Electricity and Magnetism for OCR Science: Key Revision Points | 电与磁：OCR 科学考点精讲

📚 Electricity and Magnetism for OCR Science: Key Revision Points | 电与磁：OCR 科学考点精讲

Welcome to this focused revision guide covering the key concepts of electricity and magnetism as required for OCR Gateway Science (Physics) or Combined Science. We will explore circuits, Ohm’s law, power, magnetic fields, electromagnetism, the motor effect, electromagnetic induction, and transformers. Understanding these principles, along with their practical applications, is essential for success in your examination.

欢迎阅读本考点精讲，涵盖 OCR 科学（物理或综合科学）中电与磁的核心概念。我们将探讨电路、欧姆定律、功率、磁场、电磁学、电动机效应、电磁感应和变压器等主题。理解这些原理及其实际应用，是你在考试中取得成功的关键。

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1. Electric Current and Charge | 电流与电荷

Electric current is the rate of flow of electric charge. In a solid conductor, it is the movement of free electrons, whereas in ionic solutions both positive and negative ions move. Current (I) is measured in amperes (A); one ampere equals one coulomb of charge passing a point per second. The equation linking charge, current and time is:

电流是电荷流动的速率。在固体导体中，这是自由电子的运动，而在离子溶液中，正负离子都会移动。电流（I）的单位是安培（A）；1 安培等于每秒有 1 库仑的电荷通过某一点。连接电荷、电流和时间的公式为：

Q = I × t

where Q is charge in coulombs (C), t is time in seconds (s). Conventional current flows from the positive terminal of a battery to the negative terminal, opposite to the direction of electron flow.

其中 Q 代表电荷（库仑，C），t 代表时间（秒，s）。传统电流方向是从电池正极流向负极，与电子流动的方向相反。

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2. Potential Difference and Resistance | 电压与电阻

Potential difference (p.d.) or voltage (V) is the energy transferred per unit charge between two points. It is measured in volts (V); one volt means one joule of energy is transferred per coulomb. Resistance (R) is the opposition to current flow, measured in ohms (Ω). The greater the resistance, the smaller the current for a given p.d.

电势差（电压，V）是单位电荷在两点间转移的能量。其单位为伏特（V）；1 伏特表示每库仑电荷转移了 1 焦耳的能量。电阻（R）是对电流流动的阻碍，单位为欧姆（Ω）。在给定电压下，电阻越大，电流越小。

V = I × R

Ohmic conductors follow Ohm’s law, where the current through a resistor is directly proportional to the potential difference across it, provided temperature remains constant. Non-ohmic components, such as filament lamps and diodes, do not have a constant resistance.

欧姆导体遵循欧姆定律，即只要温度保持不变，通过电阻器的电流与其两端的电势差成正比。非欧姆元件，如灯丝灯泡和二极管，并不具有恒定的电阻。

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3. Ohm’s Law and I-V Characteristics | 欧姆定律与 I-V 特性曲线

The I-V graph for a fixed resistor at constant temperature is a straight line through the origin, indicating constant resistance. For a filament lamp, the curve bends as temperature rises, causing resistance to increase. A diode allows current to flow in only one direction, showing very high resistance in the reverse direction.

在恒温下，定值电阻的 I-V 特性曲线是一条过原点的直线，表明电阻恒定。灯丝灯泡的曲线因温度升高导致电阻增大而弯曲。二极管只允许电流单向导通，反向电阻极大。

All conductors heat up when current passes through them, which changes resistance. Thermistors and LDRs (light-dependent resistors) are special resistors whose resistance changes with temperature and light, respectively; they do not follow Ohm’s law.

所有导体通过电流时都会发热，从而改变电阻。热敏电阻和光敏电阻（LDR）是特殊的电阻器，其电阻分别随温度和光照变化；它们不遵循欧姆定律。

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4. Series and Parallel Circuits | 串联与并联电路

In a series circuit, the same current flows through all components, whereas the total p.d. is shared. The total resistance is the sum of the individual resistances: R_total = R₁ + R₂ + … . If one component fails, the circuit is broken.

在串联电路中，电流处处相等，而总电压被分配。总电阻等于各电阻之和：R_total = R₁ + R₂ + …。若某一元件损坏，整个电路就会断开。

In a parallel circuit, the p.d. across each branch is equal to the source p.d., while the current splits. The total resistance decreases as more branches are added. Household circuits are wired in parallel so that appliances can operate independently.

在并联电路中，各支路两端的电压与电源电压相同，电流则分流。增加并联支路会使总电阻减小。家用电路采用并联方式，以便电器独立工作。

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5. Electrical Power and Energy | 电功率与电能

Power (P) is the rate of energy transfer, measured in watts (W). For an electrical component, the following equations apply:

电功率（P）是能量转移的速率，单位为瓦特（W）。对于电气元件，可使用以下公式：

P = I × V    P = I² × R    P = V² / R

Energy transferred E = P × t = I × V × t, where t is time in seconds. The kilowatt-hour (kWh) is a unit of energy used for domestic consumption; 1 kWh = 3.6 × 10⁶ J. A fuse protects circuits by melting if the current exceeds its rating, breaking the circuit.

