Permanent Magnets and Electromagnets
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Introductory Concepts
Permanent Magnets: As the name suggests these are the type of magnets that can retain their magnetism for a considerate amount of time. They are usually made of hard materials which are strongly magnetized. A perfect example of permanent magnet is the Bar Magnet (a magnet that is widely used to explain/understand the behaviour of magnets). In fact, Permanent magnets are most of the times referred to as Bar Magnets also.
Fig: A bar magnet
Electro- Magnets: Consider the circuit given below. When a battery is connected across a solenoid (a coil of wire with) which is wounded around a nail then the nail behaves as magnet, due to the magnetic field produced by the current flowing through the coil. The nail retains its magnetism till there is current flowing through the coil, but once there is no current, the nail loses its magnetism. This type of temporary magnets which are formed by wounding a coil of wire across an iron core is known as Electro-Magnets.
Fig: A solenoid
Difference between Bar Magnet /Permanent magnet and Electro–Magnet
Similarities between Permanent Magnets and Electro-Magnets
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Both the magnets possess imaginary magnetic field lines.
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Both the magnets have north and south-pole whose behaviour depends on the Geographic north-pole and south-pole of the earth.
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Both the magnets have the capability to exhibit the properties of magnetism.
Advantages of Electromagnets over Permanent Magnets
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In terms of material used in electro magnets and permanent magnets, the cost of materials used in electro-magnet that is, soft iron core is less compared to the hard material used in permanent magnets.
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The magnetic strength of an Electro-Magnet can be altered according to our convenience, whereas the magnetic field of permanent magnet cannot be changed.
Disadvantages of Electro-Magnets
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Electro-magnets require large number of copper coupling which makes it unsuitable for application in small space. Also, maintenance of electro magnet is difficult as it consists of too much of copper coupling. If the circuit is short-circuited, then the electro magnet is damaged and it requires re- coupling of copper coils from time to time which is not feasible.
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Electro magnets require a continuous supply of current which may at some point of time affect the magnets and its field due to various factors like ohmic heating, Inductive voltage spikes, core losses, coupling of coils, etc.
Disadvantage of Permanent Magnets
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The magnetic field of permanent magnet can be produced only below certain temperature due to which this type of magnets cannot be used for hot-device applications.
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With passing time, the permanent magnet tends to corrode which reduces the strength of the maximum magnetic field that can be achieved.
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The poles of the permanent magnet cannot be varied.
Types of Permanent Magnets
The various types of permanent magnet are as follows:
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Ceramic Magnet: Ceramic magnets are the most inexpensive permanent magnets that finds application in food processing industries, resonance imaging etc.
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Alnico Magnets: This type of permanent magnets has good temperature stability and can produce strong magnetic fields. The only disadvantage of this type of magnet is that they can be easily demagnetised. Alnico magnets finds application in sensors like electronics sensors, automotive sensors etc. An Alnico magnet consists of elements like nickel, aluminium, cobalt, copper, titanium and iron.
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Flexible Magnets: The door seals used in refrigerator are flexible magnets. This magnet is created by combining rubber polymers, plastic with magnetic powders.
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Neodymium Iron Boron Magnet (NdFeB): This type of magnet is a type of rare earth magnet which is very easily oxidised. It is a very expensive material and finds application in jewellery making, book binding etc.
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Samarium Cobalt (SmCo) Magnet: This is a type of rare earth magnet that is resistant to temperature and oxidation. This type of permanent magnet has higher Magnetic strength. It is most widely used in high end motors, turbo machinery etc.
Fig: Range of temperature of various permanent magnets.
Application of Electro-Magnets
The most commonly used application of electro-magnets is in transformer. The coils in the transformer produces varying magnetic field when current is supplied which in turn induces voltage. Transformers are used to regulate the alternate voltages in electric power system. The desired voltage as and when required can be achieved by varying the amount of current to be passed in the coil.
Other applications of electro magnets include Magnetic locks, relays, magnetic levitation, electric bells, loud speakers etc.
Q1: What is the use of Electro-Magnet?
Sol. The electro-magnet is used to produce magnetic field when current is supplied to its coil, without coming in direct contact with each other.
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