8.3.2 Uses of Permanent Magnets and Electromagnets

Magnets are everywhere in modern life, from the simple refrigerator magnet holding a shopping list to the complex electromagnets in MRI machines and particle accelerators. But not all magnets are used in the same way. Permanent magnets and electromagnets have different properties, making them suitable for different applications. Understanding these uses helps us appreciate the role of magnetism in technology and everyday life.

Definition

Permanent Magnets are magnets that retain their magnetism without an external power source. They are used in applications where a constant magnetic field is needed.

Electromagnets are temporary magnets that only act as magnets when an electric current flows through them. They are used in applications where the magnetic field needs to be controlled (turned on/off, varied in strength, or reversed in polarity).

Main Idea

The main idea is that permanent magnets and electromagnets are used in different applications based on their distinct properties. Permanent magnets are used where a constant magnetic field is needed without a power source, such as in compasses, refrigerator magnets, and permanent magnet motors. Electromagnets are used where the magnetic field must be controlled, such as in electric motors, relays, lifting cranes, and MRI machines. The choice between the two depends on the requirements of the application.

Core Concept

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."

• No power required: They maintain their magnetic field without a continuous supply of electricity.
• Reliable: They do not fail if the power goes out.
• Simple: They do not require complex control circuits.
• Cost-effective: They are often cheaper than electromagnets for simple applications.

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: Use electromagnets to convert electrical energy into mechanical energy.
• Electric fans: Use electromagnets in the motor.
• Doorbells: Use an electromagnet to strike a bell.
• MRI scans: Use electromagnets (often superconducting) to generate strong magnetic fields.
• Communication devices: Use electromagnets in speakers, microphones, and relays.
• Power circuits: Use electromagnets in relays and contactors.

• Controllable: The magnetic field can be turned on/off and its strength can be varied by controlling the current.
• Strong: They can produce very strong magnetic fields.
• Reversible: The polarity can be reversed by reversing the current.
• Versatile: They can be designed for a wide range of applications.

Comparison of Uses | Application | Permanent Magnet | Electromagnet | | :--- | :--- | :--- | | Compass | ✓ (constant field) | ✗ (needs power) | | Refrigerator magnet | ✓ (simple, no power) | ✗ (impractical) | | Electric motor | ✓ (in some designs) | ✓ (in most designs) | | Lifting crane | ✗ (cannot turn off) | ✓ (can turn on/off) | | Doorbell | ✗ (needs on/off) | ✓ (can turn on/off) | | MRI machine | ✓ (permanent magnets) | ✓ (superconducting electromagnets) | | Relay | ✗ (needs control) | ✓ (can be controlled) | | Speaker | ✓ (permanent magnet) | ✓ (voice coil is an electromagnet) |

Detailed Explanation

The choice between permanent magnets and electromagnets depends on the specific requirements of the application.

• Refrigerator magnets: Hold notes and photos on the fridge. They are simple, cheap, and require no power.
• Compass needles: Align with the Earth's magnetic field for navigation. They are permanent magnets that do not need a power source.
• Magnetic toys: Use permanent magnets for building and playing.
• Magnetic closures: Used in bags, phone cases, and cabinets.
• Loudspeakers: Use a permanent magnet to create a static magnetic field that interacts with the voice coil (an electromagnet) to produce sound.

• Electric motors: Use electromagnets to create rotating magnetic fields that turn the motor. The electromagnets can be controlled to vary the speed and torque.
• Doorbells: Use an electromagnet to strike a tone bar when the button is pressed.
• Relays: Use an electromagnet to open or close electrical contacts, allowing a small current to control a larger current.
• Lifting cranes: Use powerful electromagnets to lift and transport scrap metal. The electromagnet can be turned off to release the load.
• Magnetic levitation (Maglev) trains: Use electromagnets to levitate the train and propel it forward.
• MRI machines: Use superconducting electromagnets to generate the very strong, uniform magnetic fields required for medical imaging.

The Role of Permanent Magnets in Electric Motors In many electric motors, the stationary part (stator) uses permanent magnets to provide a constant magnetic field. The rotating part (rotor) uses an electromagnet (or vice versa). The interaction between the permanent magnet's field and the electromagnet's field produces the torque that turns the motor. This design is common in small motors (e.g., in toys, fans, and power tools).

