A-Level Physics: Electric Fields vs Magnetic Fields | 电场与磁场的核心对比

📚 A-Level Physics: Electric Fields vs Magnetic Fields | 电场与磁场的核心对比

Electric fields and magnetic fields are two fundamental concepts in A-Level physics that often appear together but behave very differently. This article provides a systematic comparison of their definitions, sources, forces, energy properties, and exam-relevant applications.

电场和磁场是 A-Level 物理中两个基本概念，它们经常一同出现，但行为方式却大不相同。本文系统对比它们的定义、来源、力、能量性质以及考试相关的应用。

1. What Is a Field? | 什么是场？

A field is a region of space in which a physical quantity, such as force or potential, has a value at every point. In A-Level physics, we study electric and magnetic fields as two distinct but related examples.

场是空间中每一个点都具有某个物理量（如力或势）值的区域。在 A-Level 物理中，我们研究电场和磁场作为两个不同但相关的例子。

An electric field exists around any charged object. It exerts a force on other charges placed within it. A magnetic field exists around moving charges or permanent magnets, and it exerts a force on moving charges or magnetic materials.

任何带电物体周围都存在电场。它对其中的其他电荷施加力。磁场存在于运动电荷或永磁体周围，它对运动电荷或磁性材料施加力。

Key idea: fields are invisible, but their effects can be mapped using field lines or test particles. The strength and direction of a field determine how charges or currents behave inside it.

关键概念：场是不可见的，但我们可以通过场线或试探粒子来描绘它们的影响。场的强弱和方向决定了其中的电荷或电流如何运动。

2. Sources: What Creates Each Field? | 来源：什么产生每种场？

Electric fields are created by electric charges, whether stationary or moving. A single positive charge produces a radial outward field, while a single negative charge produces a radial inward field.

电场由电荷产生，无论电荷静止还是运动。一个正电荷产生径向向外的场，一个负电荷产生径向向内的场。

Magnetic fields, however, are created only by moving charges or by permanent magnets (which are themselves composed of aligned moving electrons). A stationary charge does not produce a magnetic field.

然而，磁场只由运动电荷或永磁体产生（永磁体本身是由排列整齐的运动电子组成）。静止电荷不产生磁场。

Electric field source: any electric charge (positive or negative)
Magnetic field source: moving charge, current-carrying wire, permanent magnet

电场来源：任意电荷（正或负）
磁场来源：运动电荷、载流导线、永磁体

This difference is fundamental: an isolated stationary proton creates only an electric field, while a moving electron creates both an electric field and a magnetic field.

这一差异是根本性的：一个孤立的静止质子只产生电场，而一个运动的电子同时产生电场和磁场。

3. Field Lines: Visual Patterns | 场线：可视化图形

Electric field lines start on positive charges and end on negative charges. They never cross, and the spacing indicates field strength: closer lines mean stronger field.

电场线从正电荷出发，终止于负电荷。它们永不相交，间距表示场强：线越密，场越强。

Magnetic field lines form continuous closed loops. They pass from north pole to south pole outside the magnet and continue from south to north inside the magnet. Unlike electric field lines, magnetic field lines do not start or end on any “magnetic charge.”

磁场线形成连续的闭合回路。在磁体外部，它们从北极指向南极；在磁体内部，从南极回到北极。与电场线不同，磁场线不会起始或终止于任何“磁荷”。

For parallel plates, electric field lines are uniform and parallel. For a long straight wire, magnetic field lines are concentric circles around the wire. These patterns are key to solving many exam problems.

对于平行板，电场线是均匀且平行的。对于长直导线，磁场线是围绕导线的同心圆。这些图形是解答许多考试题目的关键。

4. Force on a Charge | 对电荷的作用力

In an electric field E, the force on a charge q is given by:

在电场 E 中，电荷 q 所受的力为：

F = qE

This force is parallel to the electric field direction for a positive charge and opposite for a negative charge. It acts regardless of whether the charge is moving or stationary.

