9th Physics Federal Board Notes: Unit 8 Magnetism
📋 Table of Contents
- 1. Multiple Choice Questions
- 2. Short Response Questions
- 2.1 Magnetic Field Lines Intersection
- 2.2 Freely Suspended Magnet Direction
- 2.3 Neutral Zone of Magnetic Field
- 2.4 Materials Without Magnetic Behavior
- 2.5 Proton Effect in Magnetism
- 2.6 Geomagnetic Reversal
- 2.7 Earth's Rotation Axis
- 2.8 Geographical vs Magnetic Axis
- 2.9 Paramagnetic vs Ferromagnetic Materials
- 2.10 Electromagnet Strength Factors
- 2.11 Magnetic Field Lines of Solenoids
- 3. Long Response Questions
- 3.1 Definition and Explanation of Magnetism
- 3.2 Domain Theory of Magnetism
- 3.3 Magnetic Field Strength and Shielding
- 3.4 Bar Magnet vs Solenoid Magnetic Fields
- 3.5 Induced Magnetism
- 3.6 Permanent vs Temporary Magnets
- 3.7 Uses of Electromagnets and Temporary Magnets
- 3.8 Three Types of Magnetic Materials
- 3.9 Earth's Magnetic Field
- 3.10 Animal Navigation Using Earth's Magnetic Field
🧲 Introduction to Unit 8: Magnetism
Unit 8: Magnetism explores the fundamental principles of magnetic fields, magnetic materials, and electromagnetic phenomena. This unit helps students understand how magnets work, the Earth's magnetic field, and practical applications of magnetism in technology and daily life. You'll learn about different types of magnetic materials, how to create magnets, and how animals use Earth's magnetic field for navigation.
Multiple Choice Questions
When a bar magnet is cut in half, each piece becomes a smaller magnet with its own north and south poles. The magnetic strength decreases because the magnetic domains are distributed across smaller volumes.
Placing a material inside a current-carrying coil (solenoid) is the quickest method to magnetize it because the strong, uniform magnetic field inside the solenoid rapidly aligns the magnetic domains in the material.
Earth's magnetic field intensity varies from place to place. It is strongest near the magnetic poles and weakest at the magnetic equator. Local geological features and the Earth's internal structure also cause variations in magnetic field strength.
The Earth's magnetic field is generated by the motion of molten iron and other conductive materials in the Earth's outer core. This creates electric currents through the dynamo effect, which in turn produces the planet's magnetic field.
Steel is the best material for making permanent magnets because it has high retentivity and coercivity. Once magnetized, steel retains its magnetism for a long time, making it ideal for permanent magnets used in various applications.
Soft iron is the best material for making electromagnets because it has high permeability and low retentivity. It quickly magnetizes when current flows through the coil and demagnetizes almost immediately when the current stops, which is ideal for electromagnets.
Soft iron is the best material for magnetic shielding because it has high permeability, which means it can redirect magnetic field lines around the shielded area, protecting sensitive materials from external magnetic fields.
Magnetic field lines are continuous closed loops that emerge from the north pole and enter the south pole of a magnet, then continue through the magnet back to the north pole, forming complete circuits.
When two parallel wires carry current in the same direction, their magnetic fields interact in such a way that they attract each other. This is due to the magnetic force between current-carrying conductors described by Ampère's force law.
Nickel is a ferromagnetic material, along with iron and cobalt. Ferromagnetic materials have strong magnetic properties and can be permanently magnetized. Silver, copper, and aluminum are not ferromagnetic.
Short Response Questions
1. Can two magnetic field lines intersect each other? Justify your answer.
2. A freely suspended magnet always points along north-south direction. Why?
3. What is the neutral zone or field free region of the magnetic field?
4. Is there any material which does not have any magnetic behavior? Justify your answer.
5. A proton is also a charged particle and spins like an electron. Why its effect is neglected in study of magnetism?
6. What is the geomagnetic reversal phenomenon? Explain.
7. Why the Earth spins about its geographical axis instead of its magnetic axis? Explain.
8. Why the Earth's geographical and magnetic axis are not coincident? Explain.
Earth's Geographical and Magnetic Axes
The magnetic axis is tilted at approximately 11.3° from the geographical axis
9. What is the difference between paramagnetic and ferromagnetic materials?
| Parameter | Paramagnetic Materials | Ferromagnetic Materials |
|---|---|---|
| Magnetic Behavior | Weakly attracted to magnetic fields | Strongly attracted to magnetic fields |
| Magnetic Moment | Small net magnetic moment per atom | Large net magnetic moment per atom |
| Retention of Magnetism | Do not retain magnetism after external field is removed | Can retain magnetism and become permanent magnets |
| Domain Alignment | Partial alignment in external field | Complete alignment in external field |
| Examples | Aluminum, platinum, oxygen | Iron, nickel, cobalt, steel |
| Temperature Effect | Magnetism decreases with temperature | Lose ferromagnetism above Curie temperature |
10. At what factors the strength of the magnetic field of an electromagnet depends?
- Number of Turns in the Coil: More turns of wire in the coil increase the magnetic field strength proportionally.
