电是自然界中的一种基本现象，源于电荷的存在和运动。所有物质都由原子组成，原子中包含带正电的质子和带负电的电子。当电荷发生定向移动时，就形成了电流。
初中物理电学从最基本的电荷现象出发，逐步学习电路连接、电流电压规律、欧姆定律，最终了解电与磁的关系以及安全用电。
Electricity is a fundamental natural phenomenon, originating from the existence and movement of electric charges. All matter is made of atoms, which contain positively charged protons and negatively charged electrons. When charges move directionally, an electric current is formed.
Junior physics electricity starts from basic charge phenomena, progressing through circuit connections, current/voltage laws, Ohm's Law, and finally the relationship between electricity and magnetism, plus electrical safety.
摩擦起电是指两种不同材料的物体相互摩擦后，分别带上等量异种电荷的现象。例如，用丝绸摩擦玻璃棒，玻璃棒带正电，丝绸带负电。
原因：不同原子核对电子的束缚能力不同。束缚能力弱的材料会失去电子带正电，束缚能力强的材料会得到电子带负电。摩擦起电的本质是电子的转移。
Triboelectric charging is the phenomenon where two different materials become charged with equal but opposite charges after being rubbed together. For example, rubbing a glass rod with silk makes the rod positively charged and the silk negatively charged.
Reason: Different atomic nuclei have different abilities to hold electrons. Materials with weaker binding lose electrons (become positive), while those with stronger binding gain electrons (become negative). The essence of triboelectric charging is electron transfer.
自然界只存在两种电荷：正电荷和负电荷。电荷间遵循同种电荷相互排斥，异种电荷相互吸引的规律。
验电器是利用同种电荷相互排斥的原理制成的仪器，用来检验物体是否带电。金属箔张角越大，说明带电越多。
Nature has only two types of charge: positive and negative. The rule: like charges repel, unlike charges attract.
The electroscope uses the repulsion of like charges to detect whether an object is charged. The wider the metal foil opens, the more charge is present.
电荷量用 \(q\) 或 \(Q\) 表示，单位是库仑（C）。一个电子的电荷量为 \(e = 1.6 \times 10^{-19}\,\text{C}\)，这是自然界中最小电荷单元，称为元电荷。
大量电荷的定向移动形成电流。电流方向规定为正电荷定向移动的方向，与电子实际移动方向相反。
Charge is denoted \(q\) or \(Q\), unit: coulomb (C). One electron carries \(e = 1.6 \times 10^{-19}\,\text{C}\), the smallest charge unit in nature — the elementary charge.
Mass movement of charges forms current. Current direction is defined as the direction of positive charge movement, which is opposite to the actual electron movement.
导体：容易导电的物体（如金属、石墨、人体、大地、酸碱盐溶液）。导体内部有大量自由电荷（金属中是自由电子），可以在电场作用下定向移动。
绝缘体：不容易导电的物体（如橡胶、塑料、玻璃、陶瓷、干燥空气）。绝缘体内部自由电荷极少。
半导体：导电能力介于导体和绝缘体之间（如硅、锗），是电子元件的重要材料。
Conductors: Materials that easily conduct electricity (metals, graphite, human body, earth, acid/base/salt solutions). They contain abundant free charges (free electrons in metals) that move under an electric field.
Insulators: Materials that hardly conduct electricity (rubber, plastic, glass, ceramic, dry air). They have very few free charges.
Semiconductors: Conductivity between conductors and insulators (silicon, germanium) — important materials for electronic components.
一个完整的电路由四个基本要素组成：
①电源 — 提供电压（电池、发电机），将其他形式的能量转化为电能。
②用电器 — 消耗电能（灯泡、电动机、电热器），将电能转化为其他形式的能量。
③开关 — 控制电路的通断。
④导线 — 连接各元件，提供电荷流动的通路。
A complete circuit has four basic elements:
①Power source — provides voltage (battery, generator), converts other energy to electrical energy.
②Load/appliance — consumes electrical energy (bulb, motor, heater), converts electrical energy to other forms.
③Switch — controls circuit on/off.
④Wires — connect components, provide paths for charge flow.
用统一规定的电路元件符号表示电路连接情况的图叫电路图。画电路图的要求：
• 连线横平竖直，元件分布均匀
• 不得画在拐角处
• 开关一般画成断开状态
• 元件符号使用国家标准
A circuit diagram uses standardized component symbols to show connections. Requirements:
• Lines horizontal/vertical, components evenly distributed
• No components at corners
• Switches usually shown open
• Use standard symbols
常见元件符号 Common Symbols:
电池 ─┤├─ 开关 ──/ ── 灯泡 Ⓧ 电阻 ─▭─ 电流表 Ⓐ 电压表 Ⓥ
①通路（闭合电路）：电路处处连通，有电流通过用电器，用电器正常工作。
②断路（开路）：电路某处断开（开关断开、导线断裂、接触不良），没有电流通过。
③短路：电流不经过用电器直接由电源正极回到负极。短路的危害：电流过大烧毁电源甚至引发火灾！绝对不允许电源短路！
①Closed circuit: Complete path, current flows, appliances work normally.
②Open circuit: Break somewhere (switch open, broken wire, poor contact), no current flows.
③Short circuit: Current bypasses the load directly from positive to negative terminal. Danger: excessive current burns the power source, may cause fire! Never short-circuit a power source!
下方是一个简单电路模拟。点击开关可以观察通路/断路状态。当开关闭合时电流表有读数——这就是通路；断开时电流表读数为0——这就是断路。
Below is a simple circuit simulator. Click the switch to observe closed/open states. When closed, the ammeter reads current — a closed circuit; when open, reading is 0 — an open circuit.