该力对正电荷沿电场方向，对负电荷沿电场反方向。无论电荷是运动还是静止，该力都存在。

In a magnetic field B, the force on a moving charge q with velocity v is:

在磁场 B 中，以速度 v 运动、电荷量为 q 的粒子所受的力为：

F = Bqv sin θ

Here θ is the angle between the velocity vector and the magnetic field direction. If the charge is stationary or moving parallel to the field (θ = 0° or 180°), the magnetic force is zero.

其中 θ 是速度方向与磁场方向的夹角。如果电荷静止或沿着磁场方向运动（θ = 0° 或 180°），磁力为零。

This is a major contrast: electric force works on any charge in the field; magnetic force only works on moving charges whose motion has a perpendicular component to the field.

这是一个重要对比：电场力作用于场中的任意电荷；而磁力只作用于运动方向具有垂直于磁场分量的运动电荷。

5. Direction of Force | 力的方向

In an electric field, the direction of force is along the field line direction (for positive charge) and easily predicted. For a negative charge, simply reverse the direction.

在电场中，力的方向沿场线方向（对于正电荷），容易预测。对于负电荷，只需将方向反转即可。

In a magnetic field, the force is always perpendicular to both the velocity and the magnetic field. Use Fleming’s left-hand rule: thumb points in the direction of force (motion), first finger points in the direction of field, and second finger points in the direction of conventional current (for positive charge).

在磁场中，力总是同时垂直于速度和磁场方向。使用弗莱明左手定则：拇指指向力（运动）方向，食指指向磁场方向，中指指向常规电流方向（对于正电荷）。

F ⊥ v, and F ⊥ B

The perpendicular nature of magnetic force means it does not change the speed of a particle, only its direction. This leads to circular motion when v is perpendicular to B.

磁力的垂直性意味着它不改变粒子的速率，只改变方向。当 v 垂直于 B 时，粒子做圆周运动。

6. Work Done: Energy Transfer | 做功：能量转移

Electric fields can do work on charges. When a charge moves through a potential difference, its electric potential energy changes, and kinetic energy may change accordingly.

电场可以对电荷做功。当电荷经过电势差时，其电势能发生变化，动能也随之改变。

The work done by an electric field in moving a charge q through a distance d parallel to the field is:

电场将电荷 q 沿场方向移动距离 d 所做的功为：

W = qEd

For a uniform field E between plates separated by d, this is equivalent to W = qV, where V is the potential difference.

对于间距为 d 的平行板之间的匀强电场，这等价于 W = qV，其中 V 是电势差。

In contrast, the magnetic force is always perpendicular to the displacement. Therefore, the work done by a magnetic force on a moving charge is always zero:

相反，磁力总是垂直于位移。因此，磁力对运动电荷所做的功始终为零：

W = F·s = 0 (because F ⊥ v)

This is a crucial exam point: a magnetic field cannot speed up or slow down a charged particle; it can only bend its path.

这是一个关键考点：磁场不能加速或减速带电粒子；它只能改变粒子的运动路径。

7. Potential and Potential Energy | 电势与电势能

Electric fields are conservative fields, meaning they have a well-defined scalar potential. The electric potential V at a point is the work done per unit charge in bringing a positive test charge from infinity to that point.

电场是保守场，具有定义明确的标量势。某点的电势 V 是从无穷远处将单位正电荷移动到该点所做的功。

The electric potential energy of a charge q at a point with potential V is:

电荷 q 在电势为 V 的点所具有的电势能为：

U = qV

Magnetic fields do not have a scalar potential analogous to electric potential. Because magnetic forces cannot do work, there is no meaningful “magnetic potential energy” for a charge moving in a steady magnetic field.

磁场没有与电势类似的标量势。由于磁力不能做功，在稳恒磁场中运动的电荷没有有意义的“磁势能”。

This difference explains why charged particles can gain energy only in electric fields (e.g., in particle accelerators like linear accelerators), while magnets are used to steer them.

