8.3 Induced Magnetism

Have you ever noticed that a paperclip can become temporarily magnetic when you touch it with a magnet? Or that a nail can pick up other nails after being stroked with a magnet? This phenomenon is called induced magnetism. It occurs when a material that is not normally a magnet becomes magnetized by the influence of an external magnetic field. Understanding induced magnetism is essential for understanding how magnets interact with magnetic materials and how electromagnets work.

Definition

Induced Magnetism is the phenomenon in which a material becomes magnetized when it is placed in an external magnetic field. The material is called an induced magnet (or temporary magnet). The text states: "A material that becomes a magnet when it is placed in a magnetic field is called induced magnet."

Main Idea

The main idea is that magnetic materials can become temporary magnets when exposed to an external magnetic field. The external field causes the domains within the material to align, creating a net magnetic field in the material. This induced magnetism disappears (or is greatly reduced) when the external field is removed, which is why these are called temporary magnets. Induced magnetism is the basis for electromagnets, magnetic recording, and many other technologies.

Core Concept

What is Induced Magnetism? The text states: "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."

• Induced Magnet: A material that becomes magnetized in the presence of an external magnetic field.
• Temporary Magnet: The induced magnetism usually disappears when the external field is removed.
• Magnetic Material: A material that can be magnetized (e.g., iron, nickel, cobalt).

• The external magnetic field from the magnet penetrates the material.
• The domains in the material (which were randomly oriented) begin to align with the external field.
• The material becomes magnetized, with a North pole and a South pole.
• The induced North pole is attracted to the South pole of the magnet (and vice versa), causing the material to be attracted to the magnet.

Methods of Induced Magnetism The text lists several methods for inducing magnetism in a material:

1. Stroking Method: The text states: "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."

• How it works: A permanent magnet is stroked along a bar of magnetic material in one direction. The magnetic field of the permanent magnet aligns the domains in the bar, magnetizing it.
• Result: The end of the bar where the magnet is lifted becomes the opposite pole to the magnet's pole used.

2. Hammering Method: The text states: "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."

• How it works: A magnetic material is placed in a magnetic field and hammered. The hammering helps the domains align by providing energy to overcome the resistance to domain wall movement.
• Result: The material becomes magnetized in the direction of the external field.

3. Heating Method: The text states: "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."

• How it works: Heating a magnetic material increases the thermal energy of the atoms, which randomizes the domain orientations.
• Result: The material loses its magnetization (demagnetization). However, there is also a phenomenon called "thermo-magnetism" or the "magnetic Seebeck effect" where heating can generate a magnetic field in certain materials.

4. Solenoid (Electromagnet) Method: The text states: "SOLENOID: 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."

• How it works: A coil of wire (solenoid) is wrapped around a magnetic material. When a current flows through the coil, it creates a magnetic field that aligns the domains in the material, magnetizing it.
• Result: The material becomes an electromagnet as long as the current flows. When the current stops, the material loses its magnetism (temporary magnet).

The Solenoid and Electromagnets The text states: "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."

• Electromagnet: A temporary magnet created by passing a current through a coil of wire wrapped around a magnetic material.
• DC Electromagnet: With direct current, the polarity is constant, so the material is magnetized in one direction.
• AC Electromagnet: With alternating current, the polarity alternates, causing the material to be magnetized and demagnetized repeatedly, resulting in no net magnetization.

Soft vs. Hard Magnetic Materials The text states: "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)."

• Soft Magnetic Materials: Easily magnetized and demagnetized (e.g., soft iron). Used in electromagnets, transformers, and magnetic shielding.
• Hard Magnetic Materials: Difficult to magnetize but retain their magnetism (e.g., steel). Used in permanent magnets.

Detailed Explanation

Induced magnetism is a fundamental concept in magnetism. It explains how magnetic materials interact with magnets and how electromagnets work.

The Domain Theory and Induced Magnetism Induced magnetism is explained by the domain theory. In an unmagnetized material, the domains are randomly oriented. When an external magnetic field is applied, the domains tend to align with the field. This alignment creates a net magnetic field in the material. When the external field is removed, the domains may return to their random orientation (for soft magnetic materials) or remain partially aligned (for hard magnetic materials).

The Solenoid A solenoid is a coil of wire that creates a magnetic field when a current flows through it. The magnetic field of a solenoid is similar to that of a bar magnet. When a magnetic material is placed inside the solenoid, its domains align with the solenoid's field, magnetizing the material. This is the principle of an electromagnet.