The Role of Electromagnets in Electric Motors In larger motors, both the stator and rotor may use electromagnets. This allows the magnetic field strength to be controlled, which is important for varying the speed and torque of the motor. Electromagnets are also used in generators to convert mechanical energy into electrical energy.

Intermediate Level

In physics and engineering, the choice between permanent magnets and electromagnets depends on the application's requirements. Permanent magnets are used when a constant magnetic field is needed without a power source (e.g., compasses, refrigerator magnets, permanent magnet motors). They are made from hard magnetic materials and retain their magnetism.

Electromagnets are used when the magnetic field needs to be controlled (e.g., electric motors, relays, lifting cranes, MRI machines). They are made from soft magnetic materials and require a current to maintain their field. The strength of an electromagnet can be varied by changing the current, and its polarity can be reversed.

In some applications, both types are used together. For example, in a loudspeaker, a permanent magnet provides a constant magnetic field, and an electromagnet (the voice coil) carries the audio signal. The interaction between the two produces sound.

Important Points

• - Permanent Magnets: No power required, constant field, reliable.
• - Electromagnets: Require power, controllable, strong, reversible.
• - Permanent Uses: Compasses, fridge magnets, permanent magnet motors.
• - Electromagnet Uses: Electric motors, relays, cranes, doorbells, MRI.
• - Both used in: Loudspeakers, MRI machines, some motors.

Remember

• - Permanent: No power needed, constant field.
• - Electromagnet: Power needed, controllable field.
• - Permanent Uses: Compass, fridge magnet.
• - Electromagnet Uses: Crane, doorbell, relay, motor.
• - Both: Used together in speakers.

Common Mistakes

• - Thinking permanent magnets are always better: Permanent magnets are not suitable for applications where the magnetic field needs to be turned on/off or varied.
• - Thinking electromagnets are always better: Electromagnets require a continuous power source, which can be a disadvantage in some applications (e.g., if the power fails, the magnetic field is lost).
• - Forgetting that both types are used in some devices: For example, a loudspeaker uses both a permanent magnet and an electromagnet.

Quick Revision

Permanent magnets are used in applications where a constant magnetic field is needed without a power source, such as compasses, refrigerator magnets, and permanent magnet motors. Electromagnets are used in applications where the magnetic field needs to be controlled, such as electric motors, relays, lifting cranes, doorbells, and MRI machines. Both types are used together in devices like loudspeakers and some electric motors.

60-Second Revision

• Uses: Compasses, fridge magnets, permanent magnet motors.
• Uses: Electric motors, relays, cranes, doorbells, MRI.

Frequently Asked Questions (FAQs)

Q1. What are the uses of permanent magnets? Answer: Permanent magnets are used in compasses, refrigerator magnets, loudspeakers (as the static field), permanent magnet motors, magnetic closures, and many other applications where a constant magnetic field is needed without a power source.

Q2. What are the uses of electromagnets? Answer: Electromagnets are used in electric motors, generators, relays, doorbells, lifting cranes, MRI machines, particle accelerators, loudspeakers (voice coil), and many other applications where the magnetic field needs to be controlled.

Q3. Why are electromagnets used in lifting cranes? Answer: Because they can be turned on to lift heavy metal objects and turned off to release them. A permanent magnet would be impractical because it would always be magnetic, making it difficult to release the load.

• Q4. Why are permanent magnets used in compasses?
• Answer: Because they provide a constant magnetic field that aligns with the Earth's magnetic field, and they do not require a power source.

• Q5. What are the advantages of permanent magnets over electromagnets?
• Answer: They do not require a power source, they are simpler and more reliable, and they are often cheaper.

• Q6. What are the advantages of electromagnets over permanent magnets?
• Answer: They can be controlled (turned on/off, strength varied, polarity reversed), and they can produce much stronger magnetic fields.

Q7. Where are both permanent magnets and electromagnets used together? Answer: In loudspeakers (permanent magnet provides the static field, voice coil is an electromagnet) and in some electric motors (permanent magnets in the stator, electromagnet in the rotor, or vice versa). They are also used together in MRI machines (permanent magnets for some designs, superconducting electromagnets for others).

Q8. Why are permanent magnets used in small electric motors? Answer: Because they are simpler and cheaper than electromagnets, and they provide a constant magnetic field without the need for a power source. Small motors (e.g., in toys and small appliances) often use permanent magnets.

