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Permanent magnet is a substance which at room temperature retains their ferromagnetic properties for a considerable period of time. Whereas, an electromagnet is made from a coil of wire that acts as a magnet when electric current passes through it. An electromagnet has high permeability and low retentibility.
Permanent Magnets and Electromagnets is the topic covered under the fifth chapter of NCERT Class 12 Physics i.e. Magnetism and Matter. As per the latest updates, in the revised syllabus of CBSE, no topics have been excluded from the above mentioned chapter. The whole Unit 3 will carry a weightage of 8.5 marks in the board examination. In this article, detailed description of the topic is discussed with a few previous year questions.
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Key Terms: Magnets, Permanent Magnets, Electromagnets, Magnetic Field, Magnetism, Permeability
Definition
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Magnets are the materials that can produce a magnetic field in the space surrounding them. This magnetic field of magnets is invisible and responsible for the various properties of the magnet.
The magnets having the following important properties :
- Two poles exist on the endpoints of the magnet which are termed as north and south poles.
- Two like poles repel each other while two, unlike poles, attract each other.
- When any magnet is suspended freely in the atmosphere it always rests in the north-south direction.
- The magnetic force of the magnet attracts ferromagnetic materials like iron, steel, nickel, etc.
- When the magnet comes in contact with ferromagnetic substances or fillings it can attract the fillings and they accumulate on their end poles.
Types of Magnets
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Magnets are of generally three types which are as follows:
- Temporary magnets
- Permanent magnets
- Electromagnets
Temporary magnets are those magnets that get magnetized when coming in contact with the magnetic field of the magnet and lose their magnetic behavior on the removal of the magnetic field. Iron nails and paper clips are examples of temporary magnets.
In this article, we will discuss all the details related to permanent and electromagnets. You can refer to this article to get full knowledge about permanent magnets and electromagnets.
Permanent Magnets
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A permanent magnet is an object made up of magnetized materials and creates its persistent magnetic field. This type of magnet possesses its magnetism and magnetic properties for a longer time. It can be made using various methods. One can hold the iron in a north-south direction and continuously hammer it. It is usually made up of hard materials.
Permanent magnets do not lose their magnetic properties once they magnetize due to this it is commonly used.
Read More: The Bar Magnet
Following are the ways to demagnetize the permanent magnet
- Permanent magnets can lose their magnetic properties when higher temperatures are applied to the magnet.
- The magnetism of permanent magnets loses on hammering.
- Striking one magnet with another in an inappropriate manner can lose its magnetic properties.
Types of Permanent Magnets
- Permanent magnets are of the following four types :
- Ceramic or Ferrite magnets - These magnets are economical. It is generally used in food processing industries, resonance imaging, etc.
- Alnico - This type of permanent magnet is made up of Aluminium, Nickel, and Cobalt alloys and is used in various applications.
- Samarium Cobalt (SmCo) - It is a rare earth magnet resistant to temperature and oxidation. It has high strength and used in high-end motors, turbomachinery, etc.
- Neodymium Iron Boron (NIB) - It is also a rare earth magnet and oxidizes easily. It is very costly, but higher strength. It is commonly used in jewelry making.
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Characteristics of permanent magnets
- They attract other magnets and produce induced magnetism in them.
- The attraction force is greatest at poles.
- Two like poles repel each other.
- Two, unlike poles, attract each other.
- Align to already existing magnetic fields like earth.
Disadvantages of permanent magnet
- The strength of the magnetic field of permanent magnets cannot be varied.
- Magnetic particles are attracted to poles.
- Limited applications on irregular shapes.
- Deterioration with wear.
Read Also: SI unit of magnetic field
Uses of permanent magnets
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It is used for everyday life purposes.
- Used in sensors.
- Alnico magnets are used in radar and telephones.
- Used in motors, generators, and measuring instruments.
- Used in laboratories and study institutions for performing practicals of physics and electrical engineering.
- Used for jewelry-making purposes.
Electromagnets
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When a metallic wire is wound on the soft iron core then due to passes of electric current the magnetic field is produced to its surrounding known as electromagnets. The strength of the magnetic field can be changed with varying electric current.
Read Also: Curie's law
The core magnetizes due to the flow of electric current but loses its magnetism when no electric current is to be passed. You can simply make electromagnets by wounding a coil of metallic wire (like Copper) on the soft iron core. It behaves like a magnet on flowing electric current in it.
