Magnetism and Electromagnetic Induction

Magnetism Definition

You have most likely come across magnetism in your daily lives. When you place two magnets next to one another, you'll have noticed that, if placed in the correct orientation, they experience an attractive or repulsive force on one another. Therefore we can define magnetism as the following.

Not all materials exhibit noticeable magnetic effects as magnetic forces heavily depend on a material's internal structure. We can separate materials into three categories concerning magnetism:

1. Diamagnetism - Materials that experience magnetic forces in a magnetic field, generating a field in the opposite direction to the external field.

2. Paramagnetism - Materials that experience magnetic forces in the presence of a magnetic field, generating a field in the same direction as the external field.

3. Ferromagnetism - Materials that remain magnetic when an external magnetic field is applied and then removed.

You may be familiar with a bar magnet, like the one seen in the picture below. These magnets are typically made up of iron or steel, both of which are ferromagnetic materials,

allowing them to retain their magnetic qualities even when they're not subjected to a magnetic field. This allows for fun science experiments both at home and in the lab when learning about the qualities of magnetism! Furthermore, magnets are also used in various industrial applications such as generators, motors, solenoids, and more. The term ferromagnetism stems from the Latin term "ferrum", meaning iron. As steel is an alloy of iron, ferromagnetic describes materials that have the same magnetic properties as iron.

Fig. 2 - A bar magnet made of iron or steel is an example of a ferromagnetic material.

Furthermore, magnetized materials radiate a magnetic field around them, represented in diagrams using field lines. As shown in the figure below, these field lines are directed, which is represented by arrowheads, and always point out of the north pole and towards the corresponding south pole of the magnet. The higher the density of the magnetic field lines in space, the stronger the magnetic field is in that region. Therefore, we can see the strength of the magnetic field is at its largest in the area around the poles.

We can also define another quantity that is commonly used to describe electromagnetic fields - the magnetic flux.

This can be represented in the equation

\[ \Phi = BA \cos(\theta) ,\]

where \(\Phi\) is the magnetic flux measured in Webers \(\mathrm{Wb}\), \(B\) is the strength of the magnetic field measured in teslas \(\mathrm{T}\), \(A\) is the area in which the magnetic field is passing through measured in \(\mathrm{m^2}\), and \(\theta\) is the angle between the magnetic field lines and the area it is passing through. As can be seen from the equation, we are only considering the component of the magnetic field perpendicular to the area it passes through.

Electromagnetic Induction Definition

On the other hand, we instead define it as the following.

Here, an electromotive force is similar to the potential difference across a circuit, however, it is used to describe voltage when there is no current flowing in a circuit. When we discuss an electromotive force, we refer to the induced voltage due to the induction.

In order to quantify the amount of electromotive force being induced in the conductor, we represent this as an equation

\[ \epsilon = - \frac{\Delta \Phi_{\text{B}}}{\Delta t} ,\]

where \(\epsilon\) is the induced electromotive force measured in volts \(\mathrm{V}\), \(\Delta \Phi_{\text{B}}\) is the change in magnetic flux measured in Webers \(\mathrm{Wb}\), and \(\Delta t\) is the time taken for the change measured in seconds \(\mathrm{s}\). This equation reveals a number of things; firstly, it is the combination of Faraday's law and Lenz's law, which are defined as follows.

And Lenz's law is as follows.

These two laws govern the phenomenon of electromagnetic induction. Let us first talk about Faraday's law; we can see from the equation above that the proportionality constant is given by \(N\), the number of turns in the solenoid. We can see that this equation is specifically for a solenoid undergoing induction, but it may vary depending on the type of conductor we are looking at.