- Electric Current: Higher current through the coil produces a stronger magnetic field, with field strength directly proportional to current.
- Core Material: Using a ferromagnetic core (like soft iron) significantly enhances the magnetic field compared to an air core.
- Core Geometry: The shape and size of the core affect how the magnetic field is concentrated and distributed.
- Coil Diameter: Smaller diameter coils tend to produce stronger magnetic fields for the same current and number of turns.
11. Why the magnetic field lines of a solenoid resemble with that of a bar magnet?
- Both have two distinct magnetic poles (north and south)
- The magnetic field lines emerge from one end (north pole) and enter the other end (south pole)
- The field lines form closed loops outside the magnet/solenoid
- The field is strongest near the poles in both cases
Long Response Questions
1. Define and explain the term magnetism.
Magnetism is a physical phenomenon produced by the motion of electric charge, resulting in attractive and repulsive forces between objects. It is a fundamental force of nature that manifests through magnetic fields.
Key aspects of magnetism:
- Magnetic Poles: Every magnet has two poles - north and south. Like poles repel each other, while unlike poles attract.
- Magnetic Field: The region around a magnet where its magnetic influence can be detected. It is represented by magnetic field lines.
- Magnetic Materials: Substances that are attracted to magnets or can be magnetized themselves, such as iron, nickel, and cobalt.
- Sources of Magnetism: Arises from the motion of electric charges, particularly the spin and orbital motion of electrons in atoms.
Types of magnetism include:
- Ferromagnetism: Strong attraction to magnetic fields (iron, nickel, cobalt)
- Paramagnetism: Weak attraction to magnetic fields (aluminum, platinum)
- Diamagnetism: Weak repulsion from magnetic fields (bismuth, copper)
Magnetism has numerous applications in daily life, including electric motors, generators, magnetic storage devices, medical imaging (MRI), and compasses for navigation.
2. State and explain the domain theory of magnetism.
The Domain Theory of Magnetism explains how ferromagnetic materials become magnetized based on the behavior of tiny regions called magnetic domains.
Key principles of the domain theory:
- Magnetic Domains: In unmagnetized ferromagnetic materials, the material is divided into many small regions called domains. Each domain contains billions of atoms with magnetic moments aligned in the same direction.
- Random Orientation: In an unmagnetized state, these domains are randomly oriented, so their magnetic effects cancel out, and the material shows no net magnetism.
- Domain Alignment: When an external magnetic field is applied, domains aligned with the field grow at the expense of others, and the boundaries between domains shift.
- Saturation: At high field strengths, all domains become aligned in the direction of the applied field, and the material reaches magnetic saturation.
- Retention: In soft magnetic materials, domains return to random orientation when the field is removed. In hard magnetic materials, domains remain aligned, creating permanent magnets.
Magnetic Domain Alignment
Unmagnetized (random domains) → Partially magnetized → Fully magnetized (all domains aligned)
The domain theory successfully explains various magnetic phenomena, including why heating or hammering can demagnetize a magnet (by randomizing domain alignment) and why there's a maximum strength beyond which a magnet cannot be further magnetized (saturation).
3. Define magnetic field and magnetic field strength. How can we shield a sensitive instrument from external magnetic field?
Magnetic Field: A magnetic field is the region around a magnet or current-carrying conductor where magnetic forces can be detected. It is a vector field, meaning it has both magnitude and direction at each point in space. Magnetic fields are represented by field lines, with the density of lines indicating field strength and their direction showing the field orientation.
Magnetic Field Strength (H): Also called magnetic intensity, it is a measure of the intensity of a magnetic field. It is defined as the force experienced by a unit north pole placed at that point in the magnetic field. The SI unit of magnetic field strength is amperes per meter (A/m).
Magnetic Flux Density (B): Related to field strength but also depends on the medium. B = μH, where μ is the permeability of the medium. The SI unit is tesla (T).
Shielding Sensitive Instruments from External Magnetic Fields:
To protect sensitive instruments from external magnetic fields, we use magnetic shielding based on the principle of magnetic permeability:
- Material Selection: Use materials with high magnetic permeability, such as soft iron or mu-metal. These materials provide a preferred path for magnetic field lines.
- Enclosure Design: Place the sensitive instrument inside a container made of the shielding material. The external magnetic field lines are drawn into the shield material and directed around the enclosed space.
