8.3.1 Temporary and Permanent Magnets

Not all magnets are created equal. Some magnets, like the ones on your refrigerator, keep their magnetism for years. Others, like the electromagnet in a doorbell, only act as magnets when electricity flows through them. Understanding the difference between temporary and permanent magnets is essential for understanding how magnets are used in different applications, from simple fridge magnets to complex electric motors.

Definition

Permanent Magnet: A magnet that retains its magnetism for a long period of time without the need for an external power source. It is made from hard magnetic materials (like steel or Alnico).

Temporary Magnet (Electromagnet): A magnet that only acts as a magnet while it is in an external magnetic field or while an electric current is flowing through it. It is made from soft magnetic materials (like soft iron).

Main Idea

The main idea is that the difference between permanent and temporary magnets lies in the material they are made from and their ability to retain magnetism. Permanent magnets are made from hard magnetic materials, which are difficult to magnetize but retain their magnetism once magnetized. Temporary magnets (electromagnets) are made from soft magnetic materials, which are easy to magnetize and demagnetize, allowing them to be turned on and off as needed.

Core Concept

Permanent Magnets The text states: "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."

• Material: Hard magnetic materials (e.g., steel, Alnico, iron ore).
• Characteristics: Retains magnetism without an external power source.
• Magnetic field: Relatively constant and does not require a continuous supply of energy.
• Examples: Bar magnets, horseshoe magnets, refrigerator magnets.

The text states: "Examples of permanent magnetic materials include iron ore, cobalt, nickel and Alnico."

Temporary Magnets (Electromagnets) The text states: "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."

• Material: Soft magnetic materials (e.g., soft iron).
• Characteristics: Acts as a magnet only while current flows; loses magnetism when current stops.
• Magnetic field: Can be controlled by varying the current.
• Examples: Electromagnets in doorbells, cranes, electric motors, and relays.

The text states: "Examples of temporary magnetics include iron nails, screws, metal bolts, kitchen utensils etc."

Comparison Table from Text The text provides a comparison table (Figure 8.13):

Uses of Permanent Magnets The text states: "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."

• Induction cooker: Uses permanent magnets.
• MRI machines: Use strong permanent magnets (or superconducting electromagnets).
• Particle accelerators: Use permanent magnets.
• Transformers: Use permanent magnets.
• Automotive, aerospace, medical, semiconductor, and energy industries.

The text states: "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."

Uses of Electromagnets The text states: "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 scan. Electromagnets are also used in communication devices and power circuits."

• Electric motors and generators: Use electromagnets to convert energy.
• Doorbells: Use an electromagnet to strike a bell.
• Relays: Use an electromagnet to open or close a circuit.
• Cranes: Use powerful electromagnets to lift heavy metal objects.
• MRI machines: Use electromagnets (often superconducting) to generate strong magnetic fields.
• Communication devices: Use electromagnets in speakers and microphones.

Detailed Explanation

The distinction between permanent and temporary magnets is based on the magnetic properties of the materials used.

• Hard Magnetic Materials: Have high coercivity (resistance to demagnetization). They are difficult to magnetize but retain their magnetism once magnetized. Examples include steel, Alnico, and neodymium-iron-boron (NdFeB).
• Soft Magnetic Materials: Have low coercivity. They are easily magnetized and demagnetized. Examples include soft iron, silicon steel, and Mu-metal.

• Refrigerator magnets: Hold notes on the fridge.
• Compass needles: Align with the Earth's magnetic field.
• Electric motors: Provide the magnetic field that interacts with the electromagnet to produce motion.
• Loudspeakers: Interact with the voice coil to produce sound.
• Magnetic separators: Separate magnetic materials from non-magnetic ones.

• Electric motors and generators: Provide the magnetic field that interacts with the armature.
• Relays and contactors: Use an electromagnet to open or close electrical contacts.
• Lifting cranes: Use powerful electromagnets to lift and transport scrap metal.
• Doorbells and buzzers: Use an electromagnet to strike a tone bar.
• Magnetic resonance imaging (MRI): Use superconducting electromagnets to generate the strong magnetic fields needed for imaging.
• Particle accelerators: Use electromagnets to steer and focus particle beams.

• No power required: They do not need an external power source to maintain their magnetic field.
• Reliable: They maintain their field without failure (unless demagnetized by heat or shock).
• Low cost: Simple and inexpensive to manufacture.
• No heat generation: They do not generate heat (unlike electromagnets, which can overheat).

• Fixed strength: The magnetic field strength is fixed and cannot be easily changed.
• Limited maximum strength: Permanent magnets are generally weaker than electromagnets.
• Demagnetization risk: Can be demagnetized by high temperatures, mechanical shock, or strong opposing magnetic fields.

