A material which is not a magnet in normal condition can be made a magnet with the help of some techniques and is called induced magnet and this phenomenon is called induced magnetism. Similarly a magnetic material can lose its magnetization.
There are many ways to make an object an induced magnet or a magnet to lose its magnetization like, by stroking the material with a magnet, by hammering the material in a magnetic field, by heating or by putting the material inside a coil from which direct current is flowing.
This is the way of aligning the poles of a material by a process called stroking for induced magnetism. A permanent magnet is stroked in this process from one end of a bar of some metal to the other end to magnetize it. After rubbing one pole of the magnet on the bar of material from one end to the other then lift the magnet, as shown in figure 8.8.
In this method a bar of metal is placed inside a strong magnetic field and hammered gently. The domains will begin to line up in the direction of applied magnetic field and hence metal bar becomes magnetized. This method is mainly used for magnetization of steel. The magnetization can be increased by heating the metal bar slightly before hammering. This method is shown in figure 8.9.
This method is usually used to demagnetize a material, as heat speeds up the movements of already aligned domains which results in misalignment of domains and hence material loses its magnetization. However in recent past scientists have provided the evidences of generating magnetic field by the process of heating. This phenomenon is referred to as the 'magnetic Seebeck effect' or 'thermo-magnetism'.
"A material that becomes a magnet when it is placed in a magnetic field is called induced magnet".
This is the most common method used for the magnetization of metals. A solenoid is a coil of wire wrapped around a cylindrical coil as shown in figure 8.10. The magnetic field of the solenoid resembles with the field of a bar magnet, as shown in figure 8.11. When we wrap a conducting wire (say a copper wire) around a metal with insulation the domains of the metal get aligned. In this process when a current flows through the wire it generates a magnetic field which behaves as external field to the domains of metal placed inside, which aligns the domains. The coil of wire acts as a magnet as long as the current is flowing through it. When current stops it no longer behaves as a magnet. For direct current (DC) the polarity of coil remains the same and hence it magnetizes the material in one direction. For alternating current the polarity of solenoid changes after every half cycle and hence in first half cycle it magnetizes the material in one direction then demagnetizes it. In second half cycle it magnetizes the material in opposite direction and then demagnetizes it. Materials which easily magnetize and demagnetize are called soft magnetic materials (like soft iron), on the other hand materials which cannot magnetize and demagnetize easily are called hard magnetic materials (like steel).
Electric motors used in hand-held hair dryers, electric razors, hair trimmers and many more such devices, work with the help of magnetic force. An electric motor generates magnetic field with electric current through the coil. The magnetic force then causes the movement or spinning that runs the motor.
B = μ₀ n I 8.1 Here 'B' is the strength of magnetic field having unit tesla (T) which is also equal to newton per ampere per meter (N/A m), while μ₀ is the permeability of the material (Permeability is the property of a material to its response towards a magnetic field, i.e. how much a material permits magnetic field to pass through it), 'I' is the current flowing through the solenoid and 'n' is the number of turns of the solenoid per unit length given by the relation as, n = N / L 8.2 Here 'N' is the total number of turns of the solenoid and 'L' is the length of solenoid. The value of the permeability for vacuum 'μ₀' is given as: μ₀ = 4π × 10⁻⁷ N/A² Similarly the magnetic field due to a wire carrying current can be given as: B = μ₀ I / (2π r) 8.3 Here 'r' is the distance from the center of the wire and I is the magnitude of current flowing through the wire as shown in figure 8.12.
A student takes a long straight copper wire (with insulation on it) from his physics lab, he wraps the wire on an iron rod of 50 cm, to make it solenoid of 15 turns. He connects the ends of copper wire with the battery, which provides 1.2 A current. Find the magnetic field he produces in solenoid by doing so.
Magnetic Field Strength 'B' = ?
To find magnetic Field Strength we have to find number of turns per unit length first by using equation 8.2
n = N / L = 15 / 0.5 = 30 m⁻¹ Now using equation 8.1 B = μ₀ n I = (4π × 10⁻⁷ N/A²) × 30 m⁻¹ × 1.2 A = 1.5 × 10⁻³ T = 0.0015 T
"Magnetic materials which do not retain their magnetization after removal of external magnetic field or applied current (as in case of solenoid) are called temporary magnets".
