Magnets

Magnet Definition Physics

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.

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.

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.

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.

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:

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.

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.

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