a bar magnet - this shows that the field lines depend on the shape of the magnet.
The two poles on a horseshoe magnet are very close together, resulting in a strong magnetic field between them. This means that horseshoe magnets can be used to pick up heavy objects that are made of magnetic material.
Similarly, there is a disc magnet as well with one side of the disc acting like the north pole while the other side of the disc acts like the south pole. A disc magnet along with its field lines is shown below. Its field lines are similar to a bar magnet.
There are other kinds of magnets as well like cylindrical magnets, spherical magnets, and ring magnets. However, these are all different solely due to their shape. There are also different types of magnets that behave differently even if they are the same shape. How can we classify magnets? Magnets are classified into two main categories: permanent magnets and induced magnets.
Permanent Magnets
Permanent magnets are those magnets that retain their magnetic properties for a very long time or forever. So we can define permanent magnets as:
Permanent magnets are magnets that retain their magnetism in the absence of an external field. These types of magnets produce their own magnetic fields at all times.
An example of a permanent magnetic material is Alnico, which is an alloy of aluminum, nickel and cobalt.
Induced Magnets
Induced magnets, as the name suggests, have an induced magnetic field for as long as they are in the vicinity of an external magnetic field. As soon as the induced magnets are pulled away from a magnetic field, they lose their own magnetism. The key point to note here is that the force between an induced magnet and a permanent magnet will always be attractive because the opposite poles will be induced on the induced magnet.
This explains why a paper clip is always attracted to a magnet. If the paper clip is brought near the south pole of a magnet, a north pole will be induced on the side of the paper clip nearest to the magnet and the other side of the paper clip will be an induced south pole. When the paper clip is brought away from the magnet, it will no longer be an induced magnet.
An induced magnet only becomes a magnet when placed in a magnetic field.
Examples of Magnets
We have already discussed some man-made magnets such as bar magnets and horseshoe magnets, but there are also many natural magnets!
The Earth's Magnetic Field
The earth also has a magnetic field all around it. This is due to powerful electric currents produced by flowing liquid metal in the outer core. If you suspend a bar magnet or look at a compass, both automatically align themselves in the north and south directions provided no other magnet is nearby. If you imagine an extremely large bar magnet at the center of the earth, its magnetic field lines would resemble that of the earth.
A compass always points towards the geographical north. The polarity of this imaginary large magnet buried inside the earth is such that the geographical north side will correspond to the south pole of the magnet.
The immense forces within the Earth that generate the Earth's magnetic field are constantly changing, which means its magnetic field changes too. This results in the Earth's magnetic poles shifting slightly over time, and they even switch positions roughly every 300 000 years, so if your compass suddenly starts pointing in the opposite direction, now you will know why!
MRI Scans
Natural magnets are often used to our advantage in medical applications. Magnetic Resonance Imaging (MRI) scans are based on the effect of nuclear magnetic resonance (NMR). Hydrogen nuclei have their own natural frequency, and if a very strong magnetic field is applied to one, it can absorb energy from electromagnetic radiation (in the radio frequency range) that is equal to this frequency. The controlled de-excitation of the hydrogen nuclei enables them to be located based on the radiation they emit, which allows for the body tissue to be mapped in three-dimensions while being non-invasive.
MRI scans can be used to produce images of the brain
Magnets - Key takeaways
- A magnet is an object that produces a magnetic field such that the field lines form a loop from the north pole of the magnet to the south pole.
- A bar magnet has two poles - a north pole and a south pole.
- Magnetic monopoles do not exist - magnetic poles cannot be isolated like electric charges can.
- A magnetic field forms closed loops from the north pole to the south pole.
- The density of magnetic field lines represents the strength of the magnetic field.
- Ferromagnetic materials can become magnetic by exposing them to a magnetic field.
- Permanent magnets retain their magnetism at all times.
- Induced magnets only temporarily retain their magnetic properties while in the presence of an magnetic field.
- The Earth's magnetic field resembles that of a bar magnet.
References
- Fig. 1 - "AnimacioOerstedBucle" (https://commons.wikimedia.org/wiki/File:AnimacioOerstedBucle.gif) by Antoni Salvà is licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)
- Fig. 2 - "Bar magnet crop" (https://upload.wikimedia.org/wikipedia/commons/6/6f/Bar_magnet_crop.jpg) by MikeRun is licensed by CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)
- Fig. 3 - "Magnet Experiments" (https://www.flickr.com/photos/scotthamlin/5361147971/in/photostream/) by Scott Hamlin (https://www.flickr.com/photos/scotthamlin/) is licensed by CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/)
- Fig. 4 - "magnetic field shown by iron filings" (https://www.flickr.com/photos/daynoir/2180506627/in/photostream/) by Dayna Mason (https://www.flickr.com/photos/daynoir/) is licensed by CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/)
Explanations 
Exams 
Magazine 