• B is the magnetic field.
• μ₀ is the permeability of free space.
• n is the number of turns per unit length.
• I is the current.

• Electromagnets: Used in electric motors, generators, relays, doorbells, and cranes.
• Magnetic Recording: Used in hard drives and magnetic tapes, where the read/write head induces magnetism in the recording medium.
• Transformers: Use soft iron cores that are magnetized and demagnetized by alternating current.
• Electric Motors and Generators: Use induced magnetism to convert between electrical and mechanical energy.
• Magnetic Shielding: Uses soft magnetic materials to redirect magnetic fields.

Intermediate Level

In physics, induced magnetism occurs when a magnetic material is placed in an external magnetic field. The field causes the domains in the material to align, creating a net magnetization. The material becomes a temporary magnet. When the external field is removed, the domains may return to their random orientation (for soft magnetic materials) or remain partially aligned (for hard magnetic materials).

The most common method of inducing magnetism is using a solenoid—a coil of wire wrapped around a magnetic material. When a current flows through the coil, it creates a magnetic field that magnetizes the material. This is the principle of an electromagnet. The strength of the electromagnet can be controlled by adjusting the current.

Other methods include stroking with a permanent magnet and hammering in a magnetic field. Heating a magnet randomizes the domains and demagnetizes it.

Important Points

• - Induced Magnetism: A material becomes magnetized in an external magnetic field.
• - Induced Magnet: A temporary magnet created by an external field.
• - Methods: Stroking, hammering, solenoid (electromagnet).
• - Solenoid: A coil of wire that creates a magnetic field when current flows.
• - Electromagnet: A temporary magnet controlled by current.
• - Soft Magnetic Materials: Easily magnetized and demagnetized (e.g., soft iron).
• - Hard Magnetic Materials: Difficult to magnetize but retain magnetism (e.g., steel).

Remember

• - Induced Magnetism: Magnetization by an external field.
• - Induced Magnet: Temporary magnet.
• - Methods: Stroking, hammering, solenoid.
• - Electromagnet: Solenoid with a magnetic core.
• - Soft Iron: Easily magnetized/demagnetized.
• - Steel: Hard to magnetize, retains magnetism.

Common Mistakes

• - Thinking induced magnetism is permanent: Induced magnetism is usually temporary; the material loses its magnetism when the external field is removed (unless it is a hard magnetic material).
• - Thinking any material can be strongly magnetized: Only ferromagnetic materials (iron, nickel, cobalt) can be strongly magnetized. Other materials have much weaker magnetic effects.
• - Forgetting that a solenoid creates a magnetic field: A solenoid (coil of wire) creates a magnetic field when current flows through it. This is the basis of electromagnets.

Quick Revision

Induced magnetism is the phenomenon in which a magnetic material becomes magnetized when placed in an external magnetic field. The material becomes a temporary magnet. This can be done by stroking with a permanent magnet, hammering in a magnetic field, or placing the material inside a solenoid (electromagnet). Soft magnetic materials (like soft iron) are easily magnetized and demagnetized, while hard magnetic materials (like steel) retain their magnetism.

60-Second Revision

• Induced Magnet: Temporary magnet.
• Methods: Stroking, hammering, solenoid.
• Electromagnet: Solenoid with core.
• Soft Iron: Easy magnetize/demagnetize.
• Steel: Hard magnetize, retains.

Frequently Asked Questions (FAQs)

Q1. What is induced magnetism? Answer: Induced magnetism is the phenomenon in which a magnetic material becomes magnetized when it is placed in an external magnetic field. The material becomes a temporary magnet.

• Q2. What is an induced magnet?
• Answer: An induced magnet is a material that becomes a magnet when placed in a magnetic field. It is a temporary magnet.

• Stroking: Stroking the material with a permanent magnet.
• Hammering: Hammering the material while it is in a magnetic field.
• Solenoid: Placing the material inside a coil of wire (solenoid) and passing a current through it.

Q4. How does a solenoid induce magnetism? Answer: A solenoid is a coil of wire. When a current flows through it, it creates a magnetic field. This field aligns the domains in a magnetic material placed inside the coil, magnetizing it.

Q5. What is an electromagnet? Answer: An electromagnet is a temporary magnet created by passing a current through a coil of wire wrapped around a magnetic material (usually soft iron). It acts as a magnet only as long as the current flows.

Q6. What is the difference between soft and hard magnetic materials? Answer: Soft magnetic materials (like soft iron) are easily magnetized and demagnetized. Hard magnetic materials (like steel) are difficult to magnetize but retain their magnetism once magnetized.

