📚 Electromagnets 1.1.1 – Circuits and Symbols Concept Explained | 电磁铁 1.1.1 – 电路与符号概念解析
Before you can build an electromagnet or analyse any electrical system, you must first learn the language of circuits. This involves understanding the standard symbols used to represent components, how current and voltage behave, and the fundamental relationships that govern all electrical devices. In this article, we will break down the core concepts behind circuits and symbols, laying a solid foundation for your studies in electromagnetism and beyond.
在你能够制作电磁铁或分析任何电气系统之前，你必须先学习电路的语言。这需要理解用来代表元器件的标准符号，认识电流和电压的行为方式，以及支配所有电气设备的基本关系。本文将剖析电路与符号背后的核心概念，为你的电磁学学习及更远的知识打下坚实基础。
1. What Is an Electric Circuit? | 什么是电路？
An electric circuit is a closed loop or pathway that allows electric charge to flow continuously. For a circuit to work, it must include a source of electrical energy (such as a cell or battery), conducting wires, and a load (like a lamp or motor) that converts the electrical energy into another form.
电路是允许电荷持续流动的闭合回路或路径。要使电路工作，它必须包含电能来源（例如电池或电池组）、导线以及将电能转化为其他形式的负载（如灯泡或电动机）。
If the loop is broken at any point, current stops flowing. This is called an open circuit. A switch is a simple component used to make or break the circuit intentionally.
如果回路在任何一点断开，电流就会停止流动，这称为断路。开关就是一种用来有意接通或断开电路的简单元器件。
2. Current, Charge and Conventional Flow | 电流、电荷与常规流向
Electric current (I) is the rate of flow of electric charge. It is measured in amperes (A). In metal wires, it is the movement of free electrons that constitutes the current. However, by convention, current is shown as flowing from the positive terminal to the negative terminal of a power supply. This is called conventional current direction.
电流（I）是电荷流动的速率，以安培（A）为单位。在金属导线中，是自由电子的移动形成了电流。但根据惯例，电流被表示为从电源正极流向负极，这被称为常规电流方向。
The relationship between charge (Q), current (I) and time (t) is:
电荷（Q）、电流（I）和时间（t）之间的关系为：
Q = I × t
where Q is in coulombs (C), I in amperes (A), and t in seconds (s).
其中 Q 的单位是库仑（C），I 的单位是安培（A），t 的单位是秒（s）。
3. Voltage and Electromotive Force (EMF) | 电压与电动势（EMF）
Voltage (V), also called potential difference (p.d.), is the energy transferred per unit charge as it moves between two points in a circuit. It is measured in volts (V). The power supply provides an electromotive force (EMF), which is the total energy supplied per coulomb of charge. When there is no current flowing, the terminal voltage equals the EMF.
电压（V），也称为电势差（p.d.），是单位电荷在电路中两点之间移动时所转移的能量，以伏特（V）为单位。电源提供电动势（EMF），即每库仑电荷获得的总能量。当没有电流流动时，端电压等于电动势。
Energy transferred can be calculated using:
转移的能量可以用以下公式计算：
E = Q × V
where E is energy in joules (J).
其中 E 是能量，单位为焦耳（J）。
4. Resistance and Ohm’s Law | 电阻与欧姆定律
Resistance (R) is a measure of how much a component opposes the flow of current. It is measured in ohms (Ω). For many conductors, the current through them is directly proportional to the potential difference across them, provided temperature remains constant. This is Ohm’s Law:
电阻（R）衡量元器件对电流阻碍作用的大小，单位为欧姆（Ω）。对于许多导体，在温度保持不变的条件下，通过它们的电流与其两端的电势差成正比，这就是欧姆定律：
V = I × R
A resistor is a component designed to have a specific resistance, and its circuit symbol is a rectangle or a zigzag line depending on the standard used.
电阻器是设计成具有特定电阻值的元器件，其电路符号根据所用标准为矩形或锯齿线。
5. Standard Circuit Symbols – Overview | 标准电路符号概述
Circuit diagrams use standardized symbols so that anyone can understand the design regardless of language. The most widely adopted standards are IEC (International Electrotechnical Commission) and ANSI (American National Standards Institute). In international exams, both styles may appear, but the concepts remain identical.