能量转移 E = P × t = I × V × t，其中 t 为时间，单位秒。千瓦时（kWh）是家庭用电的能量单位；1 kWh = 3.6 × 10⁶ 焦耳。保险丝在电流超过额定值时熔断，从而保护电路。

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6. Magnetism and Magnetic Fields | 磁性与磁场

A magnet has two poles, north and south; magnetic field lines point from north to south outside the magnet. Materials such as iron, steel, cobalt and nickel are magnetic. A permanent magnet produces its own persistent magnetic field, while an induced magnet only becomes magnetic when placed in a field.

磁铁有两个磁极：北极和南极；外部磁感线从北极指向南极。铁、钢、钴、镍等物质具有磁性。永磁体自身产生持久的磁场，而暂磁体仅在置于外磁场中时呈现磁性。

• A steel nail becomes an induced magnet when placed near a strong magnet, but loses most magnetism when removed.

  钢钉靠近强磁铁时会成为暂磁体，但移开后几乎完全失去磁性。

• The Earth’s magnetic field resembles that of a bar magnet, with the geographic North Pole being a magnetic south pole – it protects us from solar wind particles.

  地球磁场类似于条形磁铁，地理北极实际上是地磁南极——它保护我们免受太阳风粒子伤害。

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7. Electromagnetism and Solenoids | 电磁与螺线管

When current flows through a wire, a circular magnetic field is produced around it. The right-hand grip rule determines the field direction: grasp the wire with your right hand, thumb pointing in the current direction, then curled fingers show the field direction.

电流通过导线时，周围会产生环形磁场。右手螺旋定则可判断磁场方向：右手握住导线，拇指指向电流方向，则弯曲的四指表示磁场方向。

A solenoid (coil of wire) produces a strong, uniform magnetic field inside, similar to a bar magnet. Its strength can be increased by inserting a soft iron core, increasing the current, or adding more turns of wire.

螺线管（线圈）内部产生强而均匀的磁场，类似于条形磁铁。增强磁场的方法有：加入软铁芯、增大电流或增加线圈匝数。

Electromagnets are widely used:

电磁铁应用广泛：

• Relays – a small current controls a large current circuit through a switch operated by the electromagnet.

  继电器——小电流通过电磁铁控制开关，从而控制大电流电路。

• Electric bells – the electromagnet repeatedly attracts a striker that hits the bell.

  电铃——电磁铁反复吸引撞针敲击铃铛。

• Circuit breakers – excessive current strengthens the electromagnet, which releases a latch and opens the circuit safely.

  断路器——过大电流增强电磁铁，释放锁扣，安全断开电路。

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8. The Motor Effect and Fleming’s Left-Hand Rule | 电动机效应与左手定则

A current-carrying conductor placed in a magnetic field experiences a force – this is the motor effect. The force is maximum when the conductor is perpendicular to the field. Fleming’s left-hand rule predicts the direction: thumb = force (motion), first finger = field (N to S), second finger = conventional current.

置于磁场中的载流导体会受到力的作用——这就是电动机效应。当导线与磁场方向垂直时，力最大。弗莱明左手定则可判断方向：拇指表示力（运动），食指表示磁场（N 到 S），中指表示传统电流。

F = B × I × L

where B is the magnetic flux density in tesla (T), I is the current, and L is the length of conductor in the field. The force increases with a stronger magnet, larger current, or more wire. This principle is used in electric motors and loudspeakers.

其中 B 为磁通量密度（特斯拉，T），I 为电流，L 为磁场中导线的长度。力随更强的磁铁、更大的电流或更长的导线长度而增大。该原理应用于电动机和扬声器。

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9. Electromagnetic Induction and Generator Effect | 电磁感应与发电机效应

Electromagnetic induction occurs when a conductor cuts through magnetic field lines, or when the magnetic field around a conductor changes. This induces a p.d. and, if the circuit is complete, a current flows – the generator effect. The direction of induced current can be determined using Fleming’s right-hand rule (thumb = motion, first finger = field, second finger = induced current).

当导体切割磁感线，或导体周围的磁场发生变化时，就会发生电磁感应，产生感应电势差；若电路闭合，则有感应电流——此即发电机效应。感应电流方向可用弗莱明右手定则判断（拇指 = 运动，食指 = 磁场，中指 = 感应电流）。

The size of the induced p.d. depends on the rate of change of the magnetic field and the number of turns on the coil. Alternators use slip rings to produce alternating current (ac), while dynamos use a split-ring commutator to produce direct current (dc). Microphones convert sound waves into varying electrical signals using this principle.

感应电压的大小取决于磁场变化的速率和线圈的匝数。交流发电机使用滑

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