这一差异解释了为什么带电粒子只能在电场中获得能量（如直线加速器中的情形），而磁场用于引导粒子方向。

8. Uniform Fields and Their Formulas | 匀强场及其公式

For a uniform electric field between two parallel plates separated by distance d and with potential difference V, the electric field strength is:

对于间距为 d、电势差为 V 的两平行板之间的匀强电场，电场强度为：

E = V / d

Units: V m⁻¹ or N C⁻¹. The field is constant in magnitude and direction between the plates (ignoring edge effects).

单位：V m⁻¹ 或 N C⁻¹。在忽略边缘效应的情况下，板间的场大小和方向恒定。

For a magnetic field, a uniform field can be produced inside a solenoid or between two flat pole pieces. The magnetic flux density B is measured in tesla (T). The force on a current-carrying conductor of length L carrying current I perpendicular to B is:

对于磁场，匀强场可以通过螺线管内部或两个平面磁极之间产生。磁通密度 B 的单位是特斯拉（T）。长度为 L、电流为 I 的载流导线垂直于 B 时所受的力为：

F = BIL

If the wire is at an angle θ to the magnetic field, use F = BIL sin θ. This formula is often used in practical experiments to measure B.

如果导线与磁场成 θ 角，则使用 F = BIL sin θ。该公式常用于实验测量 B。

For a charged particle moving perpendicular to a uniform magnetic field, the magnetic force provides the centripetal force:

对于垂直于匀强磁场运动的带电粒子，磁力提供向心力：

Bqv = mv² / r → r = mv / (Bq)

This radius r is the cyclotron radius. It shows that a stronger field or a larger charge gives a tighter curve, while a faster or more massive particle moves in a larger circle.

这个半径 r 称为回旋半径。它表明：场越强或电荷量越大，曲线越弯曲；速度越快或质量越大，圆周越大。

9. Motion of Charged Particles | 带电粒子的运动

In a uniform electric field, a charged particle experiences a constant force in one direction. This produces projectile-like parabolic motion, analogous to a ball in uniform gravity.

在匀强电场中，带电粒子受到一个方向恒定的力。这产生类似抛体运动的抛物线轨迹，类似于匀强重力场中的球。

In a uniform magnetic field, if the particle enters perpendicular to B, the force is always perpendicular to the velocity, producing uniform circular motion. The speed remains constant, but the direction changes continuously.

在匀强磁场中，如果粒子垂直于 B 入射，力始终垂直于速度，产生匀速圆周运动。速度大小保持不变，但方向连续变化。

If the particle enters at an angle not equal to 90° to B, the component of velocity parallel to B is unaffected, while the perpendicular component causes circular motion. The result is a helical (spiral) path.

如果粒子以不等于 90° 的角度进入磁场，平行于 B 的速度分量不受影响，垂直分量产生圆周运动。结果是螺旋状路径。

Exam tip: always resolve velocity into components parallel and perpendicular to the magnetic field. Only the perpendicular component contributes to the magnetic force.

考试提示：始终将速度分解为平行于磁场和垂直于磁场的分量。只有垂直分量产生磁力。

10. Comparing Formulas Side by Side | 公式并排对比

Aspect	Electric Field	Magnetic Field
Force on stationary charge	F = qE (nonzero)	F = 0
Force on moving charge	F = qE (independent of v)	F = Bqv sin θ
Direction of force	Parallel to E (or antiparallel for −q)	Perpendicular to both v and B
Work done	Can be nonzero (W = qV)	Always zero
Field lines	From + to −	Closed loops, N to S outside
Potential energy	U = qV	Not defined for static field

This table is your quick revision tool. Know every row and be ready to apply it in multiple-choice and structured questions.

该表是快速复习工具。请记住每一行，并准备好在选择题和结构化题目中应用。

11. Common Exam Misconceptions | 常见考试误区

Misconception 1: “Magnetic fields can speed up a charged particle.” This is wrong. Since the magnetic force is perpendicular to velocity, it cannot change the speed. It only changes direction.

