📚 Output Devices in A-Level Physics | CIE A-Level 物理输出设备
In CIE A-Level Physics, electronics questions often ask you to describe how a sensing circuit produces a useful output, or to calculate the series resistor needed for an LED. An output device is the final transducer in a system: it changes an electrical signal into light, sound, movement or another physical quantity. This article examines the main output devices in the syllabus, their working principles, the key equations you must be able to quote, and the traps that cost marks in examinations.
在 CIE A-Level 物理中，电子学题目常要求你描述传感电路如何产生有用的输出，或计算 LED 所需的串联电阻。输出设备是系统中的最终换能器：它将电信号转换为光、声、运动或其他物理量。本文梳理大纲中的主要输出设备、工作原理、必须掌握的公式，以及考试中容易失分的陷阱。
1. What is an Output Device? | 什么是输出设备？
An output device is a transducer that receives an electrical signal and converts it into a non-electrical form of energy. The output forms examined in CIE A-Level Physics are mainly light, sound, mechanical movement and heat. In a sensing circuit the usual chain is input transducer, then processor, then output device. For example, a thermistor senses temperature, an op-amp or transistor switches the current, and an LED or buzzer gives the visible or audible warning.
输出设备是一种换能器，它接收电信号并将其转换为非电能形式。CIE A-Level 物理中考查的输出形式主要是光、声、机械运动和热。在传感电路中，通常的顺序是输入换能器 → 处理器 → 输出设备。例如，热敏电阻感测温度，运算放大器或晶体管切换电流，LED 或蜂鸣器给出可见或可听的警报。
Almost every output device can be described by three quantities taken from a data table: the voltage required across it, the current required through it, and the electrical power it dissipates. Exam questions almost always supply these values, and the resistor calculations in the later sections of this article depend on them.
几乎每个输出设备都可以用从数据表中读到的三个量来描述：它两端所需的电压、通过它所需的电流，以及它耗散的电功率。考题几乎总会提供这些数值，本文后面各节的电阻计算都依赖它们。
2. Input and Output Transducers Compared | 输入换能器与输出换能器的比较
You must be able to distinguish an input transducer from an output transducer. An LDR or a thermistor is an input device, because it changes light intensity or temperature into a change in resistance. An LED, lamp, loudspeaker or motor is an output device, because it converts an electrical signal into light, sound or movement. Some devices, such as a loudspeaker used as a microphone, can act as either, but the syllabus treats them in their normal role.
你必须能区分输入换能器与输出换能器。光敏电阻（LDR）或热敏电阻是输入设备，因为它们将光强或温度的变化转换为电阻的变化。LED、灯泡、扬声器或电动机是输出设备，因为它们将电信号转换为光、声或运动。有些器件（如当作麦克风使用的扬声器）可以两用，但大纲按它们的常规角色来考查。
| Device 器件 | Role 角色 | Energy change 能量转换 |
| LDR 光敏电阻 | Input 输入 | Light energy → electrical (resistance change) 光能 → 电能（电阻变化） |
| Thermistor 热敏电阻 | Input 输入 | Internal energy → electrical 内能 → 电能 |
| LED 发光二极管 | Output 输出 | Electrical → light 电能 → 光能 |
| Loudspeaker 扬声器 | Output 输出 | Electrical → sound 电能 → 声能 |
| Motor 电动机 | Output 输出 | Electrical → kinetic 电能 → 动能 |
3. Energy Conversion and Efficiency | 能量转换与效率
All output devices obey conservation of energy. The electrical energy delivered to the device is partly converted into the desired output form and partly wasted, usually as internal energy in the device itself. Efficiency is defined as the useful output energy divided by the total input energy, and is normally expressed as a percentage.
所有输出设备都遵守能量守恒。输送给设备的电能一部分转化为所需的输出形式，另一部分通常以设备自身内能的形式被浪费。效率定义为有用输出能量除以总输入能量，通常用百分数表示。
Efficiency = (useful output energy ÷ total input energy) × 100%
Because power is energy transferred per unit time, the same expression can be written using power, and for any electrical component the power is given by the standard equations below. These are used constantly in output-device calculations.
由于功率是单位时间转移的能量，同一表达式也可以用功率写出；对任何电气元件，功率都由下面这些标准公式给出。它们在输出设备的计算中反复出现。
P = V × I = I² × R = V² ÷ R
Typical efficiencies for comparison: a filament lamp converts only about 5% to 10% of the input energy into visible light, an LED can reach 30% to 50%, and a moving-coil loudspeaker typically converts less than 1% into sound. The remaining energy appears as internal energy, which is why output devices can become warm in use.