The solenoid as stated earlier behaves like a magnet as long as a current flows through it, just after removal of current it loses its magnetic field, hence it is a temporary magnet. It is also called as electromagnet. Electromagnet is type of magnet in which the magnetic field is produced due to an electric current. Examples of temporary magnetics include iron nails, screws, metal bolts, kitchen utensils etc.
Examples of permanent magnetic materials include iron ore, cobalt, nickel and Alnico. Temporary and permanent magnets are shown in figure 8.13. Some of the differences are listed below.
| PERMANENT MAGNET | ELECTROMAGNET |
|---|---|
| Permanently magnetized | Temporarily magnetized |
| Made of hard magnetic materials | Made of soft magnetic materials |
| Magnetism does not vary in strength | Magnetism can be varied in strength according to need |
| Magnets' poles can not be altered | Magnets' poles can be changed |
Magnets are used in many fields, depending upon the need we can use permanent or electromagnets. A permanent magnet is made from a material which is magnetized once and does not lose its magnetization by itself and has its own magnetic field. As we know that permanent magnet does not require a continuous supply of electric energy for maintaining its magnetic field. Hence it is used in those applications where continuous supply of electricity is not available or cannot be maintained for long. Although magnetic field strength of permanent magnets are lower than those of electromagnets but they can have magnetic field even in the absence of electricity. Permanent magnets are used in induction cooker, MRI machines, particle accelerators, transformers etc. and in automotive, aerospace, medical, semiconductor and energy industries. Electromagnets are kind of magnets in which the magnetic field is created by an electric current. Electromagnets can be considered as temporary magnets that function only with the flow of electric current, when the current is turned off it loses its magnetization at once.
Magnetic field of a bar magnet and that of a coil resembles with each other.
Electromagnets have a wide range of daily life applications like in electromechanical and electronics devices. Most of the home appliances use electromagnetism as the basic working principle, like electric fan, electric motors and door bells. In medical fields electromagnets are used in MRI scans. Electromagnets are also used in communication devices and power circuits.
Magnetic recording is a method of saving sounds, pictures and data in the form of electrical signals by the process of selective magnetization of some portion of a magnetic material. For writing the data a magnetic tape head is moved onto the tape which is in motion the magnetic field of the tape head aligns the pattern of magnetic domains according to the applied current flowing through the tape head, as shown in figure 8.14.
Similarly the reverse process is done for reading the data from the aligned pattern of tape. For magnetic recording the materials commonly used are iron-oxide, cobalt, chromium oxide and pure iron. The main recording media may be a magnetic tape or disk recorders which are used to store and reproduce audio, video signals and computer data. Some other magnetic recording devices are magnetic drum, core and bubble units used for computer storage units.
Speakers use magnets in their functioning. To produce sound the speaker needs to create some vibrations in the air. This can be done with the help of two magnets: one permanent magnet of strong magnetization and the other is an electromagnet. The permanent magnet is fixed in the center of the cone (diaphragm), which is a conical structure made up of some flexible material to produce vibrations. An electromagnet is attached at the center of the permanent magnet which can move to and fro. When the sound signal in the form of electric current flows through the coil of electromagnet it produces an alternating magnetic field which, due to interaction with the field of permanent magnet, vibrates it. As the cone is attached to the electromagnet, with the to and fro motion of electromagnet it vibrates and produces sound. With a larger permanent magnet, we can produce louder sound. The commonly used material as permanent magnet in speakers is neodymium. The structure of speaker is shown in figure 8.15. Microphones also use magnets in their functioning, but in reverse order. In microphones, the cone (diaphragm) vibrates due to sound and hence produces movement in the electromagnet within the magnetic field of the permanent magnet. This produces an electric current which acts as the signal for the speaker.
Door locks also use magnets in their working. They have an electromagnet fixed at the door frame and a metal plate fixed with the door, in such a way that when the door is closed the metal plate connects with the electromagnet, as shown in figure 8.16.
When the current passes through the electromagnet it attracts the metal plate with huge force, so that to open the door is difficult. When we have to open the door we disconnect the current flowing through electromagnet which releases the metal plate. As this door requires electricity for its functioning, it is mainly used in systems where a continuous power supply is available.