On the other hand, Lenz's law is represented in the equation as the negative sign on the right-hand side, showcasing that the direction of the electromotive force must be opposite to that of the rate of change of magnetic flux. This law stems from energy conservation, which states that energy cannot be created or destroyed. In order to further understand this, consider a bar magnet falling through a coil of wire; the induced current in the wire causes a magnetic field surrounding it. This field interacts with the falling magnet, exerting a force on it. As a result of Lenz's law, the direction of the force must be such that it opposes the motion of the magnet, ie. in the upwards direction. This causes the magnet to slow down and reduce the amount of induced current in the coil. Otherwise, a downwards force causing the magnet to speed up would result in a larger current being induced in the coil, an endless loop of energy being created which isn't allowed in physics!

Difference between Magnetic Induction and Electromagnetic Induction

You may have already heard of magnetic induction before you came across this article, and are asking yourself - is magnetic and electromagnetic induction the same thing? These two phenomena are actually very different things, so let us first define what magnetic induction is.

As we mentioned in the previous section, we established that all materials can be categorized into three magnetic categories. In order for these materials to be magnetized by an external magnetic field, they must either be ferromagnetic or paramagnetic. To understand the structure of magnetic materials, we zoom into the internal structure of the substance and look at its building blocks, the atoms. These individual atoms reside in magnetic domains where the atom itself becomes a magnetic dipole.

Essentially, we can think of magnetic materials as being made up of many individual bar magnets! These bar magnets are susceptible to the influence of an external magnetic field, rotating back and forth depending on the orientation of the field. Let's look at the figure below to get a clearer picture of the structure.

Fig. 4 - Magnetic materials are made up of individual magnetic dipoles. Here the material is unmagnetized as all of the dipoles are in different orientations.

In the figure above, we have the magnetic material outlined in blue, whilst the magnetic dipoles are represented by the pink arrows. In this case, the direction of the arrows indicates the north magnetic pole. Here we can see that all of the arrows are pointing in different directions, showing that the dipoles are not under a magnetic field to organize their orientations. Thus we know that this material is not magnetized.

Fig. 5 - The magnetic dipoles are aligned, showing that the material now exhibits magnetic properties due to its net magnetization.

Now in the figure above, we have the same material except that the magnetic dipoles are all orientated in the same direction. In this case, an external magnetic field has been applied to the material, causing the dipoles to rotate. The resultant alignment of the magnetic fields is what causes the magnetic properties within the material.

Therefore, now we have a clear distinction between magnetic induction, which is the process of magnetizing materials, and electromagnetic induction, which is the procedure of generating an electromotive force in a conductor. Two phenomena with very similar names but very different outcomes!

Example of Magnetism

In order to further understand magnetism, let's consider a numerical example of how to calculate the magnetic flux.

Now let's consider another example where we apply our electromagnetic induction equation instead.

Electromagnetic Induction Real Life Applications

As we mentioned earlier in the article, electromagnetic induction is a key component of many objects we use in day-to-day life. One that you may have come across recently is commonly found in the kitchen and is essential to making your food daily. Can you guess what it is?

Fig. 6 - An induction hob is an example of electromagnetic induction being used in objects we use in our everyday lives.

That's right! It's an induction hob. An induction hob is a great example of how electromagnetic induction has allowed technology to be improved and developed with added safety benefits. In the case of the induction hob, the inside of the heating element consists of coils made up of conducting material, with an alternating current running through them. The alternating current generates an oscillating magnetic field on the surface where you place your frying pan. You may have noticed that only specific types of pans work with induction cookers; this is because the pan must be made of a conducting material such that the changing magnetic field can induce a current in the bottom of the pan. Due to the internal resistance of the pan, the currents running through the pan generate heat which is then used to make your morning breakfast!

Fig. 7 - Graphic tablets also use electromagnetic induction to detect inputs from the user.

Another example of electromagnetic induction being used in real-life applications is the graphics tablet. This device is typically used by artists who want to create digital art without losing the pen-on-paper experience. As the pen is moved across the surface of the tablet, the tip of the graphics pen exudes an electromagnetic field which then interacts with the conductors beneath the surface of the tablet. Through electromagnetic induction, the computer can process those induced signals into the lines being drawn by the artists, resulting in the image you see on the screen!