• Controllable: The magnetic field can be turned on/off and its strength can be varied by controlling the current.
• Strong: Can produce very strong magnetic fields (stronger than permanent magnets).
• Reversible polarity: The polarity can be reversed by reversing the current.
• Large scale: Can be built on a large scale for industrial applications.

• Requires power: Need a continuous supply of electricity to maintain the magnetic field.
• Generates heat: The coil generates heat due to electrical resistance (I²R losses).
• More complex: Requires a power source, coils, and control circuits.
• Can fail: If the power fails, the magnetic field is lost.

Intermediate Level

In physics, permanent magnets are made from hard magnetic materials (like steel) that retain their magnetization once magnetized. They are used in applications requiring a constant magnetic field without a power source, such as compass needles, refrigerator magnets, and permanent magnet motors.

Temporary magnets, or electromagnets, are made from soft magnetic materials (like soft iron) and only act as magnets while a current flows through a coil. They are used in applications requiring controllable magnetic fields, such as electric motors, relays, and lifting cranes.

The key difference is in the coercivity of the materials. Hard magnetic materials have high coercivity, meaning their domains resist being randomized once aligned. Soft magnetic materials have low coercivity, so their domains can be easily aligned and randomized.

Important Points

• - Permanent Magnet: Retains magnetism without power. Made from hard magnetic materials (steel, Alnico).
• - Temporary Magnet (Electromagnet): Magnetized only when current flows. Made from soft magnetic materials (soft iron).
• - Permanent Magnets: Used in compasses, fridge magnets, motors.
• - Electromagnets: Used in cranes, relays, doorbells, MRI, motors.
• - Advantages of Permanent: No power, reliable.
• - Advantages of Electromagnets: Controllable, strong, reversible.

Remember

• - Permanent Magnet: Hard material, retains magnetism.
• - Temporary Magnet: Soft material, needs current.
• - Permanent: No power needed.
• - Temporary: Power needed.
• - Permanent: Fixed strength.
• - Temporary: Adjustable strength.

Common Mistakes

• - Thinking all magnets are permanent: Many magnets (electromagnets) are temporary and require a current to maintain their field.
• - Thinking permanent magnets are stronger than electromagnets: Electromagnets can produce much stronger magnetic fields than permanent magnets.
• - Forgetting that electromagnets require power: If the power fails, the electromagnet loses its magnetism (which can be dangerous in applications like lifting cranes).

Quick Revision

Permanent magnets are made from hard magnetic materials and retain their magnetism without a power source. They are used in compasses, refrigerator magnets, and permanent magnet motors. Temporary magnets (electromagnets) are made from soft magnetic materials and only act as magnets when a current flows through a coil. They are used in cranes, relays, doorbells, and electric motors. The key difference is that permanent magnets have a fixed magnetic field, while electromagnets can be controlled (turned on/off, varied in strength, and their polarity can be reversed).

60-Second Revision

• Permanent Uses: Compasses, fridge magnets, motors.
• Electromagnet Uses: Cranes, relays, doorbells, MRI.
• Permanent: Fixed strength.
• Electromagnet: Adjustable strength.

Frequently Asked Questions (FAQs)

Q1. What is a permanent magnet? Answer: A permanent magnet is a magnet that retains its magnetism for a long period of time without the need for an external power source. It is made from hard magnetic materials like steel.

Q2. What is a temporary magnet? Answer: A temporary magnet (or electromagnet) is a magnet that only acts as a magnet while an electric current flows through it. It is made from soft magnetic materials like soft iron.

Q3. What is the difference between a permanent magnet and an electromagnet? Answer: A permanent magnet retains its magnetism without power and has a fixed strength. An electromagnet requires a current to maintain its magnetic field, and its strength can be controlled by adjusting the current.

• Q4. What materials are used for permanent magnets?
• Answer: Hard magnetic materials such as steel, Alnico, and neodymium-iron-boron (NdFeB).

• Q5. What materials are used for electromagnets?
• Answer: Soft magnetic materials such as soft iron, silicon steel, and Mu-metal.

• Q6. What are some applications of permanent magnets?
• Answer: Compasses, refrigerator magnets, loudspeakers, and permanent magnet motors.

• Q7. What are some applications of electromagnets?
• Answer: Electric motors, generators, relays, doorbells, lifting cranes, MRI machines, and particle accelerators.

• Q8. Why is soft iron used in electromagnets?
• Answer: Soft iron is easily magnetized and demagnetized, which allows the electromagnet to be turned on and off quickly and efficiently.

• Q9. Why is steel used in permanent magnets?
• Answer: Steel is a hard magnetic material that retains its magnetism once magnetized, making it suitable for permanent magnets.

