📚 AP Physics Electromagnetism Key Vocabulary | AP 物理电磁学重点词汇
Vocabulary is the hidden syllabus of AP Physics. In AP Physics C: Electricity and Magnetism and AP Physics 2, students rarely lose marks because of algebra; they lose marks because of a misread term such as electric potential versus electric potential energy, or because they give a magnitude when the question asked for a signed value. This guide collects the essential electromagnetic vocabulary, symbols, units and equations in bilingual form, so that English and Chinese , helping bilingual learners can lock down the language and the physics at the same time.
词汇是 AP 物理中被隐藏起来的考纲。在 AP 物理 C：电磁学与 AP 物理 2 中，学生失分往往不是因为代数运算，而是因为读错术语——例如把电势与电势能混为一谈，或在题目要求带符号的值时只写了大小。本指南以英汉双语系统整理电磁学的核心词汇、符号、单位与公式，帮助英语与中文学学习者同时锁定语言与物理本身。
1. Electric Charge and Coulomb's Law | 电荷与库仑定律
Electric Charge (q): a fundamental property of matter that makes it experience a force in an electromagnetic field. There are two kinds, positive and negative; like charges repel and opposite charges attract. The SI unit is the coulomb (C). Charge is a scalar quantity, so it has magnitude and sign but no direction.
电荷 (q)：物质的一种基本属性，使其在电磁场中受到力的作用。电荷分为正、负两种；同种电荷相互排斥，异种电荷相互吸引。国际单位制单位为库仑 (C)。电荷是标量，只有大小和正负，没有方向。
Coulomb's Law: the magnitude of the electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them. The force acts along the line joining the two charges.
库仑定律：两个点电荷之间静电力的大小，与两电荷电量的乘积成正比，与它们之间距离的平方成反比。力的方向沿两电荷的连线。
F = k |q₁ q₂| / r²
ermittivity of free space. The force is attractive for opposite charges and repulsive for like charges.
其中 k = 1 / (4πε₀) = 8.99 × 10⁹ N·m²/C²，ε₀ = 8.85 × 10⁻¹² C²/(N·m²) 为真空电容率（真空介电常数）。异种电荷相互吸引，同种电荷相互排斥。
y −e, and every observed charge is an integer multiple of e (charge quantization, q = ne).
基本电荷 (e)：自由电荷的最小单位，e = 1.60 × 10⁻¹⁹ C。质子带 +e，电子带 −e；任何可观测到的电荷都是 e 的整数倍（电荷量子化，q = ne）。
Conservation of Charge: the net electric charge in an isolated system remains constant; charge cannot be created or destroyed, only transferred from one body to another.
电荷守恒：孤立系统中的净电荷保持不变；电荷不能被创造或消灭，只能从一个物体转移到另一个物体。
Exam note: the two forces in Coulomb's law form a Newton's third law pair, equal in magnitude and opposite in direction, even when the two charges have very different magnitudes.
考试提醒：库仑定律中的两个力构成牛顿第三定律的一对作用力与反作用力——大小相等、方向相反，即使两电荷电量相差悬殊也成立。
2. Electric Field and Field Lines | 电场与电场线
Electric Field (E): the electric force per unit positive charge experienced by a small positive test charge placed at a point. It is a vector; its direction is of the field is the direction of the force on a positive charge. The SI unit is N/C, which is identical to V/m.
电场 (E)：在空间某点放置一个小的正试探电荷时，它受到的电场力与电荷量之比。电场是矢量，其方向就是正电荷所受力的方向。国际单位制单位为 N/C，与 V/m 等价。
E = F / q₀ and E = k |Q| / r²
f a point charge Q at distance r. For a uniform field between parallel plates, E = ΔV / d.
第一式是定义式；第二式给出点电荷 Q 在距离 r 处产生的场强。对于平行板之间的匀强电场，E = ΔV / d。
Electric Field Lines: a visual tool. They begin on positive charges and end on negative charges (or extend to infinity), never cross, and their density indicates field strength. Near a conductor surface in electrostatic equilibrium, field lines meet the surface at 90°, and the field inside the conductor is zero.
电场线：一种可视化工具。电场线起于正电荷、止于负电荷（或伸向无穷远）；电场线永不相交，其疏密表示场强大小。在静电平衡状态下，导体表面的电场线与表面垂直（成 90°），导体内部的场强为零。
- +q and −q separated by a distance d. The dipole moment is p = qd, pointing from −q to +q.
- : the net force is zero, but there is a torque τ = pE sinθ, which tends to align p with E.
- ough a surface, Φ = EA cosθ, where θ is the angle between E and the normal to the surface.