电路图使用标准化的符号，这样任何人都能理解设计而不受语言限制。最广泛采用的标准是IEC（国际电工委员会）和ANSI（美国国家标准学会）。在国际考试中，两种风格都可能出现，但概念完全相同。
Below is a table of the most common symbols you will encounter in the context of electromagnets and basic circuits:
下表列出了你在电磁铁和基础电路中最常遇到的符号：
| Component (元器件) | Symbol Description (符号描述) | Function (功能) |
|---|---|---|
| Cell (单节电池) | Long line positive, short line negative (长线正极，短线负极) | Provides DC energy (提供直流电能) |
| Battery (电池组) | Two or more cells joined (两个或多个单节电池连接) | Higher voltage supply (提供更高电压) |
| Fixed Resistor (固定电阻器) | Rectangle (IEC) or zigzag (ANSI) (矩形或锯齿线) | Limits current / divides voltage (限制电流 / 分压) |
| Variable Resistor (可变电阻器) | Rectangle with arrow through it (带箭头的矩形) | Adjustable resistance (可调电阻) |
| Lamp / Bulb (灯泡) | Circle with a cross (带十字的圆圈) | Converts electricity to light (将电转化为光) |
| Switch (开关) | Break in a line with a hinged segment (线断开加铰链段) | Opens or closes circuit (断开或接通电路) |
| Ammeter (安培表) | Circle with ‘A’ (圆圈内有A) | Measures current in series (串联测电流) |
| Voltmeter (伏特表) | Circle with ‘V’ (圆圈内有V) | Measures p.d. in parallel (并联测电压) |
| Diode (二极管) | Triangle pointing to a line (三角形指向一条线) | Allows current in one direction only (只允许单向电流) |
| Light-Emitting Diode (LED) (发光二极管) | Diode symbol with two outward arrows (二极管符号加两个向外箭头) | Emits light when forward biased (正向偏置时发光) |
| Fuse (保险丝) | Rectangle with wire passing through (矩形内有导线穿过) | Melts to break circuit if current is too high (过流熔断) |
6. Connecting Components: Series vs. Parallel | 元器件的连接：串联与并联
There are two fundamental ways to connect components in a circuit: series and parallel. In a series circuit, components are connected end-to-end, forming a single path for current. If one component fails, the whole circuit is broken. In a parallel circuit, components are connected across common points, providing multiple paths for current. A break in one branch does not stop current in other branches.
电路中元器件的连接有两种基本方式：串联和并联。在串联电路中，元器件首尾相连，为电流提供唯一路径——若一个元器件损坏，整个电路便断开。在并联电路中，元器件跨接在公共点之间，为电流提供多条路径，一支路断开并不影响其他支路中的电流。
Key rules for series circuits:
串联电路的关键规则：
-
Current is the same everywhere: Iₜₒₜₐₗ = I₁ = I₂ = …
各处电流相等：Iₜₒₜₐₗ = I₁ = I₂ = …
-
Total resistance is the sum: Rₜₒₜₐₗ = R₁ + R₂ + …
总电阻为各部分之和：Rₜₒₜₐₗ = R₁ + R₂ + …
-
Supply voltage is shared: Vₛ = V₁ + V₂ + …
电源电压被分配：Vₛ = V₁ + V₂ + …
Key rules for parallel circuits:
并联电路的关键规则：
-
Current from the source splits: Iₜₒₜₐₗ = I₁ + I₂ + …
来自电源的电流分流：Iₜₒₜₐₗ = I₁ + I₂ + …
-
Voltage across each branch is the same as the source voltage: V₁ = V₂ = Vₛ
各支路电压与电源电压相同：V₁ = V₂ = Vₛ
-
Total resistance decreases as more branches are added: 1/Rₜₒₜₐₗ = 1/R₁ + 1/R₂ + …
总电阻随支路增多而减小：1/Rₜₒₜₐₗ = 1/R₁ + 1/R₂ + …
7. Ammeters and Voltmeters – Correct Placement | 安培表与伏特表 – 正确连接方式
An ammeter must be connected in series with the component whose current you want to measure. It has a very low internal resistance so that it does not significantly affect the circuit. A voltmeter is always connected in parallel across the component to measure the potential difference. It has a very high resistance to draw negligible current.
安培表必须与被测元器件串联，其内阻极低，因此不会对电路产生显著影响。伏特表必须与被测元器件并联来测量电势差，其内阻极高，因此吸取的电流可忽略不计。
Connecting an ammeter in parallel would create a short circuit with very large current, potentially damaging the meter. Similarly, a voltmeter in series would prevent useful current flow.
若将安培表并联，会造成短路而产生极大电流，可能损坏仪表。同样，伏特表串联则会阻碍有效电流通过。
8. Variable Resistors, Potentiometers and Thermistors | 可变电阻器、电位器与热敏电阻
A variable resistor (rheostat) allows you to change the resistance manually by moving a slider. It is often used to control current in a circuit, for example, to dim a lamp or vary motor speed. The circuit symbol is a resistor with an arrow diagonally across it.