• Q10. Can an electromagnet be made stronger than a permanent magnet?
• Answer: Yes. Electromagnets can produce much stronger magnetic fields than permanent magnets, especially when using superconducting coils or large currents.

Multiple Choice Questions (MCQs)

• Q1. A magnet that retains its magnetism without a power source is called a:
• A. Temporary magnet
• B. Permanent magnet
• C. Electromagnet
• D. Solenoid

• Correct Answer: B
• Explanation: The text states that a permanent magnet is magnetized once and does not lose its magnetization by itself.

• Q2. An electromagnet is a type of:
• A. Permanent magnet
• B. Temporary magnet
• C. Non-magnetic material
• D. Magnetic field

• Correct Answer: B
• Explanation: The text states that the solenoid behaves like a magnet only as long as current flows, hence it is a temporary magnet.

• Q3. Permanent magnets are made from:
• A. Soft magnetic materials
• B. Hard magnetic materials
• C. Non-magnetic materials
• D. Conducting materials

• Correct Answer: B
• Explanation: The text states that permanent magnets are made from hard magnetic materials.

• Q4. Electromagnets are made from:
• A. Hard magnetic materials
• B. Soft magnetic materials
• C. Non-magnetic materials
• D. Insulating materials

• Correct Answer: B
• Explanation: The text states that electromagnets are made from soft magnetic materials.

• Q5. Which of the following is a permanent magnetic material?
• A. Soft iron
• B. Steel
• C. Copper
• D. Aluminum

• Correct Answer: B
• Explanation: The text lists steel as an example of a permanent magnetic material (hard magnetic material).

• Q6. Which of the following is a temporary magnetic material?
• A. Steel
• B. Soft iron
• C. Alnico
• D. Iron ore

• Correct Answer: B
• Explanation: Soft iron is a soft magnetic material used in electromagnets (temporary magnets).

• Q7. The strength of an electromagnet can be varied by:
• A. Changing the color of the core
• B. Changing the current
• C. Changing the temperature
• D. Changing the shape of the magnet

• Correct Answer: B
• Explanation: The text states that magnetism of an electromagnet can be varied in strength according to need (by changing the current).

• Q8. The polarity of an electromagnet can be reversed by:
• A. Reversing the current
• B. Heating the core
• C. Cooling the core
• D. Breaking the core

• Correct Answer: A
• Explanation: The text states that magnets' poles can be changed in an electromagnet (by reversing the current).

• Q9. Which of the following is an application of permanent magnets?
• A. Electric fan
• B. Doorbell
• C. Compass needle
• D. Relay

• Correct Answer: C
• Explanation: A compass needle is a permanent magnet. The others are applications of electromagnets.

• Q10. Which of the following is an application of electromagnets?
• A. Refrigerator magnet
• B. Compass needle
• C. Lifting crane
• D. Bar magnet

• Correct Answer: C
• Explanation: Lifting cranes use powerful electromagnets to lift heavy metal objects. The others are permanent magnets.

Frequently Asked Questions (FAQs)

Q1. What is the difference between a permanent magnet and a temporary magnet? Answer: A permanent magnet retains its magnetism without a power source, while a temporary magnet (electromagnet) only acts as a magnet while current flows. Permanent magnets are made from hard magnetic materials; electromagnets are made from soft magnetic materials.

Q2. Can a permanent magnet be demagnetized? Answer: Yes. A permanent magnet can be demagnetized by heating it above its Curie temperature, hammering it, or exposing it to a strong alternating magnetic field.

Q3. Why are electromagnets used in cranes? Answer: Electromagnets are used in cranes because they can be turned on to lift heavy metal objects and turned off to release them. This makes them ideal for lifting and transporting scrap metal.

Q4. Why are permanent magnets used in compasses? Answer: A compass needle is a small permanent magnet that aligns itself with the Earth's magnetic field. It does not need a power source, making it ideal for navigation.

Q5. What is the Curie temperature? Answer: The Curie temperature is the temperature above which a ferromagnetic material loses its permanent magnetism and becomes paramagnetic. For iron, it is about 770°C.

Q6. Can an electromagnet be made without a core? Answer: Yes. An electromagnet can be made with just a coil of wire (air-core electromagnet). However, using a soft iron core greatly increases the strength of the magnetic field.

Q7. What is the difference between an electromagnet and a solenoid? Answer: A solenoid is a coil of wire that creates a magnetic field when current flows. An electromagnet is a solenoid with a magnetic core (like soft iron) that enhances the magnetic field. All electromagnets are solenoids, but not all solenoids are electromagnets (if they have no core).

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