误区一：“磁场可以使带电粒子加速。”这是错误的。因为磁力垂直于速度，不能改变速度大小。它只改变方向。

Misconception 2: “A stationary charge feels no electric force.” This is also wrong. A stationary charge always experiences an electric force F = qE in an electric field.

误区二：“静止电荷不受电场力。”这也是错误的。在电场中，静止电荷总是受到力 F = qE 的作用。

Misconception 3: “The path in a magnetic field is always circular.” Not always. It is circular only when the velocity is exactly perpendicular to B. If there is a parallel component, the path is helical.

误区三：“磁场中的轨迹总是圆。”不一定。只有当速度恰好垂直于 B 时才是圆。如果有平行分量，轨迹是螺旋线。

Misconception 4: “Electric field lines form closed loops.” False. Electric field lines start on positive charges and end on negative charges. Magnetic field lines form closed loops.

误区四：“电场线形成闭合回路。”错误。电场线从正电荷出发，到负电荷终止。磁场线才形成闭合回路。

12. Exam-Style Problem Example | 典型试题示例

Problem: A proton (q = 1.6 × 10⁻¹⁹ C, m = 1.67 × 10⁻²⁷ kg) enters a uniform magnetic field B = 0.5 T at a speed of 2.0 × 10⁶ m s⁻¹, with velocity perpendicular to the field. Calculate the radius of the circular path.

例题：一个质子（q = 1.6 × 10⁻¹⁹ C，m = 1.67 × 10⁻²⁷ kg）以速度 2.0 × 10⁶ m s⁻¹ 垂直进入匀强磁场 B = 0.5 T。求圆周运动半径。

Solution: The magnetic force provides centripetal force:

解答：磁力提供向心力：

Bqv = mv² / r

Rearrange: r = mv / (Bq)

变形：r = mv / (Bq)

r = (1.67 × 10⁻²⁷ × 2.0 × 10⁶) / (0.5 × 1.6 × 10⁻¹⁹) = 4.18 × 10⁻² m ≈ 4.2 cm

Always check units: kg × m s⁻¹ divided by T × C yields metres. If you get a nonsensical unit, you have likely mixed up formulas.

始终检查单位：kg × m s⁻¹ 除以 T × C 得到米。如果得到不合理单位，则很可能弄混了公式。

For comparison, if the same proton is placed in a uniform electric field E = 2000 V m⁻¹, the force would be F = qE = 1.6 × 10⁻¹⁹ × 2000 = 3.2 × 10⁻¹⁶ N, and the acceleration would be a = F/m ≈ 1.9 × 10¹¹ m s⁻², creating straight-line acceleration rather than circular motion.

作为对比，如果同一个质子放入匀强电场 E = 2000 V m⁻¹，则力 F = qE = 1.6 × 10⁻¹⁹ × 2000 = 3.2 × 10⁻¹⁶ N，加速度为 a = F/m ≈ 1.9 × 10¹¹ m s⁻²，产生直线加速而非圆周运动。

Conclusion: Remember the Core Contrast | 结论：记住核心对比

Electric fields act on any charge, exert forces parallel to the field, and can change both the speed and direction of a charge. Magnetic fields act only on moving charges, exert forces perpendicular to both velocity and field, and change direction without changing speed.

电场作用于任何电荷，施加与场平行的力，并能改变电荷的速度大小和方向。磁场只作用于运动电荷，施加垂直于速度和场的力，只改变方向而不改变速度大小。

When solving problems, first ask: is there an electric field, a magnetic field, or both? This single question tells you whether to calculate work, energy, or just curvature of path.

做题时，先问：这里有电场、磁场，还是两者都有？这一个问题决定了你应该计算功、能量，还是仅仅计算轨迹的曲率。

Master this comparison, and you will be well-prepared for both multiple-choice questions and long-answer structured problems in CIE A-Level Physics.

掌握这一对比，你将在 CIE A-Level 物理的选择题和长答题中游刃有余。

Published by TutorHao | Physics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

A-Level Physics: Electric Fields vs Magnetic Fields | 电场与磁场的核心对比