Lecture Notes: Magnetic Fields (Cambridge A-Level Physics 9702)

Introduction

• Presenter: Mr. K
• Focus: High-level overview of magnetic fields for Cambridge A-Level Physics 9702
• Importance: Lengthy topic with many concepts, significant for exams
• Goal: Simplify the understanding of magnetic fields quickly

Basic Properties of Magnets

• Bar Magnets: Have North and South Poles, which always exist as pairs
  • Unlike Poles: Attract each other
  • Like Poles: Repel each other
• Difference from Electric Charges: Magnets must have both poles; charges can exist as monopoles

Magnetic Field Lines

• Definition: Lines of magnetic force indicating the direction of a free North Pole
• Characteristics:
  • Start at North Pole and terminate at South Pole
  • Lines never cross to avoid ambiguity
  • Direction shows the path a free North Pole would take
  • Closeness of lines indicates field strength (closer lines = stronger field)

Defining a Magnetic Field

• Region where a moving charged particle or a permanent magnetic pole experiences a force

Motor Effect

• Explanation: When current flows perpendicular to magnetic field lines, the wire feels a force
• Fleming's Left Hand Rule:
  • Thumb: Direction of force
  • Index Finger: Magnetic field (B)
  • Middle Finger: Current (I)
• Force Calculation:
  • Magnitude: F = BILsinθ
  • Maximum force when θ = 90°

Force on a Moving Charge

• Equation: F = Bqv sinθ
• Direction: Determined by Fleming's Left Hand Rule (consider conventional current direction)

Motion in Magnetic Fields

• Charged particles moving in magnetic fields experience forces at right angles, creating circular paths
• Equation for Circular Motion: r = mv / (Bq)

Velocity Selection

• Setup: Crossed electric and magnetic fields
• Condition for Undeflected Path: v = E/B

Hall Effect

• Setup: Magnetic field applied to a conductor with current
• Result: Potential difference (Hall voltage) across conductor due to charge separation
• Equation: VH = BI / (nqt)
• Application: Used to measure magnetic flux density with a Hall probe

Magnetic Field of a Current

• Straight Current: Produces circular magnetic field
• Loop of Wire: Produces uniform magnetic field within loop
• Right-Hand Grip Rule: Determines direction of field around a current-carrying wire

Electromagnetic Induction

• Faraday's Law: Changing magnetic fields induce EMF in a conductor
  • Equation: E = -d(Φn)/dt
  • Lenz's Law: Induced EMF opposes the change causing it

Key Concepts

• Magnetic Flux (Φ): Amount of field passing through an area (Φ = BA)
• Flux Linkage: ΦN = BAN
• Units: Tesla (T), Weber (Wb)

Conclusion

• Topic requires thorough understanding due to its complexity and length
• Key points include definitions, physical laws, and applications such as the motor effect, Hall effect, and electromagnetic induction
• Practice and review are essential to mastering the topic for exams

These notes summarize the key concepts and principles discussed in Mr. K's crash course on magnetic fields, especially relevant for Cambridge A-Level Physics students preparing for exams.