Chapter 8 : Magnetism

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Chapter Overview & SLOs

What is magnetism? Chapter 8, "Magnetism," explores the fundamental force arising from moving electric charges and its diverse applications. This chapter answers fundamental questions about how magnetic materials work and how magnetism is used in technology. What is the domain theory of magnetism? You will learn the Domain Theory of Magnetism, which provides a microscopic explanation for how magnetic materials exhibit macroscopic magnetism: - Magnetic domains: Microscopic regions within a magnetic material where all atomic magnetic moments are aligned in the same direction. - Unmagnetized state: Domains are randomly oriented, canceling each other's magnetic effects. - Magnetized state: Domains align in the same direction, producing a net magnetic field. - Domain wall movement: When an external magnetic field is applied, domain walls move, causing domains aligned with the field to grow at the expense of others. How are materials magnetized and demagnetized? You will study various methods: - Magnetization methods: - Stroking method: Repeatedly stroking a magnetic material with a permanent magnet in one direction aligns domains. - Electrical method (using solenoid): Placing the material inside a solenoid (coil of wire) and passing direct current (DC) through the coil magnetizes the material. - Induction method: Bringing a magnetic material close to a strong magnet induces magnetism. - Demagnetization methods: - Hammering: Physical shock disrupts domain alignment. - Heating (above Curie temperature): Thermal energy causes domains to randomize. Curie temperature for iron = 770°C. - Alternating current (AC) demagnetization: Subjecting the material to a decreasing AC field gradually randomizes domains. What are the types of magnetic materials? You will learn to categorize materials based on their magnetic susceptibility and atomic structure: - Ferromagnetic Materials: Strongly attracted to magnets. Have permanent magnetic moments that align easily. Examples: Iron (Fe), Nickel (Ni), Cobalt (Co), Gadolinium (Gd), their alloys (steel, alnico). Used for permanent magnets and electromagnet cores. - Paramagnetic Materials: Weakly attracted to magnets. Have unpaired electrons but thermal energy randomizes alignment. Examples: Aluminum (Al), Platinum (Pt), Manganese (Mn), liquid oxygen. - Diamagnetic Materials: Weakly repelled by magnets. Have all electrons paired, creating no permanent magnetic moment. Examples: Bismuth (Bi), Water (H₂O), Copper (Cu), Gold (Au), Silver (Ag), Carbon (Graphite), Wood, Plastic. What is the difference between permanent magnets and electromagnets? You will compare: - Permanent Magnets: Made from ferromagnetic materials (steel, alnico, neodymium). Magnetic field is always present. Cannot be turned off. Used in compasses, speakers, refrigerator magnets. - Electromagnets (Temporary Magnets): Magnetic field produced by electric current flowing through a coil. Can be turned on/off by switching current. Strength can be varied by changing current or number of turns. Made with soft iron core (easy to magnetize and demagnetize). Used in electric motors, generators, relays, MRI machines, Maglev trains. What is the magnetic field of a solenoid? The notes detail the construction and physics of solenoids (long coil of wire with many turns): - Magnetic field inside a solenoid: Uniform and strong, similar to a bar magnet's field. - Formula: $B = μ₀ × n × I$ (Magnetic field = permeability of free space × number of turns per unit length × current) - $μ₀ = 4π × 10^{-7} \text{ Tm/A}$ (permeability of free space/air) - $n = \frac{N}{L}$ (number of turns divided by length of solenoid) - $I$ = current in amperes (A) - Direction: Determined by right-hand rule (grasp coil with fingers in direction of current, thumb points to north pole) What factors affect the strength of an electromagnet? You will learn: - Number of turns (N): More turns = stronger magnetic field (B ∝ N) - Current (I): Higher current = stronger magnetic field (B ∝ I) - Presence of soft iron core: Soft iron core increases magnetic field by 100-1000 times (higher permeability, easy to magnetize/demagnetize) - Length of solenoid (L): Shorter solenoid with same number of turns = higher n = stronger field What is Earth's magnetic field (geomagnetic field)? A significant focus is placed on Earth's magnetic field: - Dynamo Effect: Earth's magnetic field is generated by convection currents of molten iron and nickel in Earth's outer core, combined