Multiple Choice Questions (MCQs)

• Q1. Which of the following is a use of permanent magnets?
• A. Lifting crane
• B. Doorbell
• C. Compass
• D. Relay

• Correct Answer: C
• Explanation: A compass uses a permanent magnet (the compass needle) to align with the Earth's magnetic field.

• Q2. Which of the following is a use of electromagnets?
• A. Refrigerator magnet
• B. Compass
• C. Lifting crane
• D. Bar magnet

• Correct Answer: C
• Explanation: Lifting cranes use powerful electromagnets to lift heavy metal objects.

• Q3. A device that uses an electromagnet to open or close an electrical circuit is called a:
• A. Compass
• B. Relay
• C. Refrigerator magnet
• D. Bar magnet

• Correct Answer: B
• Explanation: A relay uses an electromagnet to open or close electrical contacts.

• Q4. A loudspeaker uses:
• A. Only permanent magnets
• B. Only electromagnets
• C. Both permanent magnets and electromagnets
• D. Neither permanent magnets nor electromagnets

• Correct Answer: C
• Explanation: A loudspeaker uses a permanent magnet to provide a static magnetic field and an electromagnet (voice coil) to produce sound.

• Q5. Which of the following is NOT an application of permanent magnets?
• A. Compass needle
• B. Refrigerator magnet
• C. Lifting crane
• D. Magnetic closure on a bag

• Correct Answer: C
• Explanation: A lifting crane uses an electromagnet, not a permanent magnet (because it needs to be turned on/off).

• Q6. Which of the following is NOT an application of electromagnets?
• A. Electric motor
• B. Doorbell
• C. Compass needle
• D. Relay

• Correct Answer: C
• Explanation: A compass needle is a permanent magnet, not an electromagnet.

• Q7. Permanent magnets are used in applications where:
• A. The magnetic field needs to be turned on/off
• B. The magnetic field needs to be constant without a power source
• C. The magnetic field needs to be reversed frequently
• D. The magnetic field needs to be very strong

• Correct Answer: B
• Explanation: Permanent magnets are used where a constant magnetic field is needed without a power source.

• Q8. Electromagnets are used in applications where:
• A. A constant magnetic field is needed
• B. No power source is available
• C. The magnetic field needs to be controlled
• D. The magnet needs to be very light

• Correct Answer: C
• Explanation: Electromagnets are used when the magnetic field needs to be controlled (turned on/off, varied in strength, etc.).

• Q9. The main advantage of permanent magnets over electromagnets is that they:
• A. Are stronger
• B. Require no power source
• C. Can be controlled
• D. Can be reversed

• Correct Answer: B
• Explanation: Permanent magnets do not require a power source to maintain their magnetic field.

• Q10. The main advantage of electromagnets over permanent magnets is that they:
• A. Are simpler
• B. Are cheaper
• C. Can be controlled (on/off, strength, polarity)
• D. Are lighter

• Correct Answer: C
• Explanation: Electromagnets can be controlled (turned on/off, strength varied, polarity reversed), which is their main advantage.

Frequently Asked Questions (FAQs)

Q1. What is the difference between a permanent magnet and an electromagnet? Answer: A permanent magnet retains its magnetism without a power source and has a fixed strength. An electromagnet requires a current to maintain its magnetic field, and its strength and polarity can be controlled.

• Q2. Why are permanent magnets used in compasses?
• Answer: Because they provide a constant magnetic field that aligns with the Earth's magnetic field, and they do not require a power source.

Q3. Why are electromagnets used in lifting cranes? Answer: Because they can be turned on to lift heavy metal objects and turned off to release them. A permanent magnet would always be magnetic, making it difficult to release the load.

• Q4. What is a relay?
• Answer: A relay is an electrical switch that uses an electromagnet to open or close contacts. It allows a small current to control a larger current.

• Q5. How is an electromagnet used in a doorbell?
• Answer: When the button is pressed, current flows through an electromagnet, which attracts a metal armature and strikes a tone bar, producing the bell sound.

Q6. What is the advantage of using permanent magnets in small motors? Answer: Permanent magnets are simpler and cheaper than electromagnets, and they provide a constant magnetic field without the need for a power source. This makes them ideal for small motors used in toys, fans, and small appliances.

Q7. How are permanent magnets and electromagnets used together in a loudspeaker? Answer: In a loudspeaker, a permanent magnet provides a constant magnetic field in the voice coil gap. The voice coil (an electromagnet) carries the audio signal, and its interaction with the permanent magnet's field causes the coil to move, producing sound.

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