典型效率可供比较：白炽灯只把约 5% 至 10% 的输入能量转化为可见光，LED 可达 30% 至 50%，而动圈扬声器通常不到 1% 转化为声能。其余能量以内能形式出现，这就是输出设备使用时会发热的原因。
4. The Light-Emitting Diode (LED) | 发光二极管（LED）
An LED is a semiconductor p-n junction. When it is forward biased, electrons and holes recombine at the junction and photons are emitted. The colour depends on the semiconductor material and therefore on the energy gap of the junction: infrared, red, yellow, green, blue and white LEDs are all available. The forward voltage is typically about 2 V for a red LED and about 3 V to 3.5 V for a blue or white LED.
LED 是一种半导体 p-n 结。当它正向偏置时，电子与空穴在结区复合而发射光子。颜色取决于半导体材料，因而取决于结的能隙：红外、红、黄、绿、蓝、白光 LED 均可获得。红色 LED 的正向电压通常约为 2 V，蓝色或白色 LED 约为 3 V 至 3.5 V。
Three facts about LEDs matter in exams. First, they are diodes, so they conduct in one direction only and must be connected the right way round, anode to the positive supply. Second, they have a very low reverse breakdown voltage, often around 5 V, so a reversed LED in a 9 V circuit can be destroyed. Third, the operating current is small, typically 10 mA to 20 mA, so a series resistor is always required to limit the current.
考试中有三点关于 LED 的事实很重要。第一，它们是二极管，只在一个方向导通，必须正确连接极性，阳极接电源正极。第二，它们的反向击穿电压很低，常约为 5 V，因此在 9 V 电路中接反的 LED 可能被损坏。第三，工作电流很小，通常为 10 mA 至 20 mA，因此必须始终串联一个限流电阻。
- Advantages over filament lamps: higher efficiency, longer life, lower operating current, no warm-up time, small size, works at low voltages.
相对白炽灯的优点：效率更高、寿命更长、工作电流更小、无需预热、体积小、可在低电压下工作。 - Disadvantages: needs a series resistor, unidirectional, sensitive to overheating, colour fixed by the material.
缺点：需要串联电阻、单向导通、对过热敏感、颜色由材料决定不可变。
5. Calculating the LED Series Resistor | LED 串联电阻的计算
The series resistor takes the voltage left over after the LED has taken its forward voltage, and at the same time sets the current. Since the resistor and LED are in series, the current through both is the same. Using Kirchhoff's second law, the supply voltage equals the sum of the voltages across the resistor and the LED.
串联电阻承担 LED 取走正向电压之后剩余的电压，同时决定电流大小。由于电阻与 LED 串联，通过两者的电流相同。由基尔霍夫第二定律，电源电压等于电阻两端电压与 LED 两端电压之和。
R = (Vsupply − VLED) ÷ I
0 − 2.0 = 4.0 V. The current is 20 mA = 0.020 A. Therefore R = 4.0 ÷ 0.020 = 200 Ω. The resistor must also be checked for power rating: P = I² × R = 0.020² × 200 = 0.08 W, so a standard 0.25 W resistor is perfectly safe.
例题：一个正向电压为 2.0 V、额定电流为 20 mA 的红色 LED 接在 6.0 V 电源上。电阻两端的电压为 6.0 − 2.0 = 4.0 V。电流为 20 mA = 0.020 A。因此 R = 4.0 ÷ 0.020 = 200 Ω。还必须校核电阻的功率额定值：P = I² × R = 0.020² × 200 = 0.08 W，因此标准的 0.25 W 电阻完全安全。
If the value you calculate is not available, choose the next higher preferred value rather than the next lower one, because a smaller resistance gives a larger current and could burn out the LED. The same method applies to any diode, and to a chain of LEDs in series, where the forward voltages simply add.
如果计算出的阻值买不到，应选择下一个更大的标准值而不是更小的值，因为电阻越小电流越大，可能烧毁 LED。同样的方法适用于任何二极管，也适用于串联多个 LED，此时各正向电压直接相加。
6. Filament Lamps and Other Light Outputs | 白炽灯与其他光输出
A filament lamp contains a tungsten wire that is heated to about 2500 K by the current. At this temperature the wire glows and emits visible light, but most of the emitted radiation is infrared, which is why the efficiency is only about 5% to 10%. The filament is sealed in an inert gas or a vacuum so that it does not oxidise.
