Permanent magnets and electromagnets, uses of electromagnets, practice problems, FAQs

Several home appliances or other electrical equipment that we use in our daily life certainly has magnets as one of the important components responsible for their proper working. Be it television, microwaves, hard drives, motors etc, these equipment use different properties of magnets while operating. These magnets are further of two types called permanent and electromagnets. Here we will be discussing both in detail.

Table of contents

• Permanent magnets
• Electromagnets
• Uses of electromagnets
• Practice problems
• FAQs

Permanent magnets

The substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets.

Permanent magnets should have the following properties:

• High retentivity (so that the magnet is strong)
• High coercivity (so that the magnetic properties are not wiped out by stray magnetic fields, mechanical treatment or temperature change)

As the material in this case is never put to cyclic changes of magnetization, hence hysteresis is immaterial.

Steel is more suitable for the construction of permanent magnets than soft iron. The fact that the retentivity of iron is little greater than that of steel is outweighed by the much smaller value of its coercivity. Recently a number of alloys, having large values of coercivity, have been developed for the purpose of construction of permanent magnets.

The four common types of permanent magnets include:.

• Neodymium magnet(Neodymium, Iron, Boron)
• Alnico (aluminium, nickel and cobalt alloy)
• Samarium Cobalt
• Ferrite or ceramic (a combination of iron oxide in cobalt, nickel or strontium)

Permanent magnets and all the materials attracted to a magnet are either ferromagnetic or ferrimagnetic in nature. The common ferromagnetic materials are iron, cobalt, nickel and most of their alloys, and some compounds of rare earth metals. Ferromagnetic materials can be classified into two types as follows.

• Soft ferromagnetic materials: These magnetic materials do not stay magnetised after being subjected to a magnetic field. E.g., Annealed iron.
• Hard ferromagnetic materials: Opposite to soft ferromagnetic materials, these will always act as a magnet once magnetised. E.g., Alnico.

Electromagnets

When an electric current passes through a coil around a soft iron core, the current produces a magnetic field that magnetises the soft iron core, displaying the characteristics of a magnet. However, there is a limitation on the magnetic property, and it lasts till the current passes through the coil. Furthermore, the strength of the magnetic field is also dependent upon the electric current.

A type of magnets, electromagnets, are usually pieces of a conductor that offer high conductivity that demonstrate magnetic properties when an electric current is applied.

Electromagnets generally consist of a wire wound tightly over a core made from ferromagnetic or ferromagnetic materials. When an electric current is passed through the wire, a magnetic field is created as a result of the electric field. This magnetic field is concentrated on the core of the electromagnet. The electromagnet functions as a magnet only as long as constant electric currents are given to it; that is, the magnetic field is dissipated when the supply of electric current is stopped.

Electromagnets have an advantage over ordinary magnets in that the amount (degree) of magnetism offered by the electromagnet can be altered by changing the electric current offered to it. The higher the electric current supplied, the higher is the magnetic property demonstrated by the electromagnet. However, unlike normal magnets, they require an electric current to operate.

Uses of Electromagnets

Based on their usage, there are two types of electromagnets. Portative electromagnets are only meant to hold an object in place. Tractive electromagnets offer a force and help in moving an object. Owing to their convenient features, electromagnets are used in various instruments and fields. Some are discussed below.

• Electric motors and generators: Motors are electric instruments that convert electrical energy into mechanical energy. Electric motors generally consist of a magnet/electromagnet, a shaft, rotor, bearing etc. among other components. When an electric current is supplied to the motor, the electric current passes through the wire windings of the electromagnet and thereby magnetises the core. The electric field created, in turn, induces a magnetic field that acts on the shaft. This shaft starts rotating due to the magnetic field and produces rotational movement and torque. Generators have a similar working principle to a motor, but in the case of generators, mechanical energy is converted to electrical energy.

• Electric bell and buzzers: When an electric current is applied to the bell and buzzers, the electromagnetic coils are magnetised by the induced magnetic field. This makes a spring-loaded arm magnetically attracted to the electromagnet. At the end of the metallic arm, a spherical ball is connected, which comes in contact with the gong. When the arm moves to strike the gong, it breaks the circuit, and the arm comes back to its original position and completes the circuit, which then magnetises the arm to strike the gong again. This process is repeated as long as the bell is rung. A buzzer uses the same working principle, but it is significantly quieter because it does not use a gong.
• Loudspeakers and headphones: Speakers and headphones primarily work by converting electric energy to mechanical energy. They have a diaphragm that can be moved by the audio signals accordingly. When an electric current is given, the electromagnets in loudspeakers and other audio devices, in general, are magnetised. When an input of an audio signal is received by the speaker, the electronics in it amplify the audio signals considerably, enabling the speaker driver to read it. In accordance with the input audio signals, the speaker driver can move the diaphragm to reproduce the sound corresponding to the original unamplified electronic audio signals.
• MRI scanners: The human body is made of tissues that contain hydrogen nuclei, which houses a single proton. Since protons are affected by external magnetic fields, they can produce minute but measurable signals. These generated signals can then be measured in terms of the density of that tissue in one particular area. MRI scanners, therefore, create strong magnetic fields, which are then targeted on the part of the body of the patient that is to be scanned.

Practice problems

Question 1. In an electric motor, why are electromagnets preferred over permanent magnets ?
Answer. An electromagnet produces a magnetic field as long as electric current is flowing through it which disappears when the current stops flowing.

The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be rapidly manipulated over a wide range by controlling the amount of electric current.

Question 2. How can we change the polarity of an electromagnet?
Answer. By reversing the direction of current flowing through the coil, we can switch the poles of the electromagnet.

Question 3. How can we find the polarity of an electromagnet?

Answer. The polarity can be found using the clock face rule. The steps are as follows:

While looking from one of the ends of the electromagnet, if the direction of current is in the clockwise direction then the face from which you are looking at the electromagnet corresponds to the South pole and vice versa.

Question 4. What is the difference between a solenoid and an electromagnet?

Answer. A solenoid is a long cylindrical coil of wire whereas electromagnet is a coil of wire wrapped around an iron core.

FAQs

Question 1. At what place in a magnet where the magnetism is the strongest?
Answer. Magnetic field is strong near the poles of the magnet while it is weak at the centre of the magnet.

Question 2. State an example of electrical instruments made by using permanent magnets.
Answer. Permanent magnets are used in generators, hard drives etc.

Question 3. Mention 3 advantages of an electromagnet over permanent magnet?
Answer. 

• The magnetism of an electromagnet can be switched on or switched off as desired. This is not possible with a permanent magnet.
• An electromagnet can be made very strong by increasing the number of turns in the coil, and by increasing the current passing through the coil. On the other hand, a permanent magnet cannot be made so strong.
• Magnetic strength of electromagnet is more than permanent magnets.

Question 4. Can a permanent magnet lose its magnetism?
Answer. 

It is possible to demagnetize the magnet by using a strong enough magnetic field of opposite polarity.

Ferromagnetic (having a high susceptibility to magnetization) materials such as cobalt, iron, magnetite, among others, could lose their magnetism if they are heated above a point that is known as the Curie temperature. Older magnetic materials such as magnetic steels and other materials are susceptible to demagnetization if a certain amount of energy is transmitted through the material via a shock, such as being hit with a hammer or dropped with a certain force.