- Gauss' Φ = Q enclosed / ε₀. Use it for high-symmetry cases such as spheres, cylinders and planes.
- 电偶极子：一对等量异号电荷 +q 与 −q，相距 d。电偶极矩 p = qd，方向由 −q 指向 +q。
- 匀强电场中的电偶极子：合力为零，但存在力矩 τ = pE sinθ，使 p 趋向与 E 平行。
- 电通量 (Φ)：穿过某一面的电场量，Φ = EA cosθ，其中 θ 为 E 与面法线的夹角。
- 高斯定律：穿过闭合曲面的净通量等于所包围电荷除以 ε₀：Φ = Q 包围 / ε₀。常用于球、圆柱、平面等高对称情形。
3. Electric Potential, Potential Energy and Voltage | 电势、电势能与电压
negative for opposite charges and positive for like charges. The SI unit is the joule (J).
电势能 (U)：电荷系统由于其位置而储存的能量。对两个点电荷，U = k q₁ q₂ / r；异号电荷时 U 为负，同号电荷时 U 为正。国际单位制单位为焦耳 (J)。
harge at a point, V = U / q. It is a scalar. The SI unit is the volt (V), and 1 V = 1 J/C.
电势 (V)，常简称电位：某点处单位电荷所具有的电势能，V = U / q。电势是标量。国际单位制单位为伏特 (V)，1 V = 1 J/C。
V = k Q / r ΔV = W / q ΔV = −E d
s in the direction of the field. Equipotential surfaces are always perpendicular to field lines, and no work is done moving a charge along an equipotential.
电势差（电压，ΔV）：把单位电荷从一点移到另一点所做的功。在匀强电场中 ΔV = −Ed，所以电势沿电场方向降低。等势面始终与电场线垂直；沿等势面移动电荷不做功。
- J, the kinetic energy gained by one electron accelerated through a potential difference of 1 V.
- Common trap: potential V is a property of a point in space; potential energy U depends on both the field and the charge placed there. Doubling the charge doubles U but leaves V unchanged.
- Sign convention: positive charges move naturally from high to low potential; negative charges move from low to high potential.
- 电子伏特 (eV)：1 eV = 1.60 × 10⁻¹⁹ J，即一个电子经过 1 V 电势差加速所获得的动能。
- 常见陷阱：电势 V 是空间某点的性质；电势能 U 同时取决于场和被放入的电荷。电荷量加倍，U 加倍，但 V 不变。
- 符号约定：正电荷自然地从高电势移向低电势；负电荷从低电势移向高电势。
4. Capacitance and Capacitors | 电容与电容器
nductors, not of the charge placed on them. The SI unit is the farad (F), and 1 F = 1 C/V.
电容 (C)：系统每单位电势差所能储存的电荷量，C = Q / ΔV。它由导体的几何结构决定，与所带电荷量无关。国际单位制单位为法拉 (F)，1 F = 1 C/V。
Parallel plates: C = κ ε₀ A / d Energy: U = ½ Q ΔV = ½ C ΔV² = Q² / 2C
the plates (κ = 1 for vacuum). Inserting a dielectric increases capacitance by a factor κ.
其中 A 为极板面积，d 为板间距，κ 为板间介质的介电常数（真空中 κ = 1）。插入电介质会使电容增大到原来的 κ 倍。
- Capacitors in Series: 1/C total = 1/C₁ + 1/C₂ + …, and each capacitor stores the same charge.
- Capacitors in Parallel: C total = C₁ + C₂ + …, and each capacitor has the same potential difference.
- Note the deliberate contrast with resistors: capacitors combine in the opposite way to resistors.
- Dielectric Strength and Breakdown: every dielectric has a maximum field it can withstand before sparking.
- 串联电容器：1/C 总 = 1/C₁ + 1/C₂ + …，各电容器所带电荷相同。
- 并联电容器：C 总 = C₁ + C₂ + …，各电容器两端电势差相同。
- 注意与电阻的刻意对比：电容器的组合规律与电阻正好相反。
- 介电强度与击穿：每种电介质都有能承受的最大场强，超过就会打火击穿。
Circuits: when a capacitor charges through a resistor, the charge follows q = Q max (1 − e−t/τ) with time constant τ = RC. When discharging, q = Q₀ e−t/τ. After one time constant the capacitor has reached about 63% of its final value.
RC 电路：电容器通过电阻充电时，电荷满足 q = Q 最大 (1 − e−t/τ)，时间常数 τ = RC；放电时 q = Q₀ e−t/τ。经过一个时间常数，电容器的量达到最终值约 63%。
5. Current, Resistance and Ohm's Law | 电流、电阻与欧姆定律
nal current is defined as the direction in which positive charge would flow, which is opposite to the electron drift direction in a metal.