可变电阻器（变阻器）允许通过移动滑片来手动改变电阻，常用于控制电路中的电流，例如调暗灯光或改变电动机转速。其电路符号是一个带对角箭头的电阻器。
A potentiometer is a three-terminal variable resistor used to adjust voltage. A thermistor is a special resistor whose resistance decreases as temperature increases. It is often used in temperature-sensing circuits. The symbol for a thermistor is a resistor with a line through it and a ‘t°’ or similar label.
电位器是一种三端可变电阻器，用于调节电压。热敏电阻是一种特殊电阻，其阻值随温度升高而降低，常用于温度传感电路。热敏电阻的符号是带有贯穿线和“t°”标记的电阻器。
9. Diodes and Light-Emitting Diodes (LEDs) | 二极管与发光二极管
A diode is a semiconductor device that allows current to pass in only one direction. It has a very high resistance in the reverse direction. The symbol is a triangle pointing to a vertical line. Current flows from the anode (triangle base) to the cathode (line) when the anode is more positive than the cathode – this is forward bias.
二极管是一种半导体器件，只允许电流单向通过，反向时电阻极大。其符号为一个指向竖线的三角形。当阳极（三角形底部）比阴极（直线）电位更高时，电流从阳极流向阴极，这就是正向偏置。
A light-emitting diode (LED) emits light when forward biased. It has a similar symbol with outward arrows representing light. LEDs are energy-efficient and are now the standard for indicator lights and displays. They require a series resistor to limit current, otherwise they can burn out.
发光二极管（LED）在正向偏置时会发光，其符号类似，外加代表光线的向外箭头。LED高效节能，现已成为指示灯和显示屏的标准配置。它们需要串联一个电阻来限制电流，否则可能烧毁。
10. Fuses, Circuit Breakers and Electrical Safety | 保险丝、断路器和用电安全
A fuse contains a thin wire that melts if the current exceeds the rated value, breaking the circuit and preventing damage or fire. The fuse symbol is a rectangle with the wire passing through. Fuses must be placed in the live wire of an appliance. Circuit breakers perform the same function but can be reset.
保险丝内含一根细丝，当电流超过额定值时便会熔断，从而断开电路、防止损坏或火灾。保险丝的符号是一个矩形，内有导线穿过。保险丝必须安装在家电的火线上。断路器功能相同，但可以复位。
Never connect a component directly across a power supply without a load, as this creates a short circuit and dangerously high current. Always include a fuse or circuit breaker for protection.
切勿将元器件不经负载直接跨接在电源上，因为这会造成短路并产生危险的高电流。务必接入保险丝或断路器加以保护。
11. Building an Electromagnet – Applying Circuit Concepts | 制作电磁铁 – 电路概念的应用
An electromagnet consists of a coil of wire (solenoid) wound around an iron core. When current flows, it produces a magnetic field. The strength of the electromagnet can be increased by increasing the current, increasing the number of turns of the coil, or using a soft iron core. This directly applies your knowledge of circuits: you need a power source, a switch to control the current, perhaps a variable resistor to adjust the magnetic strength, and an ammeter to monitor the current.
电磁铁由绕在铁芯上的线圈（螺线管）构成。当电流流过时，便产生磁场。电磁铁的强度可通过增大电流、增加线圈匝数或使用软铁芯来增强。这直接应用了你的电路知识：你需要一个电源、一个控制电流的开关、或许一个可变电阻器来调节磁力强度，再加一个安培表来监测电流。
The circuit for an electromagnet is simply a series circuit with a battery, switch, variable resistor, ammeter and the coil. The coil itself can be represented by an inductor symbol, but at introductory level, it may be shown as a coiled wire.
电磁铁的电路只是一个简单的串联电路，包含电池、开关、可变电阻器、安培表和线圈。线圈本身可用电感符号表示，但在入门阶段可能会画成绕线的图形。
12. Drawing and Interpreting Circuit Diagrams | 绘制和解读电路图
When drawing circuit diagrams, use a ruler and consistent symbols. Wires should be drawn as straight lines, with right-angle corners where needed. Avoid unnecessary crossings. Label components if necessary, but the symbols should be clear. Being able to read a circuit diagram allows you to predict voltages, currents and the behaviour of the entire system, which is essential for both exams and practical work.
绘制电路图时，要使用直尺和统一的符号。导线应画成直线，需要拐角处用直角。避免不必要的交叉。必要时标出元器件，但符号本身应清晰明了。能够阅读电路图，就能预测各处的电压、电流以及整个系统的行为，这对考试和实际操作都至关重要。
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