Electric Current and Circuits

📋 Course Outline

1. Electric Charge and Current
2. Charge Flow and Drift Speed
3. Quasi-Static Circuit Approximation
4. Circuit Topology Vocabulary
5. Kirchhoff Node Law
6. Kirchhoff Mesh Law

📖 1. Electric Charge and Current

🔑 Key Concepts & Definitions

• Electric charge : the electrical quantity carried by matter particles, with the quantized value $q = Ze$
• Electric current : the rate of change of charge through a section, given by $I = \frac{dq}{dt}$

★ Must-know

📌 The conventional current direction is the direction of positive-charge motion and is opposite to the motion of negative charges.

Further detail

• A proton has charge $+e$, a sodium ion Na⁺ has charge $+e$, a silicon ion Si²⁻ has charge $-2e$, and a semiconductor hole has charge $+e$.

💡 Memory Hook

Positive charge and negative charge move in opposite conventional-current directions.

📖 2. Charge Flow and Drift Speed

📐 Formula — For n charge carriers of elementary charge e crossing a conductor section S at speed v, the current magnitude is $I = envS$.

📐 Formula — The carrier drift speed is $v = \frac{I}{enS}$.