Types of Electromagnets
Electromagnets are of generally three types which are as follows :
- Resistant - The resistive magnets can produce a magnetic field with copper wires, it produces a magnetic weak magnetic field.
- Superconductors - The superconducting magnets can produce a magnetic field by reducing the electrical resistance.
- Hybrid - Hybrid electromagnets are the combined form of resistant and superconducting magnets.
Advantages of Electromagnets
There are various advantages of electromagnets, some of the important benefits are mentioned below :
- The main advantage of electromagnets is the strength of the magnetic field can be changed with an electric current.
- The working of electromagnets can be on and off with on and off of the electric current.
- The poles of electromagnets can be changed by reversing the direction of the electric current.
- It is a stronger magnet and can work with high currents.
- Due to its salient properties it is mostly preferred for industrial applications.
Disadvantages of Electromagnets
Some important disadvantages of electromagnets are as follows :
- It consumes a lot of energy.
- They get heat up very fast.
- It can store a huge amount of energy in its magnetic field and discharge this energy on the removal of the electric current supply.
Uses of Electromagnets
- Electromagnets are used in control switches in relays.
- Used for induction heating purposes.
- Used in MRI machines.
- Used in spacecraft propulsion systems.
- It is used for magnetic separation.
- Used in electric motors and generators.
Read More:
Difference between Permanent magnets and Electromagnets
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The difference between Permanent magnets and Electromagnets is as follows:
| Permanent Magnets | Electromagnets |
|---|---|
| The magnetic properties gets exists over a longer time | The magnetic properties exist when an electric current passes through it and loses on the removal of electric currents. |
| The strength of the magnetic field depends on the material used for the magnet. | The strength of electromagnets can be changed by changing with an electric current. |
| On the loss of magnetic properties it become useless | The electromagnets lose their magnetism temporarily and gain its magnetism again with the flow of electric current |
| There is no need of supply of electric currents to maintain its magnetic characters | The supply of electric current requires maintaining the magnetic behavior of the electromagnets. |
| It is generally made up of hard materials | It is generally made up of soft materials |
| The poles of this magnet are fixed. It cannot be changed. | The poles of these magnets can be changed by reversing the direction of the flow of electric current. |
Previous Year Questions
- The ferromagnetic substance is converted into paramagnetic substances … [JIPMER 1996]
- The relative permeability is represented by μr … [KCET 2001]
- A device which converts electrical energy into mechanical energy … [JKCET 1999]
- A circular loop of radius R, carrying current I, lies in x-y plane … [JEE Advance 1999]
- When a material is placed in a magnetic field B … [COMEDK UGET]
- Susceptibility of a magnetic substance is found to depend on temperature … [JEE Advance 1998]
- The magnetic susceptibility is negative for … [NEET 2016]
- Two short bar magnets of length 1cm each have magnetic moments … [JEE Main 2013]
- According to Curie's law, the magnetic susceptibility of a substance at an … [NEET 2003]
- An electron in motion is associated with… [ KCET 1996]
- Two parallel wires carry electric current in same direction. The wires... [ KCET 1996]
- Resistance of an ideal ammeter is...[ KCET 1996]
- If a current of 0.1 A is passed through the coil, what is the couple acting?...[CBSE Class 12 ]
- The acceleration of the falling magnet is..[NEET]
- A bar magnet is equivalent to ............[KCET 2004]
- A metal ring is held horizontally and bar magnet is dropped through….[NEET]
- The total charge, induced in a conducting loop when it is moved in magnetic field depend on...[NEET 1992]
- If a charge particle enters perpendicular in the uniform magnetic field, then...[JIPMER 2016]
- Materials suitable for permanent magnet, must have which of the following properties ?
- At what temperature, the ferromagnetic substances become paramagnetic ?
Sample Questions
Ques. What are permanent magnets? (1 Mark)
Ans. The magnets cannot lose their magnetism over a longer time. It is made up of hard materials. The poles (N and S poles) of permanent magnets are fixed.
Ques. What are the electromagnets? (2 Marks)
Ans. When a wire is wound on the soft metallic core, on the flow of electric current it behaves like a magnet. The strength of the magnetic field can be changed by the variation in the amount of electric current. The poles of this magnet can be changed by reversing the direction of the electric current.
Ques. Name the poles found in magnets? (1 Mark)
Ans. There are two poles found in magnet namely North and South Poles.