- Multiple Layers: For very sensitive applications, multiple layers of shielding with different materials may be used to provide enhanced protection.
- Complete Enclosure: The shield should completely surround the instrument to be effective, as magnetic fields can penetrate through gaps.
This method works because high-permeability materials have much lower reluctance to magnetic flux than air, so the field lines preferentially travel through the shield rather than through the protected space inside.
4. Compare the magnetic field of a bar magnet and a solenoid.
| Aspect | Bar Magnet | Solenoid |
|---|---|---|
| Source of Field | Alignment of magnetic domains in ferromagnetic material | Electric current flowing through coiled wire |
| Field Pattern | Dipole field with clearly defined north and south poles | Similar dipole field when current flows; can be turned on/off |
| Field Strength Control | Fixed strength for permanent magnets | Adjustable by changing current or number of turns |
| Internal Field | Non-uniform field inside the magnet | Nearly uniform field inside the solenoid |
| Poles | Permanent north and south poles | Temporary poles that reverse with current direction |
| Demagnetization | Requires strong opposing field, heat, or mechanical shock | Instantaneous when current is switched off |
| Applications | Compass needles, refrigerator magnets, speakers | Electromagnets, relays, inductors, MRI machines |
5. What do you know about induced magnetism? Explain.
Induced magnetism is the process by which a magnetic material becomes magnetized when placed in an external magnetic field, without direct contact with a permanent magnet. The material develops magnetic poles and exhibits magnetic properties as long as it remains in the field.
Key characteristics of induced magnetism:
- Temporary Nature: For most materials (like soft iron), the induced magnetism disappears when the external field is removed.
- Polarity: The end of the material closer to the magnet's north pole becomes a south pole, and vice versa, following the principle that unlike poles attract.
- Strength Dependency: The strength of induced magnetism depends on the strength of the external field and the magnetic properties of the material.
Mechanism of induced magnetism:
According to the domain theory, when a magnetic material is placed in an external magnetic field:
- Domains aligned with the external field grow in size
- Domain boundaries shift to favor alignment with the field
- Domains may rotate to align with the field direction
- This creates a net magnetic moment in the material
Examples of induced magnetism:
- A paperclip temporarily sticking to a magnet
- Iron filings aligning with a magnetic field
- Transformer cores magnetizing in response to alternating current
Induced magnetism is fundamental to many technologies, including electromagnets, electric motors, generators, and magnetic recording devices.
6. Differentiate between permanent and temporary magnets.
| Parameter | Permanent Magnets | Temporary Magnets |
|---|---|---|
| Definition | Magnets that retain their magnetism for a long time after being magnetized | Magnets that lose their magnetism soon after the external magnetic field is removed |
| Materials | Hard magnetic materials like steel, alnico, neodymium | Soft magnetic materials like soft iron, silicon steel |
| Retentivity | High - ability to retain magnetism | Low - quickly lose magnetism |
| Coercivity | High - resistant to demagnetization | Low - easily demagnetized |
| Domain Behavior | Domains remain aligned after external field is removed | Domains return to random orientation when field is removed |
| Applications | Compass needles, speakers, refrigerator magnets, electric motors | Electromagnets, transformer cores, magnetic relays |
| Demagnetization | Requires strong opposing field, heating, or mechanical shock | Occurs automatically when external field is removed |
7. Write down the uses of electromagnets and temporary magnets.
Uses of Electromagnets:
- Electric Motors and Generators: Convert electrical energy to mechanical energy and vice versa
- Transformers: Transfer electrical energy between circuits through electromagnetic induction
- Magnetic Relays: Control circuits using low-power signals to switch higher-power circuits
- Magnetic Lifting: In scrapyards to lift and move heavy ferromagnetic materials
- Magnetic Resonance Imaging (MRI): Medical imaging technology that uses powerful electromagnets
- Speakers and Headphones: Convert electrical signals to sound waves
- Magnetic Separators: Separate magnetic materials from non-magnetic ones in industries
- Circuit Breakers: Protect electrical circuits from overload
- Doorbells: Create the ringing sound when activated
- Particle Accelerators: Guide and focus charged particles in scientific research
Uses of Temporary Magnets:
- Transformer Cores: Essential for efficient energy transfer in transformers
- Electromagnet Cores: Enhance the magnetic field of electromagnets
- Magnetic Shielding: Protect sensitive instruments from external magnetic fields
- Induction Heating: Used in cooking appliances like induction cooktops
- Magnetic Switches: Reed switches in security systems and sensors
- Temporary Magnetic Holders: In workshops and manufacturing for holding tools or materials temporarily
- Magnetic Couplings: Transfer torque without physical contact in pumps and other machinery
8. Write a note on three types of magnetic materials.
Magnetic materials are classified into three main types based on their response to external magnetic fields:
1. Ferromagnetic Materials
Characteristics: Exhibit strong attraction to magnetic fields, can be permanently magnetized, have high permeability.
