HKDSE Physics Elective
Chapter 4: Magnetic Field | 磁場
Billy Sir’s Smart Notes: Master Electromagnetism, Magnetic Forces, and Moving Charges.
由中大物理系碩士 Billy Sir 編寫，助你極速掌握電磁學、磁力及運動電荷分析。
1. Basics of Magnetic Field | 磁場基礎
A magnetic field is a region where a magnetic material or a moving charge experiences a magnetic force. 磁場是磁性物質或運動電荷會受到磁力的區域。
| Concept (概念) | Description (描述) |
|---|---|
| Permanent Magnet 永久磁鐵 |
Retains its magnetism for a long time. Magnetic field lines point from North (N) to South (S) outside the magnet. (能長時間保持磁性。磁場線在磁鐵外部由北極指向南極。) |
| Magnetic Material 磁性物質 |
Materials that can be magnetized or attracted to a magnet (e.g., Iron, Steel, Nickel, Cobalt). (能被磁化或被磁鐵吸引的物質，如鐵、鋼、鎳、鈷。) |
| Earth’s Magnetic Field 地球磁場 |
The Earth acts like a giant bar magnet. The geographic North Pole is actually a magnetic South pole. (地球就像一塊巨大的條形磁鐵。地理北極實際上是地磁南極。) |
2. Magnetic Field of Current-Carrying Conductors | 載流導體的磁場
An electric current produces a magnetic field around it. The direction is determined by the Right-Hand Grip Rule (右手握拳定則).
🧲 Straight Wire & Solenoid | 直導線與螺線管
- Straight Wire (直導線): Magnetic field lines form concentric circles around the wire. (磁場線在導線周圍形成同心圓。)
- Solenoid (螺線管): A long coil of wire. The magnetic field inside is strong and uniform. It resembles the field of a bar magnet. (長線圈。內部的磁場強且均勻，類似於條形磁鐵的磁場。)
🏗️ Application: Electromagnet | 應用：電磁鐵
A solenoid with a soft iron core. It can be turned on and off by controlling the current. Used in magnetic cranes, electric bells, and relays. (帶有軟鐵芯的螺線管。可通過控制電流來開啟和關閉。應用於起重電磁鐵、電鈴和繼電器。)
3. Magnetic Force on a Current-Carrying Wire | 載流導線受到的磁力
When a current-carrying wire is placed in a magnetic field, it experiences a force. The direction is given by Fleming’s Left-Hand Rule (弗萊明左手定則). 當載流導線置於磁場中時，會受到力的作用。方向由弗萊明左手定則決定。
📐 Magnetic Force Formula | 磁力公式
Where \( B \) is magnetic field (Tesla, T), \( I \) is current (A), \( l \) is length of wire (m), and \( \theta \) is the angle between the wire and the magnetic field.
🔄 Turning Effect on a Coil (DC Motor) | 線圈的轉動效應 (直流電動機)
A current-carrying coil in a magnetic field experiences a pair of forces (a couple) that causes it to rotate. (磁場中的載流線圈會受到一對力（力偶）的作用而旋轉。)
- Use of Commutator (換向器的作用): A split-ring commutator reverses the direction of current in the coil every half cycle. This ensures the coil continues to rotate in the same direction. (換向器每半個週期改變線圈中的電流方向，確保線圈持續向同一方向旋轉。)
4. Magnetic Force on a Moving Charge | 運動電荷受到的磁力
A charged particle moving through a magnetic field experiences a force, provided its velocity is not parallel to the field lines. 在磁場中運動的帶電粒子會受到磁力（前提是速度不與磁場線平行）。
⚡ Force on a Charge | 電荷受力公式
Where \( q \) is the charge (C) and \( v \) is the velocity (m s\(^{-1}\)).
🌀 Circular Motion | 圓周運動
If the charge enters the magnetic field perpendicularly (\( \theta = 90^\circ \)), the magnetic force provides the centripetal force, causing it to move in a circle. (如果電荷垂直進入磁場，磁力提供向心力，使其作圓周運動。)
5. Velocity Selector | 速度選擇器
A device that uses crossed (perpendicular) electric and magnetic fields to select charged particles of a specific velocity. 利用正交（垂直）的電場和磁場來選擇特定速度的帶電粒子的裝置。
🎯 Working Principle | 工作原理
The electric force (\( F_E = qE \)) and magnetic force (\( F_B = Bqv \)) act in opposite directions. For a particle to pass through undeflected, the net force must be zero: (電場力與磁場力方向相反。要使粒子不偏轉地通過，淨力必須為零：)
Only particles with this exact velocity \( v \) will travel in a straight line. (只有速度剛好為 \( v \) 的粒子才能沿直線前進。)
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