with Earth's rotation. - Geographical poles vs Magnetic poles: - Geographical North Pole: Top of Earth's rotational axis (true north) - Geographical South Pole: Bottom of Earth's rotational axis - Magnetic North Pole: Located in northern Canada (currently), where magnetic field lines point vertically downward. Actually a magnetic south pole (attracts north pole of compass). - Magnetic South Pole: Located near geographical south pole (Antarctica). Actually a magnetic north pole. - Magnetic Declination (Angle of Declination): Angle between true north (geographical) and magnetic north (compass direction). Varies by location; approximately 11.3° between magnetic and geographical poles. - Magnetic Inclination (Angle of Dip): Angle at which magnetic field lines dip below the horizontal. At magnetic poles, dip = 90° (vertical). At equator, dip = 0° (horizontal). - Geomagnetic Reversal: Earth's magnetic field reverses polarity approximately every 200,000-300,000 years (magnetic north becomes magnetic south). Evidence from palaeomagnetism recorded in volcanic rocks. How do animals use magnetoreception? The chapter examines practical uses of magnetism in nature: - Magnetoreception: Ability of animals to detect Earth's magnetic field for navigation. - Migrating birds (e.g., Arctic Tern, European Robin): Use magnetic field to navigate during long-distance migrations. - Sea turtles: Use magnetic field to return to beaches where they were born. - Bees: Use magnetic field for navigation and communication. - Magnetic particles (magnetite): Found in tissues of many migratory animals, acting as tiny compass needles. Why do magnetic field lines never intersect? If two field lines intersected, at the point of intersection, the compass needle would point in two different directions simultaneously, which is impossible. Each point in space has only one magnetic field direction. Why is a proton's magnetic effect neglected compared to an electron's? Magnetic moment is inversely proportional to mass. The proton is approximately 1836 times heavier than the electron, so its magnetic moment is about 1836 times smaller. These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• What is the domain theory of magnetism and how are materials magnetized/demagnetized? Explain the domain theory of magnetism (microscopic regions where atomic magnetic moments align, randomly oriented in unmagnetized state, aligned in magnetized state), and describe how materials can be magnetized through stroking method (repeatedly stroking with magnet in one direction) or electrical method (using solenoid with DC current), and demagnetized through hammering (physical shock), heating above Curie temperature (thermal randomization), or alternating current (decreasing AC field).
• What are the differences between ferromagnetic, paramagnetic, and diamagnetic materials? Differentiate between ferromagnetic materials (strongly attracted, examples: iron, nickel, cobalt), paramagnetic materials (weakly attracted, examples: aluminum, platinum), and diamagnetic materials (weakly repelled, examples: bismuth, water, copper), and compare permanent magnets (always magnetic, made of steel/alnico/neodymium) with temporary electromagnets (magnetic only when current flows, made with soft iron core, can be turned on/off).
• What is the magnetic field of a solenoid and what factors affect electromagnet strength? Describe the magnetic field of a solenoid (uniform inside, similar to bar magnet) using the formula B = μ₀nI where μ₀ = 4π × 10⁻⁷ Tm/A (permeability of free space), n = N/L (turns per unit length), I = current in amperes, and identify the factors affecting the strength of an electromagnet including number of turns (more turns = stronger field), current (higher current = stronger field), and presence of soft iron core (increases field by 100-1000 times).
• What is Earth's magnetic field and how do animals use magnetoreception? Analyze Earth's magnetic field (generated by dynamo effect from convection of molten iron-nickel core combined with Earth's rotation), explain the difference between geographical poles (true north/south based on rotation axis) and magnetic poles (where field lines are vertical, inclined at 11.3° to geographical axis, undergo geomagnetic reversal every 200,000-300,000 years with evidence from palaeomagnetism), and discuss how migrating animals including birds, sea turtles, and bees use magnetoreception (detecting magnetic field using magnetite particles as tiny compasses) for navigation.

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