Behavior: In an external field, domains align strongly with the field. They retain some magnetization after the field is removed.
Examples: Iron, nickel, cobalt, and their alloys (steel, alnico).
Applications: Permanent magnets, transformer cores, magnetic recording media.
2. Paramagnetic Materials
Characteristics: Weakly attracted to magnetic fields, have small positive susceptibility.
Behavior: Partial alignment of atomic magnetic moments with the external field. Lose magnetism immediately when field is removed.
Examples: Aluminum, platinum, oxygen, tungsten.
Applications: Scientific research, magnetic separation of materials.
3. Diamagnetic Materials
Characteristics: Weakly repelled by magnetic fields, have small negative susceptibility.
Behavior: Develop induced magnetic moment opposite to the applied field. All materials exhibit diamagnetism, but it's overshadowed in para- and ferromagnetic materials.
Examples: Bismuth, copper, silver, gold, water, most organic compounds.
Applications: Magnetic levitation (e.g., superconductors), scientific instruments.
Additional Note: There are also antiferromagnetic and ferrimagnetic materials, which are more complex types with alternating magnetic moments that partially cancel each other out.
9. Write a note on Earth's magnetic field.
The Earth's magnetic field, also known as the geomagnetic field, is a magnetic field that extends from the Earth's interior out into space, where it interacts with the solar wind.
Key characteristics of Earth's magnetic field:
- Dipole Nature: The field resembles that of a giant bar magnet tilted at about 11.3° to Earth's rotational axis.
- Magnetic Poles: The magnetic north pole is currently in northern Canada, while the magnetic south pole is in Antarctica. These are different from the geographic poles.
- Field Strength: Varies from about 25 to 65 microteslas (0.25 to 0.65 gauss) at the Earth's surface.
- Magnetosphere: The region of space dominated by Earth's magnetic field, extending tens of thousands of kilometers into space.
Origin of Earth's magnetic field:
The geomagnetic field is generated by electric currents in the Earth's outer core, which is composed of molten iron and nickel. This self-sustaining dynamo effect is known as the geodynamo.
Importance of Earth's magnetic field:
- Protection: Shields the Earth from harmful solar radiation and cosmic rays by deflecting charged particles.
- Navigation: Enables compasses to work for direction finding.
- Animal Migration: Many animals, including birds, sea turtles, and some insects, use the magnetic field for navigation.
- Auroras: Creates the beautiful aurora borealis and aurora australis when solar particles interact with the atmosphere near the poles.
Magnetic Field Variations:
- Secular Variation: The magnetic poles wander over time.
- Magnetic Reversals: The north and south magnetic poles periodically swap places, with the last reversal occurring about 780,000 years ago.
- Magnetic Anomalies: Local variations in the magnetic field caused by geological features.
10. How do animals use Earth's magnetic field for navigation?
Many animal species have evolved the ability to detect Earth's magnetic field and use it for navigation, a phenomenon known as magnetoreception. This ability helps them with migration, foraging, and orientation.
Animals known to use magnetoreception:
- Birds: Many migratory birds, such as European robins and homing pigeons, use the magnetic field for long-distance navigation. They can detect both the inclination (angle) and intensity of the magnetic field.
- Sea Turtles: Loggerhead and leatherback sea turtles use magnetic cues during their transoceanic migrations and to return to their natal beaches for nesting.
- Fish: Some species of salmon and sharks can detect magnetic fields. Salmon use magnetic maps to navigate back to their birth rivers.
- Insects: Certain bees and monarch butterflies use magnetic sensing. Honeybees use it for orientation during foraging flights.
- Mammals: Bats, whales, and some rodents show evidence of magnetoreception. Bats use it for long-distance migration.
Proposed mechanisms for magnetoreception:
- Magnetite-based Detection: Some animals have tiny crystals of magnetite (a magnetic mineral) in their bodies that act like a compass needle, aligning with Earth's magnetic field.
- Chemical Compass: A light-dependent mechanism involving specialized molecules (cryptochromes) that undergo chemical changes influenced by magnetic fields.
- Electromagnetic Induction: In aquatic animals like sharks, specialized organs (Ampullae of Lorenzini) can detect weak electric fields induced by movement through Earth's magnetic field.
How animals use magnetic information:
- Compass Sense: Determining direction relative to magnetic north-south.
- Map Sense: Using variations in magnetic field intensity and inclination to determine position.
- Altitude/Latitude Detection: Using magnetic field variations that change with location.
This remarkable ability allows animals to navigate accurately over thousands of kilometers during migration, often with precision that rivals human navigation technology.
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