白炽灯包含一根钨丝，通电后被加热到约 2500 K。在此温度下钨丝发光并发出可见光，但大部分辐射是红外线，因此效率只有约 5% 至 10%。灯丝被封在惰性气体或真空中，以防氧化。
The resistance of a filament lamp increases as the current through it increases, because the temperature rises and the lattice ions vibrate more strongly, scattering the conduction electrons. This is why the V-I graph of a filament lamp curves and does not obey Ohm's law. In output-device questions, the lamp is usually treated as a fixed load with a stated voltage and current rating, but be ready to discuss the non-ohmic behaviour.
白炽灯的电阻随通过它的电流增大而增大，因为温度升高使晶格离子振动更强，对传导电子散射更多。这就是白炽灯的 V-I 图像呈曲线、不遵守欧姆定律的原因。在输出设备题中，通常把灯泡当作已给出电压与电流额定值的固定负载，但要准备好讨论它的非欧姆特性。
Other light outputs you should recognise include the filament indicator lamp in a mains-operated appliance, the seven-segment LED display, and the laser diode used in optical communication and in CD or DVD drives. A laser diode is a p-n junction like an LED, but its end faces are cleaved to form an optical cavity, which produces coherent, monochromatic light.
你应认识的其他光输出还包括市电电器中的灯丝指示灯、七段 LED 数码管，以及用于光通信和 CD、DVD 驱动器的激光二极管。激光二极管与 LED 一样是 p-n 结，但它的端面被解理形成光学谐振腔，因而产生相干、单色的光。
7. Loudspeakers and Buzzers | 扬声器与蜂鸣器
A moving-coil loudspeaker consists of a permanent magnet, a cylindrical coil attached to a paper or plastic cone, and a flexible support. When an alternating signal current passes through the coil placed in the radial magnetic field, each element of the coil experiences a force given by the motor effect.
动圈扬声器由永久磁铁、固定在纸盆或塑料盆上的圆柱形线圈，以及弹性支撑组成。当交变信号电流通过置于径向磁场中的线圈时，线圈的每一小段都受到由电动机效应给出的力。
F = B × I × L
Because the current alternates, the direction of the force alternates, so the coil and cone vibrate backwards and forwards. The frequency of the vibration equals the frequency of the signal, and the amplitude of the vibration depends on the amplitude of the current. The vibrating cone pushes and pulls the surrounding air, creating pressure variations that travel as a sound wave. Human hearing covers roughly 20 Hz to 20 kHz.
由于电流交变，力的方向也交变，因此线圈与纸盆来回振动。振动频率等于信号频率，振动幅度取决于电流幅度。振动的纸盆推拉周围空气，产生以声波形式传播的压强变化。人耳听觉范围大致为 20 Hz 至 20 kHz。
A buzzer is a compact sound source often used as an alarm. A piezo-electric buzzer contains a crystal or ceramic disc that changes shape when a voltage is applied; many types include an internal oscillator so that they sound continuously from a d.c. supply. An electromagnetic buzzer works like a miniature loudspeaker driven by an oscillating contact. Buzzers are cheap, rugged, take little current and are easy to drive from a logic or op-amp output.
蜂鸣器是一种常用作报警的紧凑型声源。压电蜂鸣器含有一片受压会变形的晶体或陶瓷圆片；许多型号内置振荡器，因此接直流电源即可持续发声。电磁式蜂鸣器的工作原理类似由振荡触点驱动的小型扬声器。蜂鸣器便宜、坚固、耗流小，易于由逻辑电路或运放输出驱动。
8. Motors, Solenoids and Mechanical Output | 电动机、螺线管与机械输出
A d.c. motor converts electrical energy into rotational kinetic energy. A rectangular coil carrying current in a radial magnetic field experiences equal and opposite forces on its two sides, so a couple acts and the coil rotates. The direction of the force is found with Fleming's left-hand rule. A split-ring commutator reverses the current in the coil every half turn, so the torque keeps acting in the same rotational sense and the coil turns continuously.
直流电动机把电能转化为转动动能。放在径向磁场中的矩形通电线圈，其两条边受到大小相等、方向相反的力，因而形成力偶使线圈转动。力的方向用弗莱明左手定则判断。分裂环换向器每半圈反转线圈中的电流方向，使转矩始终沿同一转向作用，线圈便持续转动。
In a small d.c. motor the speed increases with the supply voltage and falls as the mechanical load increases, because a larger load requires a larger current and therefore a larger torque. If the coil is held still the current is at its maximum, which is why a stalled motor can overheat.
对于小型直流电动机，转速随
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