Q7. Why does heating a magnet demagnetize it? Answer: Heating increases the thermal energy of the atoms, causing them to vibrate more vigorously. This randomizes the orientation of the domains, reducing the net magnetization.

Multiple Choice Questions (MCQs)

• Q1. A material that becomes a magnet when placed in a magnetic field is called:
• A. A permanent magnet
• B. An induced magnet
• C. A diamagnetic material
• D. A non-magnetic material

• Correct Answer: B
• Explanation: The text defines an induced magnet as a material that becomes a magnet when placed in a magnetic field.

• Q2. The phenomenon of a material becoming magnetized in an external field is called:
• A. Electromagnetic induction
• B. Induced magnetism
• C. Magnetic shielding
• D. Ferromagnetism

• Correct Answer: B
• Explanation: The text states that this phenomenon is called induced magnetism.

• Q3. Which of the following is a method of inducing magnetism?
• A. Cooling the material
• B. Stroking with a permanent magnet
• C. Breaking the material
• D. Dissolving the material

• Correct Answer: B
• Explanation: The text describes the stroking method for inducing magnetism.

• Q4. A solenoid is:
• A. A permanent magnet
• B. A coil of wire
• C. A type of magnetic material
• D. A device for measuring magnetism

• Correct Answer: B
• Explanation: The text defines a solenoid as a coil of wire wrapped around a cylindrical core.

• Q5. The magnetic field of a solenoid is similar to the field of a:
• A. Bar magnet
• B. Current-carrying wire
• C. Single pole
• D. Uniform field

• Correct Answer: A
• Explanation: The text states that the magnetic field of the solenoid resembles the field of a bar magnet.

• Q6. An electromagnet is a:
• A. Permanent magnet
• B. Temporary magnet
• C. Non-magnetic material
• D. Type of solenoid

• Correct Answer: B
• Explanation: An electromagnet is a temporary magnet that acts as a magnet only while current flows.

• Q7. Materials that easily magnetize and demagnetize are called:
• A. Hard magnetic materials
• B. Soft magnetic materials
• C. Non-magnetic materials
• D. Permanent magnets

• Correct Answer: B
• Explanation: The text states that materials which easily magnetize and demagnetize are called soft magnetic materials.

• Q8. Which of the following is a hard magnetic material?
• A. Soft iron
• B. Steel
• C. Copper
• D. Aluminum

• Correct Answer: B
• Explanation: The text states that steel is a hard magnetic material (it cannot be easily magnetized and demagnetized).

• Q9. Heating a magnet:
• A. Increases its magnetism
• B. Demagnetizes it
• C. Has no effect
• D. Reverses its poles

• Correct Answer: B
• Explanation: The text states that heat speeds up the movements of already aligned domains, causing misalignment and demagnetization.

• Q10. The magnetic field strength of a solenoid can be increased by:
• A. Decreasing the current
• B. Increasing the number of turns
• C. Using a non-magnetic core
• D. Decreasing the length of the coil

Correct Answer: B Explanation: The magnetic field of a solenoid is proportional to the number of turns per unit length (B = μ₀ n I). Increasing the number of turns increases the field.

Frequently Asked Questions (FAQs)

Q1. What is the difference between induced magnetism and permanent magnetism? Answer: Induced magnetism is temporary; the material becomes magnetized only while in an external field. Permanent magnetism is retained by the material after the external field is removed (e.g., in a permanent magnet like a bar magnet).

Q2. How does the stroking method work? Answer: A permanent magnet is stroked along a bar of magnetic material in one direction. The magnetic field of the magnet aligns the domains in the bar, magnetizing it. The end where the magnet is lifted becomes the opposite pole to the magnet's pole used.

Q3. Why are soft magnetic materials used in electromagnets? Answer: Soft magnetic materials (like soft iron) are easily magnetized and demagnetized. This makes them ideal for electromagnets, which need to be turned on and off quickly.

• Q4. Why are hard magnetic materials used in permanent magnets?
• Answer: Hard magnetic materials (like steel) are difficult to magnetize but retain their magnetism once magnetized. This makes them ideal for permanent magnets.

Q5. What is the difference between a solenoid and an electromagnet? 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.

• Increasing the current.
• Increasing the number of turns in the coil.
• Using a soft iron core (which has high permeability).
• Decreasing the length of the coil (increasing turns per unit length).

Q7. What is the magnetic Seebeck effect? Answer: The magnetic Seebeck effect (or thermo-magnetism) is the phenomenon where a temperature gradient in a magnetic material generates a magnetic field. It is a relatively new area of research.

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