Ques. Give examples of permanent magnets and electromagnets? (2 Marks)
Ans. The examples of permanent magnets are bar magnets, Neodymium magnets, etc. and the examples of electromagnets are Motors, generators, transformers, doorbells, loudspeakers, headphones, MRI machines, etc.
Ques. Define permanent magnets. Give one example. (1 Mark)
Ans. A permanent magnet is an object made up of magnetized materials and creates its persistent magnetic field. This type of magnet possesses its magnetism and magnetic properties for a longer time. For example, steel is a permanent magnet.
Ques. (i) Write two characteristics of a material that are used for making permanent magnets.
(ii) Why is the core of an electromagnet made of ferromagnetic materials? (3 Marks)
Ans. (i) The materials chosen to make permanent magnet should have
(a) High retentivity in order to produce a strong magnetic field.
(b) High permeability so that the magnet can get magnetised easily.
(ii) The core of an electromagnet is made of ferromagnetic materials because it has high permeability and low retentivity. Soft iron is a suitable material for this purpose.
Qus. Why should the material used for making permanent magnets be high in coercivity? (1 Mark)
Ans. The material used for making permanent magnets should have high coercivity so that the magnetization is not erased by stray magnetic field, minor mechanical damage or temperature variations.
Ques. (i) What are the two characteristics of a material used for making permanent magnets?
(ii) Why is the core of an electromagnet made of ferromagnetic materials? (3 Marks)
Ans. (i) The two characteristics of a material that is used for making permanent magnets are:
- It must have high retentivity so that it produces a strong magnetic field.
- It must possess a high coercivity so that its magnetization is not destroyed by strong magnetic fields, minor mechanical damage or temperature variations.
(ii) The core of an electromagnet is made up of ferromagnetic materials because they possess high initial permeability so that magnetization is large even for a small magnetising field and at the same time, low resistivity to reduce losses due to eddy currents.
Ques. The relative magnetic permeability of a magnetic material is 800. Determine the nature of the magnetic material and state its two properties. (3 Marks)
Ans. The nature of the magnetic material is ferromagnetic.
Its two properties are as follows:
(i) Their magnetic susceptibility is large and positive and they are strongly attracted by magnets.
(ii) They tend to move quickly from weaker to stronger parts of the field in a non-uniform magnetic field.
Ques. State two characteristics properties each to select materials suitable for
(i) Permanent magnets and
(ii) Electromagnets. (3 Marks)
Ans. (i) Properties of the material of permanent magnet are,
- It should have high retentivity so that it continues to be magnetised even in the absence of the magnetising field.
- It should have high saturation magnetisation.
- It must have high coercivity so that it does not get demagnetised easily.
(ii) The properties of the material of electromagnet are,
- The material must have low retentivity so that it gets demagnetised easily.
- The material should have low coercivity.
- It should have a high value of saturation magnetisation for the electromagnet to be very strong.
Ques. (a) How does an electromagnet differ from a permanent magnet?
(b) Write two properties of material which make it suitable for making electromagnets. (3 Marks)
Ans. (a) Permanent magnet: The permanent magnet is made from a ferromagnetic material which retains magnetic properties over a longer time when kept at a room temperature.
Electromagnet: The electromagnet comprises a core made from ferromagnetic material. Here, the magnetic properties exist when an electric current passes through it and loses on the removal of electric currents.
(b) The properties of the material of electromagnet are,
- Hysteresis loss should be less. It should not be heated up at the time of magnetisation and demagnetisation.
- It should have high permeability.
Ques. Write the difference between an electromagnet and a permanent magnet. How is an electromagnet designed? State any two factors on which the strength of an electromagnet depends. (5 Marks)
Ans. The difference is as follows:
| Permanent Magnets | Electromagnets |
|---|---|
| The magnetic properties gets exists over a longer time | The magnetic properties exist when an electric current passes through it and loses on the removal of electric currents. |
| The strength of the magnetic field depends on the material used for the magnet. | The strength of electromagnets can be changed by changing with an electric current. |
| On the loss of magnetic properties it become useless | The electromagnets lose their magnetism temporarily and gain its magnetism again with the flow of electric current |
| There is no need of supply of electric currents to maintain its magnetic characters | The supply of electric current requires maintaining the magnetic behavior of the electromagnets. |
The design of an electromagnet: An electromagnet is made from a coil of wire wrapped on a soft iron core which behaves like a magnet when an electric current flows through it, but when the current stops, it stops being a magnet as well.
The factors that affect the strength of electromagnet are:
(a) The number of windings in an electromagnet
(b) The amount of current supplied.
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