In this lesson, we will talk about electromagnetic induction. What is electromagnetic induction? How is it produced? And finally, we will try to understand what an eddy current is. So without further ado, let’s start.
What is electromagnetic induction?
Electromagnetic induction is the means by which electricity is produced from magnetism and vice versa. Meaning, electricity is produced using magnetism, and magnetism is also produced by the effects of electricity.
You can understand from the term ‘electromagnetic ’ that it has two words: electric and magnetism. Magnetism is produced with the help of electricity, and electricity is produced or induced with the help of magnetism. This is called electromagnetic induction.
What are the essential factors to electromagnetic induction?
There are three important factors to produce electromagnetic induction.
- The first factor is a conductor. A conductor is essential through which electric current can flow. Usually, coils are used as conductors in most electrical equipment we see around us; solid metal is rarely used.
- The second important factor is the magnetic line of force. Magnetic lines of force can be produced either from electromagnetic fields or from solid permanent magnets.
- The third important factor is the relative movement between a conductor and a magnetic line of force. There must be movement between both the conductor and the magnetic lines of force. If the conductor is static and not moving, then the magnetic field must keep changing there. Or if the magnetic field is not changing and is static, then the conductor must keep moving there, or both can move simultaneously. But there must be movement between the two; only then will electromagnetic induction be possible.
When conductor is the part of a closed circuit
There are two ways to produce electromagnetic induction.
- When the conductor is part of a closed circuit.
- When a closed circuit produces a magnetic field.
In the first condition, the magnetic field is produced by a permanent magnet. So we can understand this from the figure of the production of electromagnetic induction. You can see it is a closed circuit with a coil as part of the circuit.
When the magnet moves inside the coil, the magnetic lines of force cut across the coil. This moves the electrons inside that coil, creating electromotive force. Due to this, electricity is induced inside that coil.
An example of this is a dynamo. As a kid, you may remember the dynamos in our bicycles. The same principle is used in that dynamo. The magnet rotates at a very high speed around an electric coil, and electricity is produced in this electric coil.
Now, we know that how current is induced in such a circuit, but what will the direction of flow of current.
Direction of current
The direction of the current depends upon the direction of movement of the magnet. As shown in the first figure, when the magnet moves towards the coil, current is induced in one direction.
In the second figure, when the direction of the magnet’s movement is reversed, the direction of the induced current also reverses accordingly.
When a closed circuit produces a magnetic field
Now there is another condition in which a closed circuit produces a magnetic field. In the previous one, the magnetic field is created by a permanent magnet, but here, a closed circuit is producing a magnetic field.
Let me explain this with the figure of a transformer. There are two closed circuits. In one closed circuit, alternating current is passed. Alternating current produces a varying magnetic field, and under the influence of this magnetic field, a current is induced in the secondary circuit.
Lenz’s law
In this circuit, what will be the direction of induced electromotive force? We can understand this from Lenz’s law. According to Lenz’s law, the direction of induced electromotive force is such that it tends to oppose the force producing it. That is, it is opposite to the force producing that electromotive force.
Referring back to the figure again, notice the direction of movement of the magnet. The electromotive force that will be generated in this conductor will have a direction exactly opposite to this.
Strength of induced EMF
Now let us try to understand the strength of the induced EMF. The induced EMF depends upon two factors. The first factor is the rate of change of the magnetic field. How fast is the magnetic field changing?
Factor 1: Rate of change of the magnetic field
We can produce this by rapid movement of the permanent magnet. You can see in this figure how rapidly the permanent magnet is moving. So the rate of change of the magnetic field in it is also happening rapidly.
The second condition is that the magnet should be strong. A strong magnet produces a strong magnetic field, and if it is moving very rapidly, then the electromotive force generated by it will also be very strong.
In the circuit, where the primary circuit is producing an electric field, the strength of the magnetic field depends on the frequency of the alternating current passing through it. If we pass a low-frequency current, the magnetic field generated will be weak. But when we pass a high-frequency current, the magnetic field produced will also be very strong. Due to this, the EMF produced in this conductor will also be very strong.
Factor 2: Inductance of the conductor
The second factor that influences the strength of the current is the inductance of the conductor. By making some modifications, we can increase the inductance. So there are some principles.
- Increasing the number of turns: By increasing the number of turns in the coil, the magnetic flux linkage multiplies for the same amount of current, which directly increases its inductance.
- Placing the turns close together: Winding tightly so that the turns are close to each other, the magnetic coupling between adjacent turns is maximised, further enhancing the inductance.
- Using a soft iron core: Winding the coil around a high-permeability soft iron core concentrates the magnetic field lines significantly, which greatly boosts the overall inductance of the circuit.
Eddy current
Now, finally, let us try to understand eddy current is. Eddy currents are circulating currents induced inside a solid conductor whenever the magnetic flux passing through it changes. They are often unwanted because they heat up the metal and waste energy in electrical equipment, so they need to be reduced.
However, they are not always harmful, and we will also see a useful application in physiotherapy.
So what is an eddy current?
When a bulk conductor, such as a solid metal plate or a solid metal core, is placed in a changing magnetic field, an electromotive force is induced in the metal itself. Because the metal provides a closed path, small circular loops of current flow within it, in planes at right angles to the magnetic lines of force. These are called eddy currents.
In transformer and motor cores, the heat produced by these currents (I²R heating) is wasted energy, so the cores are built from thin laminated sheets insulated from each other, which break up the paths of the eddy currents.
However, eddy currents is also useful in physiotherapy: in shortwave diathermy, where eddy currents induced in the tissues produce deep heating.
We can understand Eddy current from this figure. You can see there is a conducting plate, a solid metal and a magnet. So, as we bring the magnet closer to this conducting plate, the magnetic flux through the plate keeps increasing, and this changing flux induces an electromotive force in the plate.
The faster we bring the magnet closer, the faster the flux changes, the larger the induced electromotive force, and the stronger the circular currents (eddy currents) it drives in the plate.
We can understand its direction from Lenz’s law. While the magnet approaches, the eddy currents oppose the increase in flux, so their magnetic field repels the magnet. When we move the magnet away, the flux through the plate decreases, so the eddy currents reverse their direction to oppose this decrease, and the plate now tends to pull the magnet back.
The current weakens as the magnet moves farther away. Eddy currents are stronger when the magnet is stronger, when it moves faster, and when the metal is a better conductor.
So friends, I hope this concept must have become clear to you. I have tried to simplify it a lot. I hope I have succeeded in it. Thank you.
The author is a physiotherapist who has been practising for the last 17 years. He holds a Bachelor's in Physiotherapy (BPT) from SVNIRTAR (Swami Vivekananda National Institute of Rehabilitation and Research), one of the prestigious physiotherapy schools in India.
Whatever he learns dealing with his patient, he shares it with the world through blogs and e-books. He also owns a YouTube channel, "Sunit Physiotherapist" with over 8 lakh active subscribers. Here, he shares everything he gets to learn serving the patient.