电流 (I)：电荷流动的速率，I = ΔQ / Δt。国际单位制单位为安培 (A)，1 A = 1 C/s。常规电流方向规定为正电荷流动的方向，在金属中与电子漂移方向相反。
where L is length and A is cross-sectional area. For most metals, resistivity increases with temperature.
电阻 (R)：对电流的阻碍作用，R = ΔV / I，单位为欧姆 (Ω)。电阻率 ρ 是材料属性；对均匀导线 R = ρ L / A，L 为长度，A 为横截面积。大多数金属的电阻率随温度升高而增大。
Ohm' ic devices, whose resistance is constant. Filament lamps and diodes are non-ohmic; their V–I graphs are curved, so resistance depends on the applied voltage.
欧姆定律：对欧姆元件 ΔV = IR 成立，其电阻为常数。灯丝灯泡和二极管是非欧姆元件，其 V–I 图像弯曲，电阻随所加电压变化。
- Electric Power: P = I ΔV = I²R = ΔV² / R. The SI unit is the watt (W).
- Resistors in Series: R total = R₁ + R₂ + …; the same current flows through each and the voltages add.
- Resistors in Parallel: 1/R total = 1/R₁ + 1/R₂ + …; the same voltage appears across each and the currents add.
- ty: I = nqv(d)A, where n is the charge-carrier number density and v(d) the drift velocity.
- 电功率：P = I ΔV = I²R = ΔV² / R，单位瓦特 (W)。
- 串联电阻：R 总 = R₁ + R₂ + …；各电阻电流相同，电压相加。
- 并联电阻：1/R 总 = 1/R₁ + 1/R₂ + …；各电阻电压相同，电流相加。
- 漂移速度与电流密度：I = nqv(d)A，n 为载流子数密度，v(d) 为漂移速度。
6. DC Circuits: EMF and Kirchhoff's Rules | 直流电路：电动势与基尔霍夫定律
Electromotive Force (EMF, ε): the energy per unit charge supplied by a source such as a battery or generator. It is not a force but a potential difference, measured in volts. Internal Resistance (r) is the small resistance inside the source.
电动势 (EMF，ε)：电源（如电池、发电机）对单位电荷提供的能量。它不是力，而是电势差，单位为伏特。内阻 (r) 是电源内部的小电阻。
Terminal voltage: V = ε − I r
Terminal Voltage: the potential difference actually available to the external circuit. When current is drawn, V is always less than ε; when the circuit is open, V equals ε.
端电压：外部电路实际可用的电势差。有电流输出时，V 总小于 ε；断路时 V 等于 ε。
- Kirchhoff's Junction Rule (conservation of charge): the sum of currents entering a junction equals the sum leaving it.
- Kirchhoff's Loop Rule (conservation of energy): the algebraic sum of potential changes around any closed loop is zero.
- Ammeter: measures current, connected in series, ideally with zero resistance.
- Voltmeter: measures potential difference, connected in parallel, ideally with infinite resistance.
- Short Circuit: a low-resistance path that draws a very large current and can damage the source.
- 基尔霍夫节点定律（电荷守恒）：流入节点的电流之和等于流出节点的电流之和。
- 基尔霍夫回路定律（能量守恒）：沿任一闭合回路，电势变化的代数和为零。
- 电流表：测电流，串联接入，理想情况下内阻为零。
- 电压表：测电势差，并联接入，理想情况下内阻无穷大。
- 短路：电阻极低的通路，会产生巨大电流并可能损坏电源。
7. Magnetic Fields, Forces and Right-Hand Rules | 磁场、磁场力与右手定则
cy unit, the gauss, equals 10⁻⁴ T. Field lines leave the north pole and enter the south pole and always form closed loops.
磁场 (B)：由运动电荷和永磁体产生的矢量场。国际单位制单位为特斯拉 (T)，1 T = 1 N/(A·m)。旧制单位高斯等于 10⁻⁴ T。磁感线从北极出发进入南极，永远闭合。
F = q v B sinθ F = B I L sinθ
The first equation gives the magnetic force on a moving charge; the second gives the force on a straight current-carrying wire. Both forces are perpendicular to both B and to the velocity or current.
第一式给出运动电荷所受的磁场力；第二式给出通电直导线所受的力。两个力都同时垂直于 B 以及速度或电流方向。
- Right-Hand Rule (force on a positive charge): point fingers along v, curl them toward B, and the thumb gives F. For a negative charge, reverse the result.
- Right-Hand Rule (field of a current): point the thumb along the current; the curled fingers show the direction of B.
- Long Straight Wire: B